Nos. 12, 13, 14 December 15, 1952 ATOLL RESEARCH BULLETIN 12 Preliminary Report on Geology and Marine Environments of Onotoa Atoll, Gilbert Islands by PRESTON E. CLoup, JR. 13, Preliminary Report on Marine Biology Study of Onotoa Atoll, Gilbert Islands by A. H. BANNER and J. E. RANDALL 14 Description of Kayangel Atoll, Palau Islands by J. L. GREssITT THE PACIFIC SCIENCE BOARD National Academy of Sciences—National Research Council Washington, D. C., U.S.A. ATOLL RESEARCH BULLETIN 12. Preliminary Report on Geology and Marine Environnents of Onotoa Atoll, Gilbert Islands by Preston E. Cloud, Jr. 13. Preliminary Report on Marine Biology Study of Onotoa Atoll, Gilbert Islands by A. H. Banner and J. E. Randall 14. Description of Kayangel Atoll, Palau Islands by J. L. Gressitt Issued by ) THE PACIFIC SCIENCE BOARD National Academy of Sciences-=-National. Research Council Washington, D.C. December 15, 1952 ACKNOWLEDGEMENT It is a pleasure to commend the far-sighted policy of the Office of Naval Research, with its emphasis on basic research, as a result of which a grant has made possible the continuation of the Coral Atoll Program of the Pacific Science Board. It is of interest to note, historically, that much of the fundamental information on atolis of the Pacific was gathered by the U. S. Navy's South Pacific Exploring Expedition, over one hundred years ago, under the command of Captain Charles Wilkes, The continuing nature of such scientific interest by the Navy is shown by the support for the Pacific Science Board's research programs, CIMA, SIM, and ICCP, during the past five aargs The Coral Atoll pega is.a part of SIM, The preparation and issuance of this Bulletin is assisted by funds from Contract No. 47-onr-291, Task Order IV. The sole responsibility for all statements made by authors of papers in the Atcll Research Bulletin rests with them, and do not necessarily represent the views of the Pacific Science Board or the editors of the Bulletin, Editorial Staff Biel) ttre Fosberg, editor M.-H, Sachet,: assistant editor Correspondence concerning the Atoll Research Bulletin should be addressed to the above c/o Pacific Science Board National Research Council 2101 Constitution Avenue, NW. Washington 25, D.C., U.S.A. NOTICE The editors of the Atoll Research Bulletin are engaged in compiling bibliographies covering several phases of the science of coral atolls as well as the vegetation of high islends. They will greatly appreciate having any papers mentioning atolls or low coral islands brought to their attention. If readers of the Builetin care to send in copies of their ow papers, this will make it more certain that they will be included in the appropriate bibliographies, and the papers will be available in the Pacific Vegetation Project files for use of those interested. It may be possible, from time to time, to issue reviews of papers thit are sent in, especially if they have a direct bearing on the work of the Atoll Research Program, or of the Pacific Vegetation Project. Such papers should be addressed to: The Pacific Vegetation Project c/o National Research Council 2101 Constitution Avenue, N. W. Washington 25, D. C., U. S. A. oe ee ee ae a we a ee ERRATA In maps accompanying several of the eerlier numbers of the Atoll Research Bulletin, especially nos. 5, 9, and 10, ratio scales, e.g. 1: 10,000, 1: 7500, were inadvertently left in the maps when they were reduced for publication. Reduction, of course, makes these inaccurate. They should be deleted or disregarded. On the title page of Bulletin no. 10 shouid be added after the author's name the following: (assisted by John Tobin and Gerald Wade). Ltosdraa’ lok ee ATOLL, RESEARCH BULLETIN eee ee No. a2 Preliminary Report on Geology and Marine Environments of Onotoa Atoll, Gilbert Islands by Preston E. Cloud, Jr. Issued by THE PACIFIC SCIENCE BOARD National Academy of Sciences--National Research Council Washington, D. C. December 15, 1952 dereeeS. Latin hdeti~~go Mnwtes ho yuehwod, Cac teli VW ‘beset “dy ox ‘4 he ad PaiP Sa bare biti NED Betis eg Naa th aL aren: | ake Ul dar cata ely : ANCL DACRARE RSH AC POUL Gel ‘cedinomeg Preliminary Report on the Geology and Marine Environments of Onotoa Atoll, Gilbert Islands SCLENTIFIC INVESTIGATIONS IN MICRONESTA Pacific Science Board > National Reséarch Council Preston E. Cloud, Jr. U. S. Geological Survey Washington, D. C. June 1952 ACKNOWLEDGMENTS This report presents preliminary results of geological field work done in 1951 under the Atoll Project of the National Research Council's Pacific Science Board. The project is supported by funds granted to the National Academy of Sciences by the Office of Naval Research, and the field work was carried out with the active assistance of the U. S. Navy epee Coast Guard, and Army. Special thanks are due for help received from Mr. Harold Coolidge, Miss Ernestine Akers, and Mrs. Lenore Smith, of the Pacific Sei- ence Board, and from Lt. M. E. Katona and ins. Lee Nehrt, of the U. S. Coast Guard cutter "Nettle." My associates in the field were Dr. EH. T. Movl, Dr. W. H. Goodenough, Dr. A. H. Banner, Mr. D. E. Strasburg, and Mr. John Randall. The field work of the Onotoa Party being in the Gilbert and Ellice Is- lands Colony, we were tne guests of the British Government, then represented by Acting Resident Conrissioner R. J. Keegan, who took a most helpful per- sonal interest in our work. Special courtesies and favors were also received from Mr. E. C. Cartland, Wr. Stanley Silver, and Mr. Alan Hart, of the Tarawa Government staff. The then Colony Lands Commissioner and Administrative Officer on Onotoa, Mr. Richard Turpin, and his wife befriended and helped the entire field party in every conceivable ED have them as "guardians" was a great help in carrying out our work among a people whose language and ways were remote from ours. Finally, I must thank the people of Onotoa themselves, who welcomed us to their island and helped us as much as they could. CONTENTS INOVPCBYONE, oe Sa Ta spstorshbletenoya aa General setting and climate —-------—~--------~---- TENUSMONS) Tau YS) General features of the lagoon -----~---~---—------~- Principal ecologicaand sedimentary subdivisions -- Islands ~-- Intertidal Outer reef Intertidal i ee a a nn ne nn em ne oe environments except reefs -------~ to lagoonal environments -~---~--- Environments of the lagoon and leeward shelf Origin of beachrock -~~------—-~-~-~---—~—--~~—~—--~-~--~-~ Hydrology ------ i ee we ee ee Ground water -—-~---~---~-~----——~--------—~--~-~-- Shallow sea and tide pools ~----~----~-------- Flow of water over the windward reef ~--~---~ Origin of reef-front grooves and surge channels -- Building and erosion of atoll islands ----~---~-—--~~ Shifts of sea level and their effects on modern reefs -----— Appendix A - List of reef building corals and hydrozoans 52 59 Appendix B -- Description of sedimentary and ecologic field units ------—--—---~-~—-~-~~-~--~--- Islands --~--~------------—-—-~--~----------=-—— aif Dune Limesands -—---—---~---~-~-------—---~— Limesands other than known dune deposits Limegravels —-----.——- eo ESS - Caliche --~----------~-----~---------~—=— Land hound areas of permanent brackish weter --—-~------~- --~-—------— -- = Intertiaal environments except reeis ~------- Uiicousolidated beach --—------~——-—--—--~ Rocky beach ---------~-~~—---------—------- ae thakesd intertidal flats Se eee Mainiy intertidal flats adjacent to ieZoon proper -----—---—----=—-~--~—~-—--—— Bars and spits -----—--—--------—-----—- Outer reef --~-----—--------—-——-~----—---=---- Intertidal to lagoonal environments -~-----—- Environments of the lagoon and leeward shelf Referer Ces) aaa an Page ILLUSTRATIONS Tables Rainfall at Government Station, Onotoa atoll Rainfall at Betio Island, Tarawa atoll Properties of ground water on Onotoa Variations in pH of shallow marine and beach-zone waters Temperature, chloride content, and hardness of shaliow marine and beach-zone waters Figures Index map, showing location of Onotoa Generalized geology and marine environments of Onotoa Island profiles, Onotoa atoll Properties of shallow water in near-shore lagoon Properties of shallow water in flow over windward reef flat Properties of water in low tide pool of windward reef flat Properties of water in high tide pool of seaward beach Temperature and pH of spray pools ff; Leal i : f hore fag ch ‘4 ! iy , saG Yoo 7 : a j aa sk vs, a alt ra $ pipiby # | vs s bes if Pex sieang A ie i ar he PRELIMINARY REPORT ON THE GEOLOGY AND MARINE ENVIRONMENTS OF ONOTOA ATOLL, GILBERT ISLANDS 1/ By Preston E. Cloud, Jr. 2/ 1/ Publication authorized by the Director, U. S. Geological Survey. 2/ Geologist, U. S. Geological Survey. ABSTRACT eition is a "dry" atoll just south of the equator and west of the inter- national date line. Its yearly rainfall averages only about 40 inches, droughts occur periodically, and ground cover vegetation is sparse. Island deposits are almost exclusively unconsolidated calcium carbonate gravel and sand, the gravel mainly toward the sea and the sand mainly lagoonward. With- in this eoeeable material and the permeabie reei-rock beneath, ground water floats in hydrostatic balance ain sea water below. Toward the center of islands more than about 1000 feet wide this water is generally potable. In narrower parts of islands, however, it becomes brackish at times of drought, resulting in the death of breadfruit, taro, and even coconut trees. Soils Ue saipey the calcium carbonate sediments, with a humus layer not exceeding about 10 inches and an average pH of about 8.1. The shape of the Lagoon bottom is Be ted cron echo sounding and direct observation. It comprises three shallow basins (maximum depth 8 fathoms) that are separated from one another and from the sea beyond by still shallower water, the whole with numerous small eaten reefs that rise to or near the surface. The near-surface framewrk of the Onotoa reefs consists primaril P of the biue sare gata Heliopora, a genus that is not extensively develoned there among now living corals. Fish are shown to be important in the produc-— tion of lagoonal sediments. The sediments, soiis, and surface waters of the island areas of Onotca, and the ecolcgic zones and cepcsits of its shallow marine waters, are here provisionally described and classified. Preliminary identifications of coral collections indicate them to include about 26 genera and 50 to 60 species. Limited observationg on the chemistry and movenent of some of the shallow marine waters show a diurnal “eeaee in pH and an out-flowing gravity current across the windward reed fiat and upper benched reer slope. During the day pH rises and precipitation of CaC03 probably occurs in very shal ew ee At nant pH falis, favoring soiution of Cal03 in intertidal environments. Dominance of solution effects in the shore zone is believed to ron fram constant flushing of precipitated Apoaueeenn The out—flowing eravity cheeens is believed an important factor in origin of offshore grooves and cones ener neis, through abrasion by debris in transit eeanard at times of bench | trimeation. : | it is argued that Maude sewosts shdiner isiadeae and \ino-precipitating pbeue are important in formation of Teg eds, ped sumably both wrouy bonding of successive guntacs layers and through interstitial precipitation of avo. Atoll islands are built on sufficiently wide reef foundations at: or near ; the surface of the sea at a distance from the reef front determined by local force of storm waves and to a iced eermninea by time and supply of sediment. First a gravel ridge or rennane Du eeeneed by storm waves on the reef flat. On the lagoon side of this gravel rampart the sandy portions of the islands grow by longshore drift of reef flat debris and by wind action. Erosion “occurs mainly at times of storm by breaching or complete removal of islands. Onotoa provides additional evidence in support of the now well-documented 6-foot eustatic fall of sea level that began probably more than 4000 and less than 7000 years ago. The evidence consists of elevated Heliopora flats and elevated cobble stripes such as are known to form only on the reef flat. The superficial appearance of modern reef surfaces in the tropical belt is at- tributed primarily to whether they were within 6feet of sea level when this recession began. INTRODUCTION This report presents some of the preliminary results of an integrated program of field studies on the terrestrial and marine botany and zoology, geology, and anthropology of Onutoa’(G no! to ane a “dry" atoll in the gouthern Gilbert Islands (the Kingsmill Group of early records). These studies were made by a field team of the Pacific Science Board during late June, July, and ihenee G2 195i. Tae Gilbert Islands (fig. 1) straddle the equator just west of the inter-— national date line, and the position of the anchorage at the west side and ‘ toward the north end of OQnotoa wes determined by Ins. Lee Nekrt of USCGS "Nettle" as 1947'33" S., 175°29'30" E. (U. S. Hydrographic Office, 1950, p. 51, states "northwestern end in 1°46' S., 175°30' S."). Onotoa is the most southerly atoll of the group, though tvo "reef islands" (Tamana and Arorae) lie still farther south. Operations were carried out from a temporary bese camp adjacent to the Government “tation on the more northerly of the two main islands of Onotoe | (fig. 2). Materials and equipment for camp and technical operations were assembled at Kwajalein end ieenanerted to Onotoa by the U. S. Coast Guard Cutter "Nettle," under command of Lt. M. E. Katona. Ali botanical names used in this report were supplied by Dr. E. T. Moul and represent either his provisional field identifications or my extensions of them. All titrations for salinity factors were made and computed in the field by Mr. D. E. Strasburg, my assistant in the geologic field studies. Preliminary identifications of corals were provided by Dr. J. W. Wells, of arthropods by Dr. F, A. Chace, and of mollusks by Dr. H. A. Rehder and Mr. R. T. Abbott. GENERAL SETTING AND CLIMATE The general setting of Onotoa with reference to currents, winds, and geography is shown in figure 1. This atoll lies between the west-—flowing south equatorial current and the east-flowing equatorial countercurrent. A local north-flowing current is suggested by the fact that during our stay there a marked swell from the south produced strong surf on exposed lee reefs that face the south (fig. 2). At the same time surf was weak along the stretch of lee reef north from the anchorage around the north end of the atoll to the large northern island (fig. 2). According to