Radionuclides in plankton near the Marshall Islands, 1956

UWFL-54

BIOLOGY AND MEDICINE

UNITED STATES ATOMIC ENERGY COMMISSION

RADIONUCLIDES IN PLANKTON NEAR THE
MARSHALL ISLANDS, 1956

By
Frank G. Lowman

February 14, 1958
Applied Fisheries Laboratory

University of Washington
Seattle, Washington

Technical Information Service Extension, Oak Ridge, Tenn.

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UWFL-54

RADIONUCLIDES IN PLANKTON NEAR THE
MARSHALL ISLANDS, 1956

by

Frank G. Lowman

Laboratory of Radiation Biology
University of Washington
Seattle, Washington

CONN

hs

Lauren R. Donaldson
Director

Win

February 14, 1958

CNN

Operated by the University of Washington under Contract No.
AT(45-1)540 with the United States Atomic Energy Commission

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ABSTRACT

Radiochemical separations were made on plankton samples
collected in and west of the Eniwetok Proving Ground in Sep-
tember 1956. Ion exchange resin column and precipitation
technicues were used. Fission products, mainly Zr?5-Nb25
and Cel44-pr144, contributed an average of 29 per cent of
the total radioactivity. The remaining 71 per cent of the
activity was contributed by the non-fisston radioisotopes
zn©5, Co57,58,60, Fe55 and Mn>+. Radioactive zinc, cobalt,
and iron accounted for averages of 24, 26 and 21 per cent
respectively of the total radioactivity. Mn54 was present
in trace amounts. Variations in ratio of occurrence for
the different non-fission products with change in geographi-
cal location was observed. Relatively high levels of gn©5
were centered in the area near Bikini and Eniwetok Atolls.
The area of high levels of radioactive cobalt and iron in
comparison to gn©5 was located approximately 480 miles west

and slightly north of Eniwetok Atoll.

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RADIONUCLIDES IN PLANKTON NEAR THE
MARSHALL ISLANDS, 1956

Introduction

During the summer and fall of 1956 members of the
Applied Fisheries Laboratory conducted two surveys in the
region of the Pacific Ocean bounded by the Marshall,
Caroline, and Marianas Islands. The purpose of the surveys
was to measure the levels of radioactivity in the water,
plankton, and fish and to determine the westward boundary
of the contaminated area.

The first survey was made June 11-21, 1956, during the
weapons testing period and included 53 collection stations
in the ocean 11° N to 14° N and 159° E to 166° FE. The re-
sults were reported in U. S. Atomic Energy Commission report
UWFL-46 (6). The second survey conducted from September 1-
20, 1956, about six weeks after the conclusion of the weapons
tests, included 74 stations between 9° N and 15° N and
approximately 145° E and 1669 E (Fig. 1). The findings were
published in U. S. Atomic Energy Commission report UWFL-47
(21).

Chemical separations for fission products, cobalt, and
zinc were made on a limited number of samples from both surveys,
the results of which were reported in the latter report (21).

According to these data fission products contributed a major

portion of the total radioactivity. Gamma spectra obtained
from the same samples, however, indicated that non-fission-
product radioisotopes contributed more of the total radio-
activity than was observed in the chemical separations,
which were designed primarily for determination of fission
products. Consequently a detailed study was made to de-
termine the radioisotopic content of several plankton
samples collected during the September survey. In addition,
gamma spectra were determined for these and most other
plankton samples collected in the latter survey. The re-

sults are reported in this paper.

Materials and Methods

The equipment and methods used for collecting the
samples at sea have been described in previous reports
(an ROS

The initial counting for beta activity was done aboard
ship soon after collection of the plankton samples, and the
remainder of the plankton from each station was preserved in
10 per cent neutralized formalin in sea water and returned
to the Seattle Laboratory.