the map on which figure 1 was based, Onotoa lies at about the northern iimit of the southeast trade winds.. During late June, July, and August of 1951 the wind blew almost steadily from a little south of east to nearly due east, with the nae of recurrent winds from the west on June 24 and 25 and of occasional squalls from the southeast to south-southeast. On one occasicn winds: of gale or near-gale velocity blew intermittently from the east and southeast for the better part of a day. The British Colonial Office (1950, p. 39) has reported that "For most of the year there is a steady easterly trade wind, but from October to March...occasional west and northwest gales occur. The wind in these gales does not reach hurricane force." An exception to the rule is found in the record of a hurricane at Butaritari in the northern Gilberts, variously dated as December 1927 and January 1928 (Sachet, in Pac. Sci. Bd ., 1951, pp. 8-9). The climate of Onotoa is warm and even. For the Colony as a whole, the British Colonial Office (1950, p. 39) reports a temperature range of 80° to 90° by day, with a minimum of 70° at night. Our party maintained no systematic 10 records of air temperature, but I Gos ee Ve a mi need high of 87° to 90° F. betreen noon and 3 p.m. on evaral occasions in one and August, and on one occasion the midday ‘temperature stood at a low of 76° F: following a period of gale and néar-gale velocity winds. At night the temperature fell into the 70's, to as low as 72° between midnight and 5 a.m. A summary of rainfall deta for the Colony as awhodleis given by the British Colonial Office (1950, p. 39) as follows: "Rainfall varies cousiderabl, not only between the islands, but also from year to year. In an average year the annual rainfall ranges from 40 inches in the vicinity of the equator to 100 inches in thé ex ~ treme northern Gilberts, with something around i20 inches in the j Hilice Islands. In the Phoenix Islands between 40 and 60 inches is a good year's figure, while the Line Islands' rainfail varies from 30 odd inches at Christmas Island to.150 or more at Washington Island. Ocean Island, the central and southern Gilberts, the Phoenix Islands and Christmas Isiend are ‘subject to severe droughts lasting many § ’ months, when the annual rainfell may fall to less than 20 inches. These droughts are said to have a rough cycle of about seven years: © In normal years the wettest months are December to February and the driest from August to October." About 40 inches may te taken as a round figure for the average annual rainfall of Onotoa. Rainfall récords locally available were kept at the Government Station on the northerm main island by Gilbertese technicians for 1938 and from January 1944 through August 1951 (table 1). These show an average of 44.2 inches pér year. Tne yearly average for the period 1924 to ~ 1930 was 38 inches, according to E. H. Bryan, Jr. (Pac. Sci. Bd., 1951, p: 2). Available records from 1924 through 1934 led Miss Sachet (Pac. Sci. Bds, 1951, p- 16) to an annual estimate of 34.42 inches. The records of table 1 show 1946 as the wettest year, with 85.1 inches, and 1950 as the driest, with only 6.6 inches. January.averages the wettest momth, with:8.6/inches, and October the™ driest, with 1.3 inches. The wettest month on record was January 1949, with 25.4 inenes, and zero rainfall has been recorded for every month in the year except July, August, September, and November. In 1950 no rain at all was re- corded from January 1 through June. TAL ee . MARSHALL oA ISLANDS Kwajalein: , : Mdju roe: Arno Equatorial Counter Current | ee el ISLA BREICEt ISLANDS "| Tabiteuea’. |: Tamanc* .2notoa . Butaritari (Makin) Northern <— = elle siiy ‘Tarawa |" Southern _— PHOENIX ISLANDS ou Limit f= man e f imitenO Canton. . e winds reget ola Palmyra Northeast Trade Winds) *— —_=§- —— |. —— —— Hawaii pee OE Christmas ls. 0? G y S. D he | NEW : HEBRIDES @& iN ~ Caledonia: : ' -1SLANDS FIGURE I. INDEX MAP, SHOWING LOCATION OF ONOTOA (From National Geographic Society, map of “Pacific Ocean", Sept. 1943.) (*4a0n TeTUOTOD YsTyTug Asequnoo) TIO eoyoup ‘ucTyeYg JUeUTAEACH ye TTeJUTeY *T STAeL ert BL°S z9°S 9e°T Eee EGae ea ESS TO Vee NE s09nc bane €9°8 “SAY W2se. Leo = OO" ts — Ol OL Swen Ggalce (COveC = 66 °Gi = te 9G. Gee wGcnve —cmeoe 69°LL = TP4OL = é é é b GE°6 MeL GEE Ge= ditec Wie 0-0) 2g°h TS6T 9°9 LL°O €7°0 0 Os5@r = Ohece OG" T 0 0 0 0 0 0 O0S6T 6°S4 7Z°O ae) T6°0 L6°O 86°0 EG OS*T YUE 20) 06° cent e-cee- ico 6°66 bere = TEs Sy fag CORO = eg= core rg = Jo “ch cl eos = Ee ON Siar S°0€ O7°E WAS 6) Lt°0 Ge ie OO = Co oe ee = 95:0 65-0 = “Gee0 90°9T LET T°S3 Gan = cont T9°€ fosgeee tics GCSE = “EG"cr —/6°9— 996 -— CGre Sle 91°% = HET Le 0) 05°E 85° Se GO Ga I I OE 0) @) Oe°t— SiGe €*s€ 0) OT'9 9L°0 eee 60,6 Oe = aCe Oe = 0) Orcr Wer L°6t SL°0 Lez tee Z 6 eee eee Gt CLO =, Veet = 7080 Sil est Tey ~ Tequaced TSquisKoy Teqoyso ceque deg ys Atnp “eunp Ken [rtd “yous, Aaenaqeg “renee Te FOL, 12 Of" Gn" C7"s 00°62 9T° 24 (°yAoH TeTuoToN ystytug Asequmoo) TTOP eMerey, “pue[ st oTqzog Fe TTesUrey °e STqeL By Pesce SCE «Gets BOTs BETO MPNTEG Fone *— STL TOnGe-="Ol LT =06"9 Oct “Ol'sy “—6°Te ~*o0'6e *-ee°Gtr Z6"EC > ‘ + = Saws SEvST ELE eons de" 67°STT 60°ST Tie. ce° OU == Tor we Fc) ac [240], ane ae 5 80) Seth ca cos se a a ey ae, er a me ee me we er ne eee & a ee ee eg AIOT. pa Tash ps 1S TT ye 06'S re: UEkS Gh dyes Oct s0cel ~ O08 65°0 nyito) CoN eg) E10) Cet, SISO) G2 ASI) Ties COL G LL°? 96°ST SO°TT WEG rit Wel 69° 710°9 66°3 e7°s L6°ST cO°8T es GHEE ENO) 92°2 T2°e 6T°S 16°6 aL € ce°t Get GG OG = ee SC OCT i a “5 j Roe sg Ag Rp eg a ra eS°47 6e°SGT = “SAW Zosce 9¢°6L = TB 40], tee 00°6 1S6t LT°O 0 ___0G6r 40°83 0*0€ 6%6T S€°6 Tete S76T 60°€ 67° ST LY6T é é 9*76T ge 13 In terms of the many characteristically "wet" atolls of the Pacific, where yearly reinfail commonly averages 100 inches or more, Onotoa is truly a "dry" atoll. This, of course, is immediately evident from its sparse grownd- cover vegetation. Its climate over a period of years shaws no clear division into rainy season and dry season--merely a slight tendency to be drier during September through November and less dry during December, January, and June. This, in turn, suggests only a slight correiation of relative dryness with the season of prevailing easterly trade winds (about late June through Novem- ber) and of relative "wetness" with the season of more variable winds (about December through early June). ven in the Gilberts Onotoa is relatively dry as compared with an atoll like Tarawa (table 2), which averaged 73.5 inches of rainfall per year from 1947 throvgh 1950. Statistics for the seeds year 1950 at Onotoa and Tarawa are given in tables 1 and 2. At such times the rainfall is insufficient to maintain a fresh-water head, permitting invasion of salt or brackish water through the pervious island sediments and rock foundation. The ground water in the nar- rower parts of the islands is soon contaminated, with resultant death of breadfruit and eventual death even of coconut trees. Taro too may be ad- versely affected, although it is ordinarily planted fer enough inland to escape the worst effects, and at least one variety found on Onotoa survives in slight- ly brackish water. To judge from field observations, the only reasonably safe answer to the loss of plant products by drought is to avoid planting bread- fruit, coconut,or taro on parts of islands less than 800 feet wide (or, better, 1000 feet wide) and to plant breadfruit not nearer than about 200 feet from standing salt water in any direction. The effects of drought on ordinary water supply are judged to be less serious than on vegetation. The fluid drunk in largest volume by natives is Ww green coconut milk, which is self purified. Water for cooking and incidental drinking can be slightly brackish without deleterious effects, end the fresh- water lens of a permeable island area + mile or more wide should ‘survive the moderate draft of an endemic island population even during drought periods, especially if washing water is dram from sources alreedy gone brackish. ‘4 Nn PLACE NAMES The importance that one attaches. to the name of a place depends on his perspective. The Gilbertese, living in his atoll universe and dependent on the sea for a living, attaches great significance to the passes in the reef through which he can safely sail his outrigger canoe, to the reefs on which he might wreck it, and to the parcels of ground on which he and his neighbors live and over which they quarrel. He is not interested in names for a whole island or islet, except as it happens that the smaller islets are commonly Single parcels of real estate. He does not go to the north day aoe end of some named island or islet, he goes to some particular named property or to the home of some fellow Onotoan at the village of Temao (TS_-m¥-3) or Tekawa (Te-Ki“na) . .On figure 2 only a few of the more important place names are given. The names of the seven villages are capitalized. Several islets that coincide with property. divisions are indicated by the names of those property divi- Sions in lower case lettering. The few reef names used are indicated by the Gilbertese word for reef, rakai (ra-ki); except for Aon te Baba (ain t&_ba-ba). and Aon te Rabata (ra-bi“ta), to the north and south, respectively of the main passage. fon means on, and te is the definite article, the two together being used in a sort of vernacular sense in combination with the designating name, as we speak of "Smith's place" or "at Joe Webbs" in English. With continued | usage such designations take on a sort of formality, and even come to be run together as a single word (like Pittsburg, Beekmantowm, Yorkshire, and Aonteuna “Obs hone) There is nothing to call the two main islands of Onotoa except the north island and the south island unless names are concocted, and nothing would be 16 gained by this - the Gilbertese would be bewildered, and Onotoa is already a small enough named subdivision in world geography. The headquarters of the Colonial administrative office on Onotoa is at a place called Buraitan (pen) However, as frequent reference is made to this place and our campsite at its south edge, and as the land areas on the larger islands are not named, it seems easier for the reader to call this place Government Sta- tion. For the same reason it seems better that the anchorage be called just that, rather than Komotu (literally anchorage, in Gilbertese). The authenticity of the names used, as well as the dozens of others not shown of the preliminary map, was checked in the field at every opportunity and finally reviewed on the last day of our sojourn by a group of six "old men" or Unimani (community elders respected for their knowledge) representing five of the seven villages. Because of the close village life and regular habits and travels of the people a man from one part of Onotoa may be quite © unfamiliar with names for natural features of other parts of the atoll (the reefs and passes especially), but the concordant judgement of the committee of six would certainly be accepted as final by most Onotoans. Pronunciation is another problem. To reduce it to its simplest im- mediately pertinent and practical terms keep in mind that the sound indicated by b is almost that of the letter p, the combination ma sounds like mwa with an almost imperceptible w, the terminal ti is pronounced like an s, and other terminal i's are silent. Rules for syllabification and emphasis are more com- plicated, but pronunciation is indicated for important words upon their first use in this report. aby) GENERAL FEATURES OF THE LAGOON Reference to figure 2 will show the general shape of the Onotoa lagoon bottom as contoured from 21 echo sounding traverses, a few spot soundings, and >a details visible on air photographs. This chart may not be re- lied upon in detail for navigation, however. In general, the underwater contour lines refer only to the general depth of bottom between patch reefs. Although a few large patch reefs are individually contoured, no indication is given of the positions of the numerous small reefs that reach to or near the surface over a large part of the lagoon. Unless one has learned some particular channel or is completely familiar with the lagoon, he should not attempt to negotiate its waters in any kind of boat (including canoes) without keeping a very sharp lookout for reefs and shoals, and he should avoid travel on the lagoon at night. For ordinary ships' boats the only reasonably clear shore approaches are within the segment defined by lines of fathometer traverses A and 8 to the jetty at Government Station ) and about along or a little south of the line of traverse G to the Heneaba (iwa“6-4ba) at Aiaki (1%). A course along traverse G would need to evade linear patch reefs between 4000 and 5000 feet offshore, and there is no : anchorage for ships off the outer reef here. There are no navigation lights or buoys enywhere, and. the only good sighting points are the ends of islands, the white stone monument on Aonteuma, and the churches and large community meeting houses, or Maneabas, show on figure 2. The only suitable anchorage for larger vessels at Onotoa is on the Vee ward shelf outside the gap in the outer reef opposite a Seetesas This is a mell-protected anchorage except at the rare times of westerly ane. It has a good holding bottom and adequate swinging room. It is reported 18: (U. S. Hydrographic Office, 1940) that small ships may anchor near "Taburari" (Tab-ti-ar’6r-i) at the south end of the island. However, the only possible anchorage at this place is a very narrow shelf right against the reef and generally swept by a rolling swell from the south--an undesirable anchorage except at times of dead calm. ‘To my knowledge, no vessel of any size has ever entered the Onotoa lagoon. It would be possible, however, by careful manipulation, to work a vessel of less than 9-foot draft into the lagoon and anchor it there, and it might be worth doing if one were to be there beyond a few days. It would also be possible to clear channel and enchorage in the lagoon for regular use by vessels up to 9-foot draft. The intended reference datum for the depth contours in figure 2 is mean low low tide. This datum can be only rovghly approximated, as the U. 5S. Coast and Geodetic Survey "Tide Tables" for 1951 give no correction factors for Onotoa tides. They do give records for several other Gilbert atolls, and I have arbitrarily assumed the same corrections as for Nonouti (nd-nuch) , with Kwajalein as reference point. This gives 6.2 feet as the spring range of tide | and 4.4 feet as the mean range of tide. No effort was made to make a precise check on these data, but the assumed ranges and times seemed about right in the field, with considerable local lag in enclosed tide flats and tidal inlets. Depth traverses were made with Navy Model NK-7 portable echo sounding equipment, which consists of a magnetostrictively actuated transmitter- receiver unit in an outboard wooden fish and a recorder unit that produces a continuous graphic record on a strip of sensitized chart paper. This fath- ometer was carried in a 20-foot flat-bottomed dinghy driven by a 7-horsepower outboard motor. It was operated by two parallel-connected 6-volt automobile batteries which proved of inadequate capacity to maintain the sensitivity required to operate at depths below 200 feet for more than very short periods. iy) EXPLANATION Estimated 10 fothom Limesand Limegravel EOS a _—————— ae KAUKK Kerk SLEEK Natural depression 0040 o00 goco Green alga zone of windward reef flot ‘a Red algo zone of windword reef flot 7 LELL ube, \ tite tr an Leeward reef flot oo, A\ (Green algae lagoonword , red algae and eA sturdily bronching forms of the corals " Acropora and Pocilfopora seaward) > S S — e =) 2: bY, / A Dead reef surface,generally with gravel veneer (nay firt Foraminiferal flots (Living Golcarina ond Marginopora matted in soft olgae) J c3; Jo 7%, 0 4 0/7 Tronsect A? ° ent Wien Reef area of abundont living coral of few types Bi Reef oreo of scattered living coral of many types (Varied coral types scattered on bottom of dead coral-algal rock thot 1s veneered with limesand and limegrovel) eee. PPPPPR Abanekeneke Tidal inlet 12 vy Atl vvvy yyy i vive Se cae) te VN Sy sn SS \ LoeG yyy Tide flots ond shoal woler oreos of sond, gravel, dead coral-algal rock or ony combination of these UAV vv v we (Locally with patches of turtle grass 3 307 Thalassia, green algal growth and sparse living coral) High tide line PLANO Margin of wave-breaking reef AN Th TM, |) Front of submerged reef area Boundaries between Intertidal and related shoal units Se 6 Number and guide line to geology- soils profiles Foy Designation ond course of fothometer traverse AO TONE oe a Approximote location and depth in fathoms be- low meon low low tide of generalized under - water contour line. (Hotchures point toward closed depression. Surface indicated is generol- ized bottom surface - above this rise numerous patch reefs, some awash at low tide) X 6-28, XS-7, X L2 Selected collecting locality of P, € Gloud Jr S sediments, L lithology B-| to B-B Collecting localities of A H. Bonner Ttox Fish collecting localities of John Randoll >>>>> Tronsect A 2 Maneabo (village meeting house) a Church APPROXIMATE MEAN DECLINATION 1951 ANNUAL MAGNETIC CHANGE 2" INCREASE Bose compiled f Bs from aerial photographs, using shi taped traverses for control. Adjusiment” by ee a FIGURE 2. GENERALIZED GEOLOGY AND MARINE ENVIRONMENTS OF ONOTOA ATOLL GILBERT ISLANDS By Preston E. Cloud yr, 0 ve = 2Miles — —— eS Se 2 ae a, asia is veneer Gp on Sc Profile 2 Profile 4 Profile 5 middie _subzone green algo zone ——________,. __ red aiga zone outer reef flot , N.N.E. gubzone subzone of ania back ridge trough corolline ridge and SISO-—SIcO SUIDI p4OH Bt api Lusi OO0LI 009i OOS! 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A pH reading as low as 7.4 was recorded colorimetrically in a high windward spray pool at 6 a.m. and one as high as 9.4 in a lagoonside spray pool at 3:30 p.m. Importance is attached to figure 7, representing a high seaward tide pool, because it shows an essentially regular diurnal variation curve of pH (colorimetric) through a period of fluctuating temperatures and dilution by rain (2.05 inches rainfall in 24 hour interval recorded on figure 7). Concen- tration of Cl” in this tide pool fell 6000 ppm during hard rains from 3:15 to 5:15 a.m. However, it jumped back 4500 ppm with the first flushing wave of the high tide after the rains stopped and was kept at this concentration as iong as the tide pool was reached by an occasional high wave. Concentra- tion fell 15,000 ppm during a day of rains but jumped from 2,000 to 19,000 ppm as the tide reached peak and flushed the pool again. Chlorinity fell off markedly again at 9 p.m. as the tide receded and the pool was beyond reach of waves, but this drop must be explained by dilution from accumulated rain water seeping and trickling down from the irregular rock surface above the pool, for there was no rain at this time. Concentration in ppm of CaC0., MgC03, Ca*t+, and Mgt* varied directly with Cl”, and none of these concentra- tions showed any relation to temperature of pH. Clearly, the pH of this tide pool is not significantly affected by or related to either temperature, chlorinity, or any of the variables that change with chlorinity. However, pH, temperature, and chlorinity do vary together in other situations (figs. 4 - 6, 8), and it looks as if they may vary in re- lation to some common factor. Sunlight provides a suitable common factor for temperature and pH, but its possible relation to the measured variations in chlorinity is mot clear. Rain and variation in outflow from the fresh water lens with the tides are probably more important in accounting for chlorinity variations. 38 The general periodic variations of pH from relatively high during the day to relatively low at night is well brought out by table 4. During hours of sunlight marine plants (both attachec and planktonic) .use up COp in photosynthesis, causing relative acidity, as measured by hydrogen ion con- centration, to decrease, and pH, the inverse measure of hydrogen ion concen- tration, to rise. The reverse is true at night, when plants are not using C05 for photosynthesis, but both plants and animals are producing cO5 through respiration. The C09 content of the water increases, hydrogen ion concentra— tion rises, and pH falls. This is true of the high seaward tide pool, with- a out regazd to the extraneous factors that affect calcrinity and temperature, presumably becavse the veriation in pH is organically controlled. Emery ‘ (1946, ps 221, fig. 2) clearly shows that diurnal varietion of pH in tide pools at La Jolla is inversely related to CO> concentration. At the same place, he notes that variations in partial pressure of C05 from greater than in air at night to less than in air auring the day indicate a larger range in C05 actually released and used then is indicated by measurements obtained. Special interest in the diurnal cycle of pH variation derives-from the part that tropical marine waters appear to play in ‘solution and precipita- tion. of Cal03. On the one hand, it is now common knowledge that such waters are normeily saturated or supersaturated with CalO3 and therefore not capable of taking it into solution. On the other hand, the physical evidence of pitting and undercutting of tropical limestone shores is convincing to some ‘(including myself) that normal tropical marine waters, under some conditions, can dissolve CaCO3. Data obtained at Onotoa substantiate the conclusion al- ready reached by Emery (1946, pp. 225-226) that these conditions are rélated to diurnal variation of pH in intertidal or very shallow waters with a high biotic density. During the day, CO. in shallow waters and tide pools is 39 being used in photosynthesis, pH rises toward a maximum of $.6 to 8.8 or 4.92! in open shoal water and 9.1 to 9.12! 4m tide pools and spray pools, and precipitation of CaCO, should take place. At night, when the CO content of 3/ these same waters is increasing, pH falls toward a minimum of 7.6 to 8.3~ in Highest readings colorimetric and probably in the range of 0.3 high. both open shoal water and tide pools, and it is probably at times of lcower- ing of pH below about 7.8 to 8.0 that solution occurs. Emery (1946, pp. £22- 225) has made the necessary calculations to show for similar data, though in a temperature range about 10° ¢ lower, that solution at night and precipita- tion during the day is in fact possible within the observed range of pH. In arriving at the foregoing figures, cata from the windward spray pool of figure 8 are discounted, because this pool was found to contain decaying flesh that doubtiess accounts for its low pH. Of course, such things are common in tide pools and spray pools and would account for accelerated solution there. That the effects of solution in shore zone areas are commonly more in evidence then precipitation is exoléined by the susceptibility of the minute aragonitic needles of the precipitated Caco, to being flushed away by waves—— or even blown away by wind at low tide from parts of the reef and tide flats that are exposed long enough to dry. Precipitated Caco. in and near the shore zone of Onotoa appears to be preserved only on the elevated rims of certain tide pools and probably as part of the white encrustations on the surfaces of sediment-binding algae. Naturally, rain water, both as solvent and as flushing medium, accentuates the process of pitting and formation of tide pools and spray pools, and the effect of decaying organic matter is also in- portant. However, neither rain nor decaying organic matter can have much L0 effect on the production of the undercut notches that are so common around limestone islands cf the tropical seas.. Flow of water over the windward reef The movement of powdered fluorescein marker dye was observed at several places over the reef flat, in surge channels, and over the benched reef slope seaward of the reef evar along the windward shore near Government Station. Observations wers made during e receding tide at a time of moderately strong surf, and all time intervals end quantities of dye were estimated. &bout midway of the reef flat, which is about 800 feet wide here, a patch of dye about 20 feet in diameter cn application spread out to about 80 feet wide by 100 feet long (clongated normal to shore) and moved altogether past the point of application in about 30 seconds. It then surged inward and out— ward with onshore surge and recession of waves but sinking as it moved and with dominant movement seaward along the bottom. Within about 10 minutes after application the dye was foaming in the surge channels of the coralline ridge. About half a cupful of .the powdered dye was applied just behinc the coralline ridge and then ovserved from a raft anchored about 110 feet beyond the ridge on the benched reef slope. Traces of this dye foamed in the upper waters of the surge channels for a long while, but the bulk of it continued to sink and drift seaward for about 30 minutes.,- It gradually worked dom to a basal layer of water and streamed cut over the seaward sloping bench. Dye was added to surface water about 50 feet seaward of the coralline ridge and surge channels. This dye worked outward and dowmward, streaming to the bottom at about a 30° angle in about 5 minutes. Within about 15 minutes it was all seaward of the shelf. AL H About three~quarters of a cupful of powdered dye was released at the bottom of a 6- to 8-foot-wide surge channel near its midlength, in abovt 10 feet of water. This dye surged up and down and spread to adjacent grooves, 16 1% stayed in the surging waters for about 10 to 15 minutes before begin- ning to stream definitely seaward. It then streamed outward and dovmward across the sloping bench. The foregoing observations show that there is a definite outward-moving bottom current in the shallow water over the reef flat and upper reef slope, at least at times of receding tide. Time did not permit repetition of the observations with an incoming tide, but I would expect the same pattern--the water that runs onshore at the surface because of the breaking waves must move offshore at the bottom. ‘The fact of most importance is that this cur- rent is downmard as well as outward, literally dragging the bottom, and at times of sien between swells at the ree¥ margin its force is memorable. Moreover, as this movement is perceptible even beyond the reer fronv at times of only moderately strong surf, it is probably considerable during storms. This is of importance in Conmecrien with the origin of reef front grooves and surge channels. ORIGIN OF REEF-FRONT GROOVES AND SORGE CHANNELS The fronts of most organic or limestone reefs that are exposed to the ~ sea somewhere show a comd-tooth pattern of closely spaced grooves that are — separated from one another by rocky buttresses. The parts of these grooves: that transect the surf zone (and the coralline ridge if one is present) are called surge channels (Tracey et al, 1948, p. 867). The origin of these groove-and—buttress systems is a vexing question, . for they show features attributable to both biogenic construction and me- | chanical erosion. Ladd and others (1950, p. 413) have emphasized the import- ance of outgrowth of algal spurs to form the buttresses at Bikini atoll. They believe that aitnough there probably "is mechnenical abrasion during periods of exceptionally heavy weather...this does not seem adecuate to ex- plain the grooves as erosional figures." David and Sweet (1904, p. 81) ex— plained them by a hypothesis of comkined growth and erosion factors and Kuenen (1933, p. 80-81) believed that they were mainly constructional. Newell et al (1951, p. 25), with reference to the Bahama Islands, in-_ clined to the view that "the grooves are cut," and to judge from the fact z that the grooves observed by them "are incised in oolitic country rock they evidently are erosional features." Before learning of Newell's views, studies of grooves and surge channels on Onotoa and previous observations of Similar features on Guam, Saipan, and elsewhere had lead me to recognize ero- sion as important in the formation of the grooves. I have also seen, but not studied, grooves similar in plan to more conventional surge channels in the face of a basalt-floored bench just west of Haena point in northwestern Kauai, of the Hawaiian Islands. In my opinion the grooves in many places are initially cut by outflowing undercurrents that carry tools of abrasion not available to the more spec- tacular inrushing surf. This produces the characteristic radial pattern of gravity fiow. it is further suggested that most of this cutting followed falls of sea level, when reduction of bench surfaces provided maximum quan— tities of detritus for abrasion. Under proper light conditions air photo- graphs of some shores (e.g., north Saipan) show several levels of offshore and even elevatec grooves, not closely matching at their boundaries. These indicate groove-cutting at successive stands of