The concentration of plankton in the tropical seas and
the levels of radioactivity in the plankton are both limited.
thus plankton samples from adjacent stations were combined
to provide sufficient material for analysis. Figure 1 shows

the track of the September survey and the groups of stations

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from which samples were combined for analysis.
Five plankton samples were analysed in detail for their
radioisotopic content eleven months after the date of col-

lection. These included

Sample No. Stations
I (Ave
1g 23,4 5
a gat 43, 44,45, 46
IV 50,51,52,53
V 57,58,59

The samples were filtered through No. 1 Whatman filter
papers and the plankton and filtrate fractions dried sepa-
rately. Gamma spectra were made from both fractions. The
plankton samples were then wet ashed with concentrated
nitric acid and hydrogen peroxide and converted to the
chloride form by evaporating the sample to partial dryness
repeatedly with hydrochloric acid. The moist salts were
dissolved in 0.2 N HCl, filtered, and the filtrate passed
through a cation-exchange column (5 mm x 100 mm) of Dowex 50
(IS eure Ieee

Two hundred and fifty mgm or less of the sample in 0.2 N
HCl solution were passed through the column at a4 rate of 0.2 -
0.5 ml per minute, then the resin was washed with 50 ml of
0.2 N HCl. The eluate from the sample and the subsequent
hydrochloric acid wash contained those ions not adsorbed to

the resin, 1.e. the anions. These included Zr?95-Nb95 and/or

-5-

Rul06-ppl06, The cations that were adsorbed onto the resin
were differentially removed next by passing 40 ml of 0.5
per cent oxalic acid and 40 ml each of 5 per cent ammonium
citrate solutions at pH 3.5, 4.1, 5.1, and 6.1 through the
column. Oxalic acid removed cationic Zr95-Nb95 and Fe55.
Ammonium citrate at pH 3.5 removed Cel44_ppl 44 mn 4 |
Co>?,58,60, ana zn©5*, Ammonium citrate at pH 4.1 removed
Cs137, No radioisotopes were detected in the pH 5.1 or

6.1 fractions when the fractions were counted for beta or
gamma activity.

The pH 3.5 fractions were dried, wet ashed and re-
dissolved in 5 ml of concentrated HCl (12M) and run through
a column (7 mm x 250 mm) of an anionic resin, Dowex l, to
separate Mn, Co, and Zn from each other according to the
method of Kraus and Moore (15). The sample was added to the
column and the fractions were eluted at a flow rate of 0.1
to 0.2 ml per minute. The elutrients included 10 ml each of
10M, 6M, 4M, 2.5M, and 0.5M HCl, 20 ml of .005M HCl, and 10
ml of Ho0. Under the conditions of the experiment Mn>4 was
eluted in the 12M wash, Co57,58,60 in the 6m fraction, and
zn©5 in .005M HCl.

From the fractions eluted out of the Dowex 50 column

one-ml aliquots of the HCl fractions were dried, treated with

*These radioisotopes were subsequently separated from each
other on a second column.

26e

concentrated nitric acid. dried and diluted with water, then
plated on one and one-half-inch stainless steel planchets.
One-ml aliquots of the oxalate and citrate fractions were
plated directly on the planchets and dried. The samples were
counted for beta activity in a windowless methane gas-flow
chamber. The oxalate plates were counted after drying, after
flaming, and with a filter of 4.7 mg per cm@ aluminum in
order to identify the radiation from Fe?> (16).

The remainder of each fraction was dried and analysed
with a gamma spectrometer equipped with a two-inch, well-type,
sodium iodide crystal. The isotopes were identified by their
gamma energies, maximum beta energies, half lives, and elution
patterns.

Beta and gamma counts were converted to disintegrations
per minute (d/m) by the use of correction factors previously
described (16). Disintegration rates for the individual
radioisotopes were corrected to the date of collection.

The fractions eluted from the second column (Dowex 1)
were counted in solution for gamma activity and the fractions
containing the peaks of radioactivity from mn54 , CodT, 60, or
zn©5 were combined into three groups (Mn, Co, Zn), dried, and
the disintegration rate of each group determined.

Radiochemical separations in duplicate for 8r90-y99 were
made on each of the five samples and their filtrates according
to the method outlined in the chemical procedures of the Health

and Safety Laboratory (New York Operations Office) (11).

-7-

Results

The non-fission products C057, Co58, C062, zn©5 and Mn54
contributed almost all of the activity in gamma spectrum
curves made from filtered plankton samples eleven months after
collection, masking the weaker peaks from the fission products
Zr95-Nb95, Rhl06, Bal37m(csl37) ana Cel44 (Fig. 2). The gamma
peaks of the latter isotopes were detected only in the frac-
tions eluted from the ion-exchange columns (Fig. 3).