sea level related to bench formation. Once a bench is redvced to equilibrium level, however, growth factors become relatively important. The abraded upper sides and crests of spurs then become veneered with growing coralline algae and corals, and the grooves may be masked cver and. generally closed or partly closed at the sur- ‘face. This produces 1mder-reef caverns and blowholes. Growth of algae and corals subsequent to groove cutting may be so extensive as to mask completely the evidences of abrasion, but the grooves and surge channels are found at so many places, and the radial pattern is so like the normal gravity pattern found on rilied rock beaches and elsewhere, that abrasion by outflowing -* gravity currents probably determined the basic pattern at many places where organic growth is the prevailing modern feature. Many grooves and surge channels observed on Onotoa and elsewhere are undercut at their basal sides and floored with gravel, and many on the lee- ‘ward coast of Saipan end in submarine potholes containing coarse gravel. The grooves are ordinarily most abundantly developed on windward reefs, but they have been observed in all quarters of the wind and at places are common on leeward reefs. Their degree of prominence is believed to be controlled by strength of outflowing current, and thus sre) and by guanine abrasive materials in transit. On the other hand, there are places siiérs growl Sie nae produce the Donbetno tt nena Both mechanical erosion end organic growth must be considered dinpontane in the origin of groove-and—buttress sys-— tems, the part played by each probably (eens eesoesene to local conditions. on Onotoa the grooves of the windward reef are elie limited to the surf zone and are thus synonymous with uagactinee channels, but traces of them run across the benched slope of the upper reef, masked by coral avout and | debris. The front of he reef at the landward side of this bench is about 12 feet high and from the seaward side looks like the truncated spur-and- canyon topography of a steep-fronted and flat-topped mountain range or plateau. The surge channels range in eden from about 50 to 80 end rarely as much as 120 feet. They are about 6 feet deep at midlength, and deepen gradually to about 8 feet at the reef front, with a dowmward dip of another 2 to 4 feet as they pass Peyend the wave-breaking front of the reef. They range from 2 to 8 feet in width at the reef front and are undercut up to 1 foot on each side at their eee Living algae and corals are abundant only at Ave assis co upper sides of intervening buttresses. The surge channels are floored with goRiNepa mostly slabby gravel. At the reef front above this gravel during 2 period of relatively strong surf (swell measurea 6 feet high, combers averaged an estimated 8 feet), the only movement experienced was an up and down with the ever. Down in the lower part of the channels, However, the swimmer is carried’ back “and forth with’ tne sdeee eos aaeentaclis i620 eeu a anenetes Under these conditions only small pieces of the gravel were observed to move, the maximum size observed in movement being a slab about 8 inches in diameter that rocked gently back and forth without being transported from its original position. Slabs this size and larger, although well rounded, are mostly 45 coated with a fairly luxurious felt of living green algae, and it is evident that their rounding occurs only at times of storm or very heavy surf, with plenty of time between for growth of algae. Whereas there is apparently enough movement of the boulders and smaller gravel and sand to prevent grovth of coral and corailine algae on Ene icok and lower parts of the surge channels (except locally at their mouths), the srooves are probably not being signifi- cantly enlarged at the present time. It is suggested that most of the groove cutting in the reef front at Onotoa occurred during beveling of the present reef flat after the recent 6- foot eustatic fall of sea level. © sera sae BUILDING AND EROSION OF ATOLL ISLANDS On Onotoa, evidence for Recent lowering of dedi level of the order of 5 or 6 feet is found in remmants of an elevated Heliopora reef flat that occurs up to about 2§ feet above the inner edge of the reef flat, both on the beach and in wells (e.g., profile 5, fig. 3). The inner edge of the reef flat, in turn, is estimated to be 2 to 3 feet above present mean low low tide. At pre-— sent the upper limit of living Heliopora flats is about at low tide level. Similer occurrences of relatively elevated Heliopora flats are also found at Funafuti (Sollas, 1904, pp. 21-24; David and Sweet, pp. 67-68 and plates). Further evidence of a fall of sea level of about 6 feet at Onotoa is provided by elevated cobbie stripes of a sort that I have observed only on reef flats. These cobble stripes rise about 2 or 3 feet above a surface of cobble gravel that is about 6 or 7 feet above the present reef flat at the northwest end of Onetoa and are separated from the lower—lying present reef flat by a gravel rampart. As a Recent world-wide 6-foot fall of sea level may be amply documented, the evidence on Onctoa is only part of the broad picture. The higher stand from which the present Bentres receded is provisionally attributed by Stearns (1941, p. 780) to the postglacial optimum temperature cycle of 5000 to '7000 years ago, when water previously and now tied up in the polar ice caps was in the ocean. Fall to present sea level probably took place in two steps, the first a 3- or 4-foot drop and the second 2 or 3. Evidence for the second drop consists of a bench about 2 or 3 feet above the present reef flat at Onotoa and elsewhere (see Kuenen, 1933, pp. 66-70; Dana, 1872, pp. 333-346). No at— tempt will be made here to summarize the large literature on the question of recent eustatic falls of sea level. Ltth Atoll islands characteristically consist of unconsolidated debris rest- ing on a solid foundation. This foundation must be broad enough anda high enough so that this unconsolidated debris can accumulate beyond the reach of strong wave action and.be preserved there. The foundation may consist of a reef that has grow to the surface of the sea, or which, having grow.to the surface, is left somewhat above normal sea level by recession of the sea. On a surface which is exposed between tides, lime-precipitating and sediment-binding green and blue-green algae flourish, and even coarse clastic materials are quickly and firmly bonded together by interstitial calcium car- bonate. This is demonstrated by the cementation of blocks in the stone-ring fish treps on the outer reef flat and by firmly welded bars of boulder con- elomerate at Aonteuma and at the northwestern extremity of the atoll. Upon such an intertidal surface, also, debris tossed by the waves has a good chance of remaining im position at a distance from the reef front that varies with the transporting power of storm waves. The first step in the building of an atoll island, then, is the erection by storm waves of a ridge or rampart of coarse gravel on a living reef flat or wave-cut bench. Seaward additions may, of course, be made to such a ram- part by subsecuent storms. However, evidence that the structure is essentially stable along a given line and under prevailing strength of waves is found in the fact that the gravel rampart is a single ridge at most places. Building of land on the lagoon side of this rampart is harder to under-— stand. Thet much of the work is done by wind is evident from the prevalence of dune sands at many places, but from where does the sediment come?- In the sands of Onotoan islands it is clear from the abundance of the reef-flat dwelling foraminifer Calcarina that mach if not most of the sand is derived Le from the reef. The tests of Calcarina anc other Foraminifera that inhabit the algal mats of the reef flat apparently were washed across the reef and drifted around the ends of and along the lagoon side of the gravel rampart by local currents. The washing of water across the reef through breaks in the vampart is a sufficient explanation of the currents, but they may be locally emphasizec or negated by other factors, such as wind. Im the job of island building these currents will be aided by wind—borne sand from tide flats or from bars projuced by the currents along the growing shore in the lee of the gravel ramoart. The island: shovld continue to grow in width as long as there is a base for it to spread lagoonward on and a suppiy of sediment for building. The latter is providec by Foraminifera end clastic particles of Cel03. Eventually, if the process continues, and currents do not keep the lagoon swept free of sediment, the lagoon must fiil up and 4 large land area develop, as at Chris+- mas Island, in the northern Line Islands. The height of an atoll island, insofar as it is not attributable to fall of sea level or to rampart building, depends on the height to which wind can build dunes on the base provided and from material at hand. Most atoll islands are relatively narrow and low,” seldom anywhere exceeding 12 to 18 feet above the reef flat. In my opinion this indicates that they are also relatively modem phenomena. Several av- thors have suggested that the building of atoll isiands has been accelerated by and perhaps dates from the Recent 6-foot eustatic fall, and such an inter- pretation would help to explain much of what is known of these islands, their biotas, and human migration in the Pacific. This recession of sea level would have resulted in an apparent elevation of near-surface reefs, providing excel- lent bases for land construction of the type the atoll islands show. aa The common presence of a lengthwise depression or depressions within atoll islands is explained by the outlined manner of growth. In the early stages of the process the currents from the ends of the islands would tend to swing a little away from the gravel rampart and build a longshore bar on the lagoon side. Subsequent additions are made mainly to the lagoon side of this longshore bar, and sediment is added to the depression areas only as it may blow in or wash over bar or gravel rampart. On Onotoa the inner depression is only locally present. However, the process that results in an inner Cce- pression is perhaps exemplified at both ends of the atoll islands by the ares of land whose sandy exter.sions curve around tidal inlets (fig. 2). The general pattern of distribution on Onotoa of sand toward the lagoon and gravel toward the sea, and of isiands mainly to windward, also is consistent with the patterns of other atolls and with the process suggested. Storms that either washed across or broke through the gravel ramparts or swept in gravel from the lagoon may be called upon to explain gravel deposits lagoonward of the rampart. Stages in island growth, according to the scheme outlined, seem to be illustrated by the longitudinally paired island strips of Marakei Atoll in the Gilberts (Agassiz, 1903, pls. 149-150) and by the filling since 1900 of lakes in the central depression of Putali Island on Addu Atoll in the Indien Ocean (Sewell, 1936a, p. 77). Sewell also shows (Gigeh Neate) ay reference to pumice lines, that "the inner beach of the island has advanced toward the lagoon by some 10 yards" between about 1885 and 1934. The gravel rampart itself is commonly capped and at places completely concealed by a veneer or thick ae of fine-grained younger dune sands, blom ashore from the reef—flat area so recently as to show no humus layer, or thinly to veneer a humus layer below. This sand contains few Foraminifera and 59 is thovght tc be mostly cerived at times of low tide from the fine Caco, particles that adhere to the drying surfaces of the green and blue-green algae of the inner reef flat. The probability fee even extensive windward beach-zone dune belts cap gravel ramparcs seems strong enough to ween showing of inferred ramparts beneath such dunes on the island profiles of figure 3. If the islands of Onotoa were mainiy ouilt on a platform residual from the 6-foot stand of the sea, and if this stand of sea is proverly correlated with the postglacial optimum, all of these land-building events have taken place in about the last 4,000 to 7,000 years. \toll islands appear to ve eroded primarily at times of great storms by breaching of islands or by the complete removal of islands and other sediments on stretches of the reef flat. If at least the seaward portions of the uncon- solidated atoll sediments rest on a bench surface ae higher level than the reef flat, as at Onotoa, destructive processes should be retarded. Remnants of beach rock on denuded reef flats and buried or outcropping besch rock within tend areas provide the best basis for reconstructing siages in the building and erosion of atoll islands, once given a foundation. 51 SHIFTS OF SEA LEVEL AND THEIR EFFECTS ON MODERH REEFS The reef flats of Onotoa on which islands are situated are truncated surfaces. Green algae thrive on the inner reef flats. A few corals and abundant red algae are found on their seaward vortions. lvidence that this surface has been truncated is found in the elevated Heliopora flat that dips under the islands. This surface is continuous, at places observed carefully, with en old, truncated Heliopora flat that runs across the present reef and is merely veneered with algae and the sediments which they bind and cement to rock. Hvidence of a former stand of the sea about 6 feet above present sea level is found in the elevated area of reef-flat cobble stripes at the northwest end of Onotoa, and also in the elevated and truncated surface of the old Heliopora reef. At Arno Atoll, in the southeastern ilarshall Islands, coral growth flourishes at least on many parts of the reef flat. Of this atoll Wells (1951, pp. 4-5) has stated that there is no evidence of fall of sea level, and the same is commonly reputed to be true of atoll islands. On the other hand, evidence of fallen sea level has been recorded at Bikini (Ladd et al, 1950, pl. 4, p. 413), Funafuti (David and Sweet, 1904, p. 67-68), and Hors- burgh atolls (Sewell, 1936b, p. 121). Regardless of the fact that indepen- dent confirmation cannot everywhere be found, there is widespread and impressive evidence not only of 2 recent 6-foot eustatic fall of sea level, but of a very recent fall of roughly 14 to 3 feet and of one or more former sea levels in a range of 16 to 35 feet above the present one (Daly, 1920; Daly, 1926, pp. 174-179; Kuenen, 1933, p. 66-70; Stearns, 1941, p. 779-780; Stearns, 1945). The 16- to 35-foot zone is obscure, and its effects on modern reefs can only have involved shoaling preparatory to later events of 52 more significance to their present aspects. The 14- to 3-foot fall seems best considered as a temporary stand in the ines of the sea from the 6-foot level. There have also been local and perhaps eustatic positive move- ments of sea level, but positive eustatism for any given level is hard to demonstrate and relates only indirectly to the question here considered. The evidence at hand suggests that the present superficial aspects of + reefs are related to rhether their surface was within 6 feet of sea level at the time of the 6-fcot eustatic stend. If they lay below 6 feet, the drop in sea level would not nave affected them markedly, and, if not sites of islands, they would presumably be flourishing organic reefs today. At such places ne evidence of eustatic fell wovld be found except, in an indirect wey, islends themselves, the ccnstruction of which would be facilitated by the shoaling of their potential foundations. If the surface of a reef were within 6 feet of sea level at the time of the 6-foot eustatic stand, it would be abraded and truncated with fall of-the sea. It would be an area poor for grovith of corals and crustose corailine algae, and veneered with clastic debris and soft algee or articulate corallines. Such reefs are found at Onotoa, Taraw, and Butaritari in the Gilbert Islands as well as in many other parts of the Pacific. In my opinion they are in themselves evidence of recent fall of sea level. Of course, it is to be expected tnat nontruncated reefs will be found in areas of truncation, for it is highly unlikely that all reefs of a given area or ali parts of a given reef would have grorn to uniformly shoal depths prior to the $-foot fall. A second feature of interest in-connection with the Recent 6-foot fall of sea level is the already discussed development of grooves and surge chan-_ nels in the present reef rim. It is here considered that such features at many or most places originaliy result from abrasion by gravity currents flowing 53 outward acress the reef and equipped with abrasive tools provided by trunca-— tion of a relatively elevated reef flat. When such a reef flat is reduced to a stable level, or before, if conditions are favorable, growth of coralline algae and corals at the beveled reef margins is accelerated and eventually masks or even eradicates evidences of abrasion. The east end of Tarague Beach, at north Guam, is believed to exemplify an elevated bench in process of such reduction. For some unknown reason it, alone of all reef-flat areas seen cn Guam, preserves numerous remnants of the older level between grooves that extend across the entire reef flat--as, of course, they should do until sucn time as lateral cutting processes reduce them to a general level. A corollary of the contention that the 6-foot eustatic fall exerted a controlling influence on the superficial aspects of modern organic reefs is that one should be able to state, from the nature of its surface, whether or not any given reef area was within 6 feet of sea level at the time of the 6-foot eustatic stand. If it is sparse in living coral and veneered with green algae and clastic debris, and particularly if it is also a relatively smooth surface, it was probably truncated. If coral growth is vigorous and the surface irregular, it was probably not within 6 feet of the old sea level, or else it has gromn up from a very severely beveled reef margin. D4 APPENDIX A--LIST OF REEF BUTLDING CORALS AND HYDROZOANS for the following preliminary identifications of corals and reef build— ing hydrozoans from Onotoa I am indebted to Dr. J. W. Wells. The list given is composite for all localities and environments collected. Altogether it includes 26 genera and 50 to 60 species of corals end 2 genera and species of hydrozoans. Scleractinia Acropore numilis (Dana) Acropora spp. Astreopora sp. Coscinarea columna /Dana) Culicia Cyphastrea micropthalma (Lemarck) Echinophyllia aspera (Ellis and. Solander) Echinophyliia sp. Echinopora lamellosa (Esper) Favie stelligera (Dana) Wavia spp. Favites sp. Fungia concinna Verrill Fungia scutaria Lamarck Fungia valida Verrili--a new record Goniestrea pectinata (Ehrenberg) Goniastrea retiformis (Lamarck) Halomitra philippinensis Studer Herpolitha limax Esper Hydnophora microconos (Lamarck) 55 ——eorrr Hydnophora rigida (Dana) Leptastrea purpurea (Dana) Lobophyllia sp. Merulina sp. Montipora caliculata (Dana) © Montipora foveolata (Lamarck) Montipora verrucosa Lamarck Montipora spp. Pavona clavus (Dana) Pavona varians Verrill Pavona sp. Platygyra rustica (Dana) Platygyra sinensis (Edwards and Haime) Plesiastrea versipora (Lamarck) Plesiastrea sp. Pocillopora caespitosa Dana Pocillopora damicornis (Dana) Pocillopora danae Verrill Pocillopora elegans (Dana) Pocillopora meandrina Dana Pocillopora modumanensis Vaughan? Pocillopora spp. Porites andrewsi Vaughan Porites lichen Dana Porites lobata Dana Porites lutea Edwards and Haime Porites superfusa Gardiner Porites spp. Psammocora (Plesioseris) sp. 56 faoranad) nti lowre?. hae Be. fis r ai: Hydrozoa dovackd peoeT One Millepore tenera Boschma the “oR wo Stylaster sanguineus Edwerds and Haime PAR, APPENDIX B--DESCRIPTION OF ECOLOGIC FIELD UNITS Recognition of contiguous ecologic field units within a given general environment amounts to designating segments of a continuously variable se- quence. Such units in large part express real central tendencies, but their boundaries are mostly indefinite, and to draw boundaries at all may be mis- leading. How to define the particular continuous variables in question and express them suitably on a map without recognizing suites of intergrading units is a problem yet to be satisfactorily solved. Pending such solution, or a reduction oi categories on completion of laboratory studies and re- evaluation of field data, the following descriptions may give the interested reader a more particular idea of the ecology of Onotoa. Islands Dune lLimesands Younger dune sand. Mostly fine- to medium-grained, angular Caco. sand. Humus layer incipient, thin, or absent. Older dune sand. Similar to "younger dune sand," but with humus layer weakly to moderately well developed. In part rich in tests of foraminifer Caicarina. Indurated dune sand. Indurated phosphatized (?) older dune sand. Limesands other than known cune deposits (Gravel intervals localiy included in all types. Generally comprising mest arable land and supporting thickest vegetation on Onotoa.) Younger limesand. Fine- to coarse-grained sand, with humus layer thin or absent; locally includes gravel and wind-blown sand. According to local reports, the area of younger limesand and gravel on the point at Tabusrorae hes been built since 1900. 58 Calearira limesand. Sand of which 50% to 99% of the individual grains are tests of the foraminifer Calearina. Genevaliy with well-developed humus layer. Forms taoses etl apadaed soil vith goed capiaaeey system. Pavoned sees es pits and breadfruit where ecard water is suftieienmee fresh. | Graveliy iimesand. Sand vith less than 50% Calcarina and with intermixed Hel gravel (abandent small Cardium, etc.) and snail peeene gravel. Undifferentiated limesani. Fine- to coarse-grained sand with cone well- developed humus leyer, with less than 502 Calcarina, and with littie or no shelly gravel. Limesilt gracing to limesand. Mapped only in low, permanently damp a Generally wet snd stiff. Humus layer poorly to moderately well developed. At places encrusted with celiche-like hardpan. Supports sait-—tolerating shrub Pemphis (As well as poor coconuts, sparse Pandanus, etc.). Favored for retting pits beceuse generally brackish vater lies close to surface. Limesrevels (Intervals of mostly angular sand iocally included in all types) Blevated flat—cobble stripes. Low ridges or stripes of cobbles oriented normal to beach line, similar to ridges that develop on modern gravel—veneered reef flats. No humus, few fines. Stripes are about 3 feet high, and bases of troughs between them are about 6 feet above present reef flat (hand level data). This is taken as evidence of a recent relative ele- vetion of about 6 feet and correlated with the now well-documented Recent world-wide 2-meter eustatic fail of the sea. Coarse coralliferous gravel. In part composed of large meandriform and astraeiform coral heads. Has little or no humus and few fines. . Grades to "coralliferous pebble gravel." 5g Coralliferous pebble gravel. Fragments of branching Acropora conspicuous-—~ also includes Heliopora and other corals, corraline algae, and mollusk shells and fragments. In coarser range grades to "coarse corallifer— ous gravel" and at many places includes areas or intervals of such gravel. In finer range srades to sands by increasing proportion of fines ard reduction in size of gravel, and in such places approaches seil and vegetation characteristics of limesands. Caliche Caliche. Caiiche-like limestone, not similar to beachrock. Found at one locality ahout 3 feet above reef flat level and behind sea-facing boulder rampart (north end of northern large island). Very thin crusts of caliche also occur at the surface of the enclosed Pemphis flats near this locality and in low places that are flcored with wet limesiit. Brackish water ponds. Maximum depth 3 to 4 feet, blue-green algae abundant. ue-green algae flats. Areas of very fine Caco, sediments rich in moderate- BL ly to slightly brackish water cover nowhere exceeding 1-foot depth at formal tide level and in places barely enough to keep the ground wet. eeiad with cauliflower-shaped nodules or mats of sediment-binding and lime-secreting blue-green algae. Intertidal environments except reefs Includes sand beach, pravel beach, sand and grevel beach, bovider beach, and outer beach. Quter beach. Sand beach off lagoon side of southern main island that extends 60 beach proper beyond normai tide range and is exposed only at low low tides. Similar to "limesand flats" but narrower and sloping te oe Rocky beach (Some units described here also occur inland and above normal tide range) Concordant beachrock. Conformable with present beaches and certain tide flats. In large part little eroded, but commonly rilied and pitted with tide pocls. Comprises limesandstone with dips 5° to 7° lagocnward on lagoon beaches and nearly horizontal on protected tide flat areas. On sea-facing beacnes is limesandstone or coralliferous and algal congiom— erate dipping 7° to 10° seaward. Nonconcordant beachrock. Greater age than "concordant beachrock" suggested by occurrence at abnormally high levels, marked unconformity with pre- sent beach orientation, or unusually high degree of solution pitting in well indurated limesandstone. As meaoped, probably in part includes "elevated reef—flat rock." Bonded limesands. Weakly to strongly bonded limesands, commonly with a sur- face felt of sogsnenie panes (and lime-precipitating?) blue-green algae. ft places consisting of successive layers separated by thin films of chlorophyll-rich sand that mark former exposed surfaces. Genera of algae provisionally identified from honded limesands in the field by Dr. Edwin Moul are Chroococcus, Gomphospheria, Gleocapsa?, and other genera of the Chroccocales, as well as Lymgbya and Scytonema. At places the bonded limesands show aberrant dips, some up to 30° landward, where they apparently have formed as depression fillings or perhaps slumped into cavities by coilapse from beneath. 61 Elevated reef-flat rock. Old Heliopora-flat rock or rock consisting of frag- ments of coral and coralline algae in limesand matrix. The matrix may be partly or entirely a beachrock, but it lacks dip, is unbedded or very obscurely bedded, and is thus more suggestive of indurated reef-flat detritus. Enclosed intertidal flats — ee Inclosed limesand, Jinesilt, or Limemuc tide flats. Fiddler crab (Qca) bor- ye ings abundant, and. ocor of HS commonly strong in freshly exposed sedi- ments. Permanently demp and seline, but flooced only at highest tide. "Wind" is used provisionelly and. in the sense of probable grain size only; it has not yet acivaily been determined that any of this material is a limemud. Pemphis filets. Similar tc "enclosed limesand, limesiit, or limemud flats," but with cover of the salt—tolerating shrub Pemphis. Found at shore- ward margins of "enclosed flats." The shrub Pemphis, of course, also grows upon the land itself, at the edge of the beach or even iniand in low places that are subject to periodic flooding or where the ground water is brecxish. Mangrove flats. Similar to "enclosed limesand, limesilt, or limemud flats," but with cover of the mangrove Rhizophora. Generaily flooded at same stage of all tides, but mostly "dry" at lowest low tides. Sediments generally in the limemud to limesilt size range and high in Ho5. Mainly intertidal flats adjecent to lagoon proper (Units under this heading grade to lagoon, reef, and beach units) Coral-alzal rock flets. Dead coral—algal bottom veneered to a large extent with limesand and with local pockets where the sand is thick. Displays 62 occasional concentrations of the turtle grass Thalassia (and mostly un- attached Microdictyon) and in areas of standing water, sparse living coral that consists mostly of stubbily branching Acropora, Pocillopora, and smallish, hassock-Llike Porites. Coral-algai rock and sand flats with Zoanthus. Similar to "coral-algal rock flats" just described, but with sand veneer somewhat more conspicuous and supporting extensive growths of the colonial anemone Zoanthus as weil as consideracle numbers of varied green algae. Limesand flats. Relatively "cleen" sand-covered tide flats, with generally sperse megafauna of burrowing sipunculid worms, ghost crabs (Ccypode sp.), the snail Polynices, occesicnal cones and terebras, and, at places, the anemone Zoantaus and the common holothurian Holothuria atra Jager. Plants are scarce, but algae occur locally on erratic rocks, and Enteromorpha has been tentatively recognized. Zone extends beyond beach proper to the zero fathom line (mean low low tide) or slightly deeper. Send and gravel flats. Tide flats of calcareous sand end gravel with green algae resembling Cladophora and Cladophoropsis, Dictyosphaeria, and Valoniopsis abundant in portions that remain wet at normal low tide. A fev Living corals are present locally. Sand_and_sravel flats with coral. Similar ic and gracing to "sand and gravel flats" just described, but with scattered living coral, chiefly hassock- like Porites. Invariably wet when seen, and presumably water-covered excent at lowest low tides. Cobble prevel flats. Cobble-veneered areas mostly lagoonward from reef flats, inclucing occasional boulder or pebble fractions. Components mostly angular. Unit also includes indurated cobble conglomerate flats, adja- cent to or continuous with reef flats (as adjacent to Aonteyma and at north end of reef flat beyond this islet). 63 Pebble grave] flats. Areas veneered mainly with pebble gravels, but with some cobbles. Individual coarse fragments primarily angular. Beacnrock ribbed tide flats. Low ridges of oid beachrock interspersed with dirty limesand flats, incipient beachrock patches, and cireular patches of Thalassia (and Microdictyon). The common sea cucumber Holothuria atra Jager very abundant locally in pools and permanently wet depressions. Bars and spits (Continuously exposed or inundated only at highest high tides) Includes sand bars and spits, pebble gpravel bars and spits, boulder gravel bars, and bars of sand and gravel. Outer reef Grooved reef slopes. Upper slope of either leeward or windward reef front marked with conspicuous grooves normal to reef front and senarated by buttresses veneered with living coral. Papillated reef slopes. Upper slope of leeward reef front papillated with scattered, but more or less linearly arranged, patch reefs of living coral snd coralline algae. ; Benched reef slope. Upper slope of windward reef front, comprising a bench that slopes about 15° seaward from 4 depth of about 2 fathoms to the upver part of a 30° to 40° undersea slope at atout 9 or 10 fathoms. Bench generally veneered with a mat of living and dead coral, the pre- dominant types being stoutly branched Pocillopora elegans (Dana). Reef front. Coralline ridge and surge channels prominent on windward side, but ridge is weak or absent on leeward side. The coralline ridge is lor, purplish-red in color, and thickly crowded with masses and crusts of coralline algae such as Porolithon and Uoniolithon. It runs along the 64 surf edge of the reef, is exposed at low tide, and is intersected by numerous channels through which surges the white water of the breaking surf. Presumably it was casual view of this reef front that led Setchell (1928, ». 1849) to state "the atoll of Onotoa...vas composed, so far as visible,- entirely of nullipore...largely if not entirely...Porolithon craspedium (Foslie) Foslie." Ped alga zone of windverd reef flat. A permanentiy wet area of red algal growth Lendvard from reef front. The outer nart or subzone, an area of permanent stancing water and iocus of tidal fish traps, is called the back ridge trough. Here are scattered cabbage-shaped and brenching masses and crusts of corailine algae such as Forolithon and Goniolithon and scattered large living heads of astraeiform and meandriform corals, as well as stubbily branching Acropora and Pocilionora. The green al~ gae Caulerpa and Halimeda are found locally and sparsely in the back ridge trough. The inner vart, or Jania subzone, of the red alga zone slopes up and grades to the green alga zone of the inner reef flat, their point of juncture being approximately defined by the inner edge of the fish traps. Biota of the Jania subzone dominated by articuiate coral- line Jania, with living Foraminifera of the genera Calcarina and ene opora locaily abundant. At places Jania subzone shows scattered, rolied coral boulders up to 16 inches in diameter, these boulders probably being broken loose within the back ridge trough. Green alga zone of windward reef flat. Inner reef flat characteristically matted with green algae. Commonly divisible into outer, middle, and inner subzones. In the outer subzone the red alga Jania is an abundant holdover from the rec alga zone, but greer algae predominate. ‘the intermediate subzone is one of flourishing green algae, and the inner 65 subzone is one wherein the green algae are whitened by encrusting bonded sediments or at places absent from the bare dead coral—algal rock below. At a distance these subzones seem sharply defined because of color dif- ferences, but they actually intergrade over rather wide intervels. Characteristic genera of algae throughout the green alga zone include Cladophora or Cladophoropsis, Valoniopsis, and Dictyosphasria. At many places this zone is strewn with scattered, rolled meandriform and astraeiform coral heads up to:-i6 inches in diameter, these boulders probably being derived from tne back ridge trough of the red alga zone. Leeward reef flats. Lagoonward portion generally dominated by green algae; seaward portion characterized by abundance of articulate coralline Jania, erustoss corallines, and scattered sturdily branched Acropora and Pocillopora. Gravel and sand veneered reef-flat areas. Uecd or decadent reef fiat veneered with angular gravel of pebbles, cobbles, or boulders, and with a conspic-— uous fraction of sand. Living corals few. Cobble and boulder veneered reef-flat areas. Dead or decadent reef flat veneered with cobbles and boulders. ‘and inconspicuous. Flat—boulder veneered reef-flat area. Chaotic coralliferous flat—boulder gravel on windward reef flat. Gravel veneer on dead reef—breccia. Rough, angular, corailiferous cobble- pebble gravel with some boulders. Veneers surface of coral debris breccia that presumably represents old reef flat. At places old reef- breccia is bare, with no veneering gravel. Mostly covered only at high tide. Developed primarily between the two main islancs. 66 Cal carine-Marginopora reef-flat areas. Protected reef-—flat areas matted with living Foraminifera of the genera Calcarina and Marginopora, and with green algae, the Foraminifera commonly entengled in the algae. Scattered boulders and cobbles are common iocally. A few specimens of the common bleck see cucumber Holothuria atra Jager are found in perma~ nently wet pockets. Heliopore reef zone. Living Neliopora in essentially continuous and generally thickly arborescent reef growth, with Acropora and Porites secondary and other coral types minor. Porites reef zone. Living reef area dominated by large flat-tonped heads of Porites. Irregular coral grcwth on bottom having depths of several feet Acrovora—Pocillopora reef zone. Living reef area of varied coral types dominated by varieties of Acrovora anc Pocillopora; corals thin cut from reef flat toward lagoon or tide flats With increase in area of limesand bottom. Varied reef zone. Reef area of abundant to scattered living coral growth of varied types on bottom of dead coral-algal rock that is at places exten- sively veneered with coral-algal gravel and limesand. Dominant living corel types are Acropora, Porites, Orbicella, and meandriform genera. Heads of coralline algae and pavement—type corallines locally abundant. Depths less than 1 fathom at low tide. Heliopora flats. Living Heliopora scattered over and rising 1 to 2 feet above limesand bottom. Upper tips of Heliopora barely exposed at low tide. liinor gravel patches occur locally. The sea cucumber Holothuria atra Jager is common. Echinoids recorded include a large poisonous Diedema and the harmless Tripneustes cf. 1. gratilla. 67 Decadent Heliopora flats. Includes scraggly truncated Heliopora, a few other species of coral, and green algae, interspersed on surface of limesand and gravel. Dead Heliopora flats. Elevated, truncated, dead Heliopora reef flats. Es- sentially the same as the foregoing, but inundated only at high tide and thus with no living Heliopora. Heliopora—Porites reef zone. Living reef area, mainly Heliopora, in large flat-topned heads crusted with Porites and crustose corallines. Sandy reef zone. Mostly clean limesand with occasional living and dead coral at lagoonward margins of extensive leeward reef areas. Intertidai to lagoonal environments Thalassia flats and shoals. Dirty limesand with clusters or continuous mats of the turtle grass Thalassia. Commonly also with much of the green. alga Microdictyon, the latter mostly unattached. the sea cucumber Holothuria atra Jager is locally very abundant. Rocky flats and shoals. Bottom mostly of dead coral-algal rock patchily | veneered with gravel and sand. Scattered but fair representation of living coral dominated by stubbily branching Acropora and Pocillopora and locally by hassock-like Porites. Circular patches of the marine grass Thalassia and the green alga MMicrodictyon occur locally at the beachward margin in the inner lagoon, and the brown alga Turbinaria is abundant at places. Holothuria atra is locaily abundant. Corslliferous rocky shoal bottom. Bottom similar to that of "rocky flats and shoals," but with fairly abundant living coral patches wherein stubbily branching Acropora and Pocillopora are dominant. 68 Enclosed inlet. Area walled off as pair of fish ponds. Supports thick growth of turtle grass Thalassia and many fish, including small sharks and an unknown fish that is much feared by the natives (apparently not a barracuda, to judge from the description, but was not seen by our field party). This area was not explored er sounded, but it is reported by the native, Kane, to be generally under 4 feet and nowhere more than 9 feet deep at low tide. Environments of the lagoon and leeward shelf The following units comprise a continuously variable sequence with more than usually indefinite boundaries: Limesand_ bottom. Mostly clean limesand bottom at depths greater than 2 fathoms, living coral present locally. Conspicuous lagoon patch reefs. Patch reefs of varied coral types and subor- dinate coralline algae, over 200 feet in diameter. Reef symbol on figure 2 used to indicate parts that are awash or nearly awash at low tide. Limesand with scattered patch reefs. Mostly clean limesand floor, above which rise small scattered coral-—algal patch reefs and pinnacles. Purely ar-— bitrary and grades imperceptibly to limesand and patch reefs. Limesand and patch reefs. Smail patch reefs of varied coral types and sub- ordinate coralline algae abundant but areally exceeded by limesand floor. Grades to "varied patch reefs and limesand," "Heliopora patch reefs and limesand," and "limesand with scattered patch reefs." Varied patch reefs aad limesand. Small patch reefs of varied coral types and subordinate coralline algae very abundant and only narrowly separated by areas of limesand floor. 69 Algal patch reefs and limesand. Abundant patch reefs of massive coralline algae and varied coral types presumably rising above limesand floor (bottom between reefs not observed or sampled). Heliopora patch reefs and Jimesand. Abundant patch reefs consisting mainly of Heliopora in tree- and cendelabra-lilte growths that produce a forest— like underwater scenery. In vart the Heliopora patches are extensively asked by overgrowth of otner coral types, and locally the patch reefs are of varied coral types. For the most part, intervening limesand bottom only narrowly separates individual patch reefs. Varied bottom with scattered larger patch reefs. Subcircular vatch reefs 100 to 300 feet in diameter scattered on bottom of limesend and iimesiit with irregular low patches and small patch reefs of living coral and locally with abundant Halimeda. Depths between natch reefs mustly more than 3 fathoms, ranging to more than 7 fathoms locally. At shallow margin are several ridge-like patch reefs up to half a mile long. Coral plantations. Coral and subordinate coralline algee essentially con- tinuous or intimately intermingled with areas of dead coral on irregular bottom. Acropora the cominant genus in ereés observed. Limesand patenes in cori plantations. Extensive areas of limesand and minor patches of coral within coral plantations. 70 REFERENCES Agossiz, Alexander, 1903, The coral reefs of the tropical Pacific, Mus. Comp. Zool. Mem., vol. 28, 410 pp. British Colonial Office, 1950, Report on the Gilbert and Ellice Islands Colony for the year 1949, B. C. 0., London, 52 pp. Carey, L. R., 1918, The Gorgonaceae as a factor in the formation of coral reefs: Carnegie Inst., Dept. Marine Biol., vol. 9, pp. 341-362, pls. 100-105. Carey, L. R., 1931, Studies on the coral reefs of Tutuila, American Samoa, with special reference to the Alcyonaria: Carnegie Inst., Papers from Tortugas Lab., vol. 27, pp. 53-98, figs. 1-14, pls. 1-7. Couthouy, J. P., 1844, Remarks upon coral formations in the Pacific; with suggestions as to the cavses of their absence in the same parallels of latitude on the coast of South America: Boston Soc. Nat. History Journ., vol. 4, pp. 66-105, 137-162. Daly, R. A., 1920, A general sinking of sea-level in recent time: Nat. Acad. Sere Prec... vol. 6, 0 Sen DOs 246-250. , 1926, Our mobile earth, Chas. Scribner's Sons, N. Y., 342 pp. Dana, J. D., 1872, Corals and coral islands, Sampson Low & Co., London, 398 pp. David, T. WW. E., and Sweet, G., 1904, The geology of Funafuti: Royal Soc. London, Rept. of Coral Reef Comm., The Atoll of Funafuti, sec. 5, pp. 61--124, pls. B-E in text, pls. 1-19 in separate portfolio. Enery, K. O., 1946, Marine solution basins: Jour. Geology, vol. 54, no. 4, pp. 209-228, figs. 1-15. Finckh, A. E., 1904, Biology of the reef-forming organisms at Funafuti Atoll: Royal Soc. London, Rept. of Coral Reef Comm., The atoll of Funafuti, sec. 6, pp. 125-150, fig. 19. 71 Freeman, O. W., et al., 1951, Geography of the Pacific, John Wiley & Sons, _Inc., N.¥.3 Chapman & Hall, Ltd., London, 573 pp. Kuenen, P. H., 1933, Geology of coral reefs: The Snellius Exped. (Eastern Neth. E. Ind. 1929-30), vol. 5 (Geol. Results), pt. 2, pp. 1-125, figs. 1-106, pls, 1-11, Kemink En Zocon, N. V. Utrecht. Ladd, H. S., et al., 1950, Organic growth and sedimentation on an atoll: Jour. Geology, vol. 58, no. 4, pp. 410-425, figs. 1-2, pis. 1-7. Newell, N. D., et al., 1951, Shoal-water geology and environments, eastern Andros Island, Bahamas: Am. Mus. Nat. History Bull., vol. 97, art. 1, pp. 1-29, figs. 1-5, pls. 1-8. Pacific Sci. Board, 1951, Handbook for atoll research: mimeographed, numerous gmail units by varicus authors, Nat. Res. Council, Washington, D. C. Safford, W. E., 1905, The useful plants of the island of Guam: U. S. Nat. Mus., Contrib. from.U. ©. Nat.. Herbarium, vol. 9, 416 pp. Setchell, W. A., 1928, A botanical view of coral reefs, especially of those of the Indo-Pacific region: Third Pan-Pacific Sci. Congress, vol. 2, pp- 1837-1843. Sewell, R. B. S., 1936a, An account of Addu Atoll: British Mus. (Nat. Hist.), The John Murray Exped. 1933-34, Sci. Repts., vol. 1, no. 3, pp. 63-93, Peers ON Sis nee Sewell, R. B. S., 1936b, An account. of Horsbergh or Goifurfehendu Atoll: British Mus. (Nat. Hist.), The Jotm Murray Exped. 1933-34, Sci. Repts., Toews 55.0 po. VO9S25.) fie. 1) pis. We6. Sollas, W. J., et al., 1904, The Atoli of Funafvti: Royal Soc. London, Rept. of the Coral Reef Committee, Harrison & Sons, London, 428 pp., 19 pls. in separate portfolio. (5) a Stearns, H. T., 1941, Shore benches on North Pacific islands: Geol. Soc. 7 Awerica Bull., vol. 52, no. 6, pp. 773-780, figs. 1-2, pls. 1-3. » 1945, Eustatic shore lines in the Pacific: Geol. Soc. America Bull., vol. 56, pp. 1071-1078. Tracey, J. I., Ladd, H. S., and Hoffmeister, J. E., 1948, Reefs of Bikini, Marshall Islands: Geol. Soc. America Bull., vol. 59, pp. 861-878, Bia. oe, pls. 1-11, U. S. Coast and Geodetic Survey, 1951, Tide tables, central and western Pacific Ocean and Indian Ocean:. U. 5. Dept. Commerce, Coast and Geodetic Survey, Washington, D. C., 243 pp. U. 8. Hydrographic Office, 1940, Sailing directions for the Pacific Islands, | Vol. II: U. 5. Navy Dept., Hydrographic Office Pub. No. 166 (Onotoa, p.« 380)!, 522 pps 0. S. Hydrographic Office, 1950, Supplement to Hydrographic Office Pub. No. 166, U. S. Navy Dept., Hydrographic Office Pub. No. 166, U. S. Navy Dept., Hydrographic Office Pub. No. 166-S, 85 pp. Wells, J. W., 1951, The coral reefs of Arno Atoll, Marshall Islends, Nat. Res. Council, Pac. Sci. Bd., Atoll Res. Bull. No. 9, 14 pp., 16 figs. | Wentworth, ©. K., 1947, Factors in the behavior of ground water in a Chyben- Herzberg system: Pacific Sci., vol. 1, no. 3, pp. 172-154, figs. 1-4. ATOLL RESEARCH BULLETIN No. 13 Preliminary Report on Marine Biology Study of Onotoa Atoll, Gilbert Islands Part i by A. H. Banner Part II by John E. Randall Issued by THE PACIFIC SCIENCE BOARD National Academy of Sciences=--National Research Council Washington, D. C. December 15, 1952 i Thy RR NE es Bye AY vary Riereyth Koree nod Late Lhetinwenietet ko woebeoh Leno tte iD 60 cediuabra KOS 9h reenter oat PRELIMINARY REPORT ON MARINE BIOLOGY STUDY OF ONOTOA ATOLL, GILBERT {SLANDS SCIENTIFIC INVESTIGATIONS IN MICRONESIA Pacific Science Board National Research Covncil Part I Dr. A. H. Banner University of Hawaii Honolulu, Hawaii February 20, 1952 Pert IL Mr. John E, Randall University of Hawaii Honolulu, Hawaii March 4, 1952 ACKNOWLEDGMENTS This field work was carried on in connection with the Coral Atoll Project of the Pacific Science Board of the National Research Council and was supported by funds granted to the National Academy of Sciences from the Office of Nawal Research. Generous cooperation was received from the Military Air Transport Service and the United States Coast Guard in assisting with transportation, the University of Hawaii in supplying much needed equipment, the adminis- trative officials of the Gilbert and Ellice Islands Colony. The author is particularly grateful for the agsusGenrs extended by Miss Emestine Akers and Mr. Harold J. Coolidge of the Pacific Science Board. Thanks are also due to Dr. Preston E. Cloud, Jre, the leader of the expedition, for the use of the map he made of the island, and to R. Tucker Abbott of the U. S. National liusevm for the identification of some of the molluscs. He: PREFACE The marine biological work on Onotoa is divisible into five portions: le 26 3e he 5 The investigation of shallow water ecological associations, re= ported herein: A. The ecology of the windward reef. B. The ecology of the lagoon reefs and shores. The investigation of the deeper water ecological associations, to be reported by Dr. Preston E. Cloud, Jr. The investigation of the marine algae, to be reported by Dr. Edwin Moul. The investigation of the ichthyofama, reported by Mr. John Randall and appended to this report. The native use of the marine invertebrates for food, reported hereine liy portion of the study, the marine invertebrates, was severely limited by an attack of blood poisoning and a subsequent attack of influenza that resulted from attempting to do field work when not fully recovered from the first illness; as a result of these two illnesses, over five of the ten weeks spent on Onotoa were lost and the investigations made were neither as thorough nor as extensive as planned. The following reports are preliminary, and should be taken to show merely the extent of the work done. The identifications are rield idmtirications and must be confirmed by experts, with the exception of some of the molluges which have already been identified by R. Tucker Abbott of the U. S. National Museums and no conclusions are incorporated in the reports. When these reports are published the deficiencies will be corrected. apa eee’ hs Et tas tt EN ayant bites a BOE A SETA: OE oe Reed oxi. yd hetsonne ad og weenie: magehogane: aes ember! bo Pigeror: .ghae WH) RODE Tale Re Bier ois bi etki’ LATO S & Case chat arnigadeoyrs Sitio Oct ¢qehe cone ieee Tea. sermtiol ‘to. agate deprpandiass foe yalanetad heo.tel fey ene K bury roses CAC ome cies bftoy felt cb oF yntdqmotca me SVO2) SHO gtesmenLih cyt osei? to PF himeg ys ie 86 tnvies uve soem emiitapiinerns orld hie # nod ehariradsy: wa ov OF magind ot Chaos bee gienecliniteny ots ae iepey petwo fel Sm Maal ttimsl blett eee wictiahish foebh ot) Geet sow edd he ae Ps id bernie aeod vbsecia ei HOMO T FE. Dee | oy aka eae sas | en fe 4 pela Tip ha 322 Ee Bp RTE USD, ay wer Ay LAT oRTto@ ae, ee! anLons.te.c tad: wid bela ts ie ‘ PART I a WINDWARD REEF TRANSECT The windward reef on Oaetas ais ound along the northern, eastern and southern shores of the atoll, presenting an aineet unbroken barricade against the force of the prevailing waves. It varies in width from three or four hundred feet to over a quarter of a mile and is more extensively developed eeound the BOL eeern island than around the northern. As it is of quite miform height, structure and biotic zones, a single transect across its surface was deemed to be indicative of the general ecology of the reef. Conditions of tho Reef The inshore border of the reef is composed either of consolidated and eroded coral rock or moderately fine sand with the uwoper edge extending to the maximun height of the storm waves and the lower edge varying but usually about the 2.0 to 2.5 foot tide level. Beyond this steep shoreward area the reef flat extends to a uniform area of slight slope, with freauent smail to large Ee ies tools of water left at low tide. the reef flat in the trangect studied was 650 feet broad. Seaward of the reef flat is a 2 depression, the backeridge trough, between 50 and 100 feet wide and ranging in depth from about the # 0.2 to the - 1.5 foot tidal level. The final edge of the reef is the coralline ridge (or Lithothamnion ridse by previous workers), a rampart between 1.0 and 2.0 feet above the zero tide and 50 - 100 feet broad. Its shoreward edge presenvs an almost continuous front of recdish coralline algae, but on its seaward side soon develop deen fissures or surge channels at right angles to the shore that reach six or more feet below the surface of the reef? and eae are of varying width, widening as they reach seawarde The seaward edge of the coralline ridge thus separates into a series of separate and depressed fingers that finally slope rapidly dow to the growing reef surface below, The outermost reef or the reef shelf is relatively narrow, about three hundred feet wide, and slopes rather rapidly from about ten feet deep on the shorevard side to over thirty or thirty five feet deep on the seaward side; it consists of living coral growing in-irregular moumds with areas Hebieen the heads strerm with dead coral fragments. Beyond this reef shelf the eee Grops suddenly away, at a slope of perhaps more than 5° and soon disappears in the turbid waters; this last zone was not explored at all. | The windward reef facing the trade winds sustains the almost continuous — beating of the waves. At low tide the waves are broken against the coralline. ridge and only slight waves are. felt in the backridge trough. However, when the tide is Went only a portion of the strong waves is expended against the coralline ridge and the adjacent trough and moderate sized waves sweep across the reef flat, carrying enough energy to move coral rocks a foot or two in diameter e The reef flat from the coralline ridge back is the evident result of the © souueiiaaaan of a living coral reef, chiefly of Heliopora, by coralline algae; in almost all areas the old Heliovora is completely dead and covered with the algae to make an aliiost table=like top, This top, however, is pitted with small to large depressions, and in many areas perforated by burrous leading dorm among the old coral fronds. Animals a pis pens the flat are subjected to many biological vicissitudes in addition to the action of waves. In the inshore. area especially the reef flat is exnosad to the air for several hours at a time at the lower low waters, and those animals that cannot migrate to the shallow pools: must be abie to withstand this period of dessication. Those :animals in the pools, as well as those exposed to the air must also be able to withstand great changes in > Se salinity of their environment, for the high tide has the normal ocean salinity, while the low tide may expose them to torrential rains which would lower the salinity of the topmost layers at least to almost zero. However, because of the difference in specific gravity and the absence of agitation in these small ace of water it is likely that the bottoms of the pools and the burrows in the rock especially maintain their normal salinity. | Probably the most We ee ree change the animals are subjected to is the change in temperature for the cark reef surface on low tides is exposed for long periods to the tropical sun. At these times the water in the inshore pools become hot to the touch (studies on temperature made by Strasburg will _be renorted by Cloud): yet with the flooding tide the temperature will drop perhaps 15° in a few minutes. . Previous studies have shorm that the oxygen content of the water over the reef at high tide and in the pools at low tide is always near if not above its saturation value. But as the temperature rises this saturation value, in grams of oxygen per liter of sea water, decreases rapidly, so the reef inhabitants must be able to adjust to less than normal oxygen. Tio biological conditions of the pee flat should be mentioned as in- fluencing its ecology. in the first place the reef surface not in the small tide pools is covered in most areas by a dense algal mat that affords botna food and Boeken for the inhabitants; this was particularly true in the middle and outer portions of the reef flat. Secondly, while fe larger predators and scavengers like fee fish, lobster and crabs were found while the survey was conducted at low tide, ae moved onto the reef at high tide. Methods and Limitations of the Study: | The objects of the investigation were to find the transition of dominant forms over the reef surface, and, if possible, to designate sharply delimited -3- zones on the reef through a eee study. On the main reef flat the study was conducted by laying out a series of | continuous stations, twenty fect wide and fifty feet long, and within thea Se extending the length of the station one or two feet wide. ‘“lithin the smaller area all animals were collected and comted; the larger area was then inspected for larger but less common animals like the larger snails, sea cucumbers, etc. Then areas in the same tidel zone adjacent to the studied area were superficially exarrined to see if the zone selected was typical; it was found so in all cases. In the inshore beach area, in the packridge trough, and over the offshore shelf no quantitative study was attempted because of difficulty in obtaining either enough animals in a typical area or because of the difficulty in laying out an area for study and collecting it (as in twenty to thirty feet of water). Because of poor tides and poor weather conditions when it was possible for me to do field work, almost no study was made on the coralline ridge at all, | The limitations of the study are: 1. The study is limited to macroscopic invertebrates; no microscopic — forms of life nor any fish are considered. lire Randall did a parallel study on fish and will report it separately, 2. Concerned as it is with the dominant animals, this study omits the more rare animals. 3. All identifications of animals are but field identifications, and will be corrected uvon the identification by experts. he The study is limited by necessity to the ore superficially occurring animals; it wes impossible to explore the tubes reaching dowm from the con- solidated surface of the reef, 5. No statistical checks have been applied to the quantitative results, and they should be accepted merely as rovgh indications rather than accurate stabistics; in other words, a similar section two hundred feet away might give different figures, but would show the same trend. Transect ee Area A-O3; Shoreward beach, The well-denarlced cr extends from about 205 feet to about 8-10 feet above the zero tide zone. It is divisible into sa different habitats, the sand beach composed of loose and shifting sand, and the rock beach consisting of consolidated grea and beach rock, eroded and with some small tidal pools. ‘The sand beach is the habitat only for Ocypode ceratophthalma, the "shost crab" that lives in deep burrows by day; also at night terrestrial . hermit crabs migrate dorm to the upper zones of the beach. the rock Beach inhabited by Grapsus grapsus in fair numbers, some identified hermit crabs, and large numbers of *Nerita plicata (species marked * indicates the identification has been confirmed by R. Tucker Abbott.) Areas A=1 to A=1). | These stations covered the reef-flat and present roughly the same type of substrate. The surface is relatively smooth, being built up by the con- ‘solidation of the individual heads and fronds of coral by coralline algae, Its surface is pitted with small shallow depressions in which water stands at low tides; these are usually less than a square foot in area and not over about three inches deepe The exposed surface of the coral and in sane areas the tidal pools, are usually covered with a more or less dense growth of algae (to be reported by Dr. Moul). ‘The exceptions to these generalizations are in the back-ridge trough (areas A-13 and A-1),) where the surface is below the level of the lowest tides. Areas A-7 and A-8 and A-9 were at least in part covered by a single extensive tide pool; in these areas a few living Pieces of Heliopora were still growing uncovered by coralline algae. Liles - TRANSECT, WINDWARD REEF FLAT Stations A-1 to A-1 eee ee cee In the tabulations below those animals not quantitatively estimated and those animals that are rare, scattered or very irregular in their occurrence (as would be those fomd only in the occasional loose coral boulders) are in- 4 dicated by P for present. t 1 eu eeaeent) t 7 1 1 oy t 1 1 1 1 1 t 1 H t t ' 1 ! 1 ! i Station Lies Od gail ogo Gel Taw 9 Bs ger or © oqgt Aa gaa 1 ! 1 1 1 ' t i ’ t t 1 1 Distance Q=150-1100=1150—=1 200~1250—1300—1350-=1!100—!1:50~- '500- ' 550~'600—'650~ from beach 5011001 1501 200: 2501 3001 3501 oo! 50! 500 ' 550 ' 600! 650! 700 i 1 Chen a | ! 1 1 Reng i] 1 1 ot Tat i t 1 ! ! t 1 ! ! ! ! ’ 1 ee ea ee $$$ ite 1 ! 1 ! 1 ! 1 1 1 1 1 Height Pieeen2.0 1 Tsot aor Le teat 1_OV LON 7Oge 1 Osout O.4'40.4'-0,2 above 0,0 1 1 ' t ' 1 1 1 1 1 1 t6-b to ! 1 tao 2 ta L ol 1 I tide zone 1 1 1 1 ' i 1 1 Approxi- 1 ‘ mate per-= : centage ou ' ‘ covered 70%130%130% 150% by tidal 1 t 1 pools. ! ; s ! 1 t t 1 1 1 1 J 1 t i 1 1 1 { LJ q Ui 1 J t { 1 ! 1 1 1 ! ! 1 ! 1 1 1 1 1 ' ! 1 1 1 1 1 1 1 1 1 1 1 1 ! 1 1 1 ! 1 ! 1 1 ! ! 1 1 ! 1 ! 1 1! Lien ! 1 1 ! 1 1 1 ! 1 3 30%! 30%: 80%1100%Z' 90%! 70% ' 80% ' 70% '20% '100% 1 Lf 1 1 1 ! ! 1 1 1 I EE a eS see ae a 1 1 1 ! PORTFERA t t t 1 ! 1 | 1 1 ! Black Sponge -' = = <=! 1 80 20t}0 60 EM eb et 9240 Sipe J i] 1 1 i] Purple Sponge 6111 3 1:1)0100 4O' = = -1 = o Bue = 1 i 1 1 1 Yellow Sponge -' = = -='t = 2 et = = ~t = = PAF Lol TB 1 i t i 1 1 ae 1 T 1 T T COELENTERATA ' I t ! 1 t 1 1 f 1 Heliopora spp -' - = = a a a ee rr | ‘d ‘J JH { ‘J ‘J ia] 8 8 i} ia] Sipunculus sp. = a} CRUSTACEA ‘J L LY Stomatopoda -- (Pseudosquilla estzata)(?) é i] ‘J ~ —-— = ees P{L(P (Op ib20r - .20P ! 20P P fee ae) eet a) (me om) me Tee ies ioe) re Crangon spe = aa 1 t ' t t 1 1 ! 1 1 1 1 1 1 1 1 t 1 1 t 1 i] 1 1 1 1 1 ! 1 Synalpheus SPp= t- a =! - —! =! - - - To. - -= 1P 1 t 4 1 1 Shrimps, other=1' = - =! 3p - - ! OP — Aig ae = = es 1 ! t . 1 Callianassaspe ' = =- pt .« = = = a5 ae ie ee a a - 1 1 . t . aompetons Di aE Ae Lica La 7 Sere ! 1 1 1 Paribaccus spe ! = - bast ea = fi 1 1 ! ' 1 ! Hermit crabs 800'87 689 1500'3),00 3900 1720'111 60 ~ : : 1 1 Wee a ce t i] t Prema Spa = fia; .< es Ps = t Thalnita , ' ' t 1 edwardsii at os - 3P1 hop Seiwa tata ig A eg - -!t = : , ee ! 1 tide Trapezia —t = is mg pees) ae Mee S oF ee in ge t t ' t te Grapsoid crabs -' - - ape os = OP {Uo6 ae = etek ee BOUIN MINIONS esc Me UU 2 UR ee t ! i ' aT ' F F Lybia tessa- 1 ' ; ; ; - - lata =f = = Fees ‘ss ae as eis uf UE t ' 1 ! t Crabs,other 20P! P P Pit Wh P PyVAOP SaP P, 20P - -: P GASTROPODA ' t Patelloida spe -! = =) W2OM ' ; 1 Patella ! a ‘stellaefor- . ‘ aoe et - 1 - Nerita ! pPlicata 50,00016000: t I { t t i i I J t t =] = i ee ee ee ee er i] i I 1 1 { = | - cs e— we lt lw lle le ~ he ' xCerithiun 1 concisum 11001 200 28° ————— . Cerithium ' obeliscus ait - ais ine) fo) t Saas 1 t i - I 1 { | (pe) (eo) pe) [e) mM (eo) § 4 ee a a) xCerithium U : columa Ar9000!: W200 NCOO1 20 Y20.. i= ——_— a ee ee ee ee er er en ) ine) (e) 8 4 ee ee ee fee ee A eS SS, AS *Nautica SPe = 3 3 20 1 (0) - ~1 — 5 = —1 - ~= | = *Monetaria moneta 6 1 t j “ft 1 18 30 50:10 160, 0,180,100 80; 5 se Siig ' t t ; f ' *Ranularia ‘muricina - -— ~ = = = = se : : Us t ! : iO l= Wee = 088. 080/080 «10 ~ 10m — te gaee = faemsttin ae pes dengan epi i on eae aM ! ai ron 1 H ! 1 xCymathium chlorostomun - Bursa bufonia <= = iyo) ISS) Had Gyroscala - perplexa ° { Oe) ee i : & ine) (@) Thais hippocastanun h ~j BR nw (@) Vasum ceraiti.cum litterata ine) WwW je) (eo) WL Less \O *Mitra virgata 5 *Engina ; mendi.caraia 150 +«Drupa erossularia = ne) foo) = Nh b A —J Ly io} ee Drupa ricina I Lk +Morula granulata 1 Abe) Ww (e) > Oo On —-— —-— me tees *Morula fiscella 150 I t (pe) 7 {2) *xConus hebraeus & Ce spon= dylus ine) ine) ee ee ee ee ee ee ee ee ee ey ~ *Conus miliaris = 2 o (OL 500' 160 I > ie) 81 100 500! 1),0 t LO 60 t { L} moe & I t rf 80 100- i lel hl! i nN Wy 2P —-— eee , =o eee mele me) me? Se) fee co! Se) me oa) oe) ‘es! we] we! a0) eel ae) Tet? eet ow) eel ee). me Kegs malt Sen el. ep ren) > oa Ree ieee) sae Conus flavidus ~' ~~ - -! ~ 20 - 20 = - AL - 1P - Ta aan ae ; yee Conus sD. PS - - - =f) 160 = = ae ~ 4 ‘Cythara sp. -' «= - 200! 20 280 ~ - - - = ea = - 1 1 ‘in| *Torinia - : f } - Warigata ~ ay Para BI BAO TAXON 2X0) SU i - _ - = = - ave 1 ! ! Vermitidae 2 Mut = - -! = = ee ass te = = 3 a STRELA? 1 t ! Nudibranch — ' = ab -! aL - -— t - I P - ~ —= = | com PELECYPODA } *Barbatia | tenella 1 Volsella auriculata ~ | *Isognomon ‘perna 3 x*Gafrarium pectinata - | | ECHINODERMATA Tripneustes “gratilla = Echinometra mathael - Diadema paucispinus - Distichopis sp.e- —-— se me ele ry — ~~ Ss = we = os lo | | | | Ophiocoma brevipes - Other brittle stars —_ Holothuria atra —_ Actinopyra mauritana - Other Holo- thurians - CHORDATA Ptychodera sp. 1 1255 a | : cy ee er) WW 20 O 360 1390 1h00 72 t ' { | | | | | | | | - |= = = we = wow em =o — — -— =~ wee lll ba) | | | | | i I i 1 t 3 —-— = «= «= = ee ee ee ee ee ee ee 20: ee ee ee re jes) oO bh oo 260 - 120P — iP ee Oe ee ed Coralline ridges Ia situs 8 ; | The topographical features of this ridge are described above. Unfor-= tunately aes and waves Wace ee perme an examination, neither quantitative. nor per ae of ne Eas ‘of this gone. The relatively shes surface of the coralline algae did Tom omeee any..protection for animal life; the shifting "rocks at the bottom of the surge charinels offered less. However, reaching into the ass of the coralline algae were numerous openings, and within the os were chambers in which many animals lived. In: this habitat Wee found such animals as Echinometra mathel, Heterocentrosus: SPs and several species of xanthid crabse Reef Shelf, . A) Ape This area, lying beyond the outer edge of eis Woralline Fides, was estimated to:be.about 300 feet.:ride, from 8-10 feet deep at the coralline ~ ridge to about 30 feet deep where the bottom begins to drop away abruptly. In this area no invertebrates other than corals were observed, and no facilities were available to transport heads of coral to shore for Porcher examination; however, numerous, holes were noted in the coral floor where crustaceans, worms and other forms could have lived. | The coral on the shelf was voughly zoned, with the goranane species in the shallower water near the coraltine ridge being Pocillopora meandrina, and in the deeper water of the middle and outer shelf, Seeeties of Acropora.s In the middle and outer mometede of the shelf ebbeaine heads of Porites lobata | were conspicuous. Among the onner corals fone aap this area were all of those reported froii the backeridge trough and some small specimens of ' Stylaster growing on the mdersides of coral heads in twenty feet or more of water. Large areas of the bottom were covered. with dead, loose fronds of Acropora. - 12 « Inshore Shallow Water MARINE ASSOCIATIONS Re ONOTOA, GILBERT /S. Explanation in Windward Reef =| Leeward Reef Heliopora Flats Coral Shingle Mud Flats yA Incipient Beachrock \ ; t : EL Sand and Beachrock Foreshore h Sand Flats oy Z el — r Turtle Grass Decadent Coral Reef | fathom line 10 fathom line ,~ (approx.) ‘-. (Qpprox.) ~~ 2 Miles Adapted from map made by PrestonE. Cloud ur. A.HBanner ieee Hine ake aaah ge As (i " Peis si mys Yat at we Ae onan’ , dN Pk ; ; G foe adeiedy eal Pach plone a el eae jt | II SHALLOT WATER LAGOON REGIONS AND ADJACINT AREAS” AREAS NOT IN LAGOON. Ey Leeward Tsland Reef. This area lies to the lee of the ends of the alee, een ana Rycpclniasiese of Tabaurorae and its northern reef, The regions faunisticelly approach the windward reef but on them there is not a We asvelioned reef flat and no backridge Hele or coralline ridge whatsoever, but instead cuanges at places quite abruptly into conditions similar to the reef shelf off the tees reef. In water of moderate depths -= tio to five feet -- the major elements of the fauna are the same as the backridge trough on the windward reef. The major exception to these generalities “aS in the region northward of the , lieLiopora flats off Anteuma; here, ne conditions are similar to the area. Ce the reef to the zest of Abenecnec Island (to be described by Bes Cloud). if. Seg OE ES The BEES s designated 4 as Heliopora fens are found in a _ protected region behind the. amend reef at ane sou th end of the north island and. northnest of ; the tip of the Hoth island, | | | The Sommers Helionora flat consists of an Seren save tide pool avout 800 feet in diameter, protected on the oceanside by a cele eepeiine poulder ridge, and by elevated sand and boulder Wereesa Tefeisuits) (o)9\ vhe other sides. The bottom ce the pool is eka re to be about the 0,0 y uidat level, and the water stands about trelve se) eighteen inches fee The bottom is sand. The oat animal is Jieldoporay vith one heac about every square Joos Porites SP. is perhaps a ee as plentiful. Other corals, all infrequent, include ia Ae a Orbicella, Pocillopora, Leptoria, On the exposed sand bottom no animals ex« cept Holothuria atra-are conspicuouse Other invertebrates are found in two habitats » A. Under coral heads. Here are fowmd stomatopods (Pseudosaut la ciliata), Tethys, two species of tmicates, four species of holothumaadel Thais hippocastanum, and several species of breehrenen crabs. B. In coral heads. Here the dominant forms are crangonid shrimps and small xanthid crabs. Encrusting sponges of various types are sae black ; palendal. tmicates are plentiful; one head only showed numerous Seid sea anemones. Annelid worms, both Errantia and Sedentaria, are noder geome common » Several species of clams, including *Isognomon spe and sBarbatia benelen eee found between the inner branches of the coral. The northern Heliopora flat is famistically similar to the southern, with the same eeu ici, However, it shows the transition, on its inner side, between a eaten Heliopora flat as peseriven above with infrequent heads of Heliopora BE OULS from the sand bottom, through a condition where the Heliopora is growing thickly and the top ends were ‘being consolidated = coralline algae, to a consolidated condition like thee described in saetien - A-8 of the windward reef. In the labyrinthian prenek below the ounreee con= solidation are numerous small fish. (n the outer fee tess piees orachentee “change in a moderately deep water coral association eh passages between the coral six or more feet deep». | III. Gravee Flats. These areas of shingle -- Pinpesied and waveworn coral rocks ee in regions where the waves and the currents are strong enough to sweep aay the sand. These conditions are found in the passes between’ the islands, as in the three passes ‘between North Tsland Ane South Island, and He. two passes Wnt of South Island. The size of the rocks varies with location, being large ashe where there is an wmbroken sweep of the water, as betireen the windward side of the Abenecnec passes, and gradually changing into fine gravel on the more protected extensions of the current, as to the west end of the southern tip of North Island, which in turn is replaced by the fine sand characteristic of the Zagoon. All shingle areas inspected were above the 0.0 tidal zone, and in places extended up to the edge of the terrestrial flora. In some portions of the passes there were developed broad shallow.tidal pools, with a bottom of finer rocks or sand. \ithout exception these actual pass areas were found to be devoid of larger animals; even the tidal pools appeared lifeless, However, where there was slight protection either from islands or fron bars, there was a feeble “fauna developed, inith some xanthid crabs, a few sponges and heads of Porites “in the tidal pools. In the fine gravel zones, transitional between the shingle ‘and the lagoon sand, some life was found in the levéis near tne zero tide zone. Burrowing into the dead coral reef under these areas were foumd sipunculids and annelids; in the small shallow tidal pools were fod occasional brittle stars, solitary zooanthids and small crabs under the scattered loose boulders. LAGOON AREAS IV. Sand Foreshore. Along the lagoon side of the island the foreshore, from about the two foot tidal level up alternates bettreen fine sand ane consolidated beach rock with more areas of beachrock off the northern island and more sand off the southern. Only near the tips of the islands and around smaller islands like Anteuma and Abenecnec are these two characteristic beach fommtion replaced by coral shingle. The sand foreshore is devoid of life except for occasional ghost crabs Ocypode ceratophthalma the same species that is found much more - 16- ee venti ally on ee vrindvard sand beach, | Ve Beach-t rock Foreshore. Alternating with Sie ane. foreshore are areas where the elevated eee ’ rock of the islandts base is exposed by wave action. This slab is eroded on the top surface Lite the typical cupped pattern, and often is mdercut along the lower edge by wave action and ‘possibly solution by fresh water from the island isnt At places, especially in the lower tidal zones, the mdercutting has proceeded far enough so that slabs up to several. feet or more long have broken off from the base rock and lie free on the substrate of either beach- rock or of sand. Animal communities in this nabitat when the tide is out are subjected to | dessication aa heat, to rain and especially to the flowing fresh water, common all along the shore; thon’ the tide is in, to moderate wave action (except, possibly curing periods ‘of storms from the vest when the wave action would be vigorous). The rocks can be subdivided into four. associations: AG etirhe id esabicachanoek areas This is above about the 2.5 foot tidal | } zone and is almost devoid of life except for“Nerita plicata and Grapsus prapsus, | neither as common as on the similar eke on the a ial; ya LEGEND FOR MAPS 8.0 Coral heads LHS Edge of coral reef Navianys Staghorn coral = Solid reef limestone 1 3/4 Soundings in fathoms eae Sand exposed at low tide ashy Coral gravel 20% Loose coral rocks SUN Beech sandstone Dey Ve Dense vegetation (semi-natural) ko &o Taro pits M. Megapode nests Coconut palms sO Coconut palms killed by coconut rhinoceros beetle wa Well ao Houses, a-bais or school =e Paths === Main avenues | abt wal ta boscpes hase, . he . ok " nae 8 yor Lizad poo ebree ‘oautt ‘ ( Larne Lowe) sins padenew, aecatl | | | er io orunt alesm wboageMt ania tyros elteed werebontiy tunnecs ut Soli amisg dumopod uF ti . Lint eeodss vo abod-es asawall alte | Beuiers bent } euoteendl, torr pelo? Be eli ee Feet ge tac) * KAYANGEL ATOLL 3 42 i yy zi Hi, ggg tT, 352 g05 £05! pan 315 SIPS kite ee) aa 262 123 276 5 16, Yee 235 G4 8°02'30" MAP NO. | ISLETS OF KAYANGEL ATOLL DENSE VEGETATION NGARIUNGS NGARAPALAS Lo aR GORAK MAP NO. ee bie A i ; i ih }AYAN 4081S se ) ig) Melee ae ae a a" mua 3 90