Although the non-fission products, in each instance,
contributed the major part of the gamma activity in the plank-
ton, they were not always present at a constant ratio to each
other in the different samples. Thus in some samples the
amount of zn65, based on the gamma spectra, was high in com-
parison to radioactive cobalt, (Fig. 2, plankton stations 50,
Saale -242) and in others the reverse relation

gn©5 1.0
isted (Fi 5 lankt tation 42 Corl = 5-5)
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variation was related to the site of collection. A similar
variation was observed in the levels of some other non-fission
radioisotopes and will be discussed later.

Detailed descriptions of the elution patterns from ion-
exchange resin columns and the identification of the separated
fission and non-fission radioisotopes appear in an earlier
paper (16) and are not repeated here. However, the gamma
spectrum curves for plankton samples 50,51,52,53 and the sepa-

réted fractions are shown in Figure 3 to illustrate the small

1000

GAMMA
ACTIVITY

100

——PLANKTON STATIONS 50,51,52,53
Co’ / Zn = 0.19/ 1.0

10 J] PLANKTON STATION 42

57 _ 65
Co/Zn = 5.5/1.0

60
Co SPIKE

OZ) O04 O68 10:8 LOC rl4.
MEV

Fig. 2 Gamma spectrum curves from three areas west of
Eniwetok Atoll showing the variation in ratio
of Co>T /2n®, The gamma peaks of Mn 54 C098,
and Co°9 are also indicated. The gamma peaks
of a Co? spike are shown for comparison.

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amount of cross-contamination between the radioactive ele-
ments separated by a combination of the Dowex 50 cation-
exchange method and the Dowex 1 chloride complexing tecmmidque.

The results of the raditoisotopic separations are summa-
rized in Tables 1 énd 2. In these samples the predominant
fission-product radioisotopes were the relatively short-
lived Zr?5-Nb?5 (65 a), Cel44-prl44 (285 a), and in one
sample, Rul0€E-Rpl6 (3 yr). The fission products accounted
for 22 to 37 per cent of the total radioactivity in the three
analyses from which cerium determinations were made. In the
other two samples fission products contributed at least 5.7
per cent (stations 57,58,59) and 44 per cent (stations 13,14,
15) of the total radioactivity.

gr90-y990 were not detected in the ion-exchange sepa-
rations nor were they found in fuming nitric acid precipi-
tations made on duplicate aliquots from both the five samples
and their filtrates).

Of the non-fission radioisotopes, Fe>5, CoD7, CoD8 ana
zgn©5 contributed almost all of the activity. Although C060
was present at an average level of only about one per cent,
it has the longest half life (5.3 yrs) of the reported non-
fission radioisotopes and is therefore important.

The variability, mentioned previously, between the
ratios of Co! to 2n©5 also was evident between Co?! and C060,
and between Co?! and Fe>. In Figure 4a, the ratios of

CodT /C060 in the five plankton samples are shown; also, these

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151° 152° 153° 184° §=1559

Rigs 4a Ratio rare in parentheses) at collection date
from stations 7,5,9; 13,14,15; 43,44,45,46; 50,51,
52,53; 57,58,59; Bikini Heel and Eniwetok AGouaNS

151° 152° 153° 154°— «156°

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StS eter Ch Ty

165°

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Fig. 4b Ratio CodT /gn©5 (in parentheses) at collection date

from the plankton stations; Bikini Atoll; and Eni-
wetok Atoll.

151° = 152° 153°. 154°—s« 155°

160° ee [|

gc

= aa, Os a
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.. SURVEY TRACK
a 4 13.4

Fig. 4c Ratio Co2!/Fe25 (in parentheses) at collection date
from stations 7,8,9; 13,14,15; 43,44,45,46; 50,51,
Be oo5 and: 57,56, 59).

Sai

ratios for other biological samples from within Eniwetok and
Bikini Atolls, reported in an earlier paper, are given (16).
Co5T and Co60 were present at Bikini Atoll in a 1:1 ratio; at
Eniwetok in a 5.331 ratio. A similar difference in levels of
the two cobalt isotopes also occurred in the plankton taken
in the open sea in the vicinity of the Pacific Proving Ground:
Thus, at stations 57, 58, 59, approximately 200 miles northwest
of Bikini Atoll, the ratio Co57/co®° is 4.6:1.0 and the value
150 miles northwest of Eniwetok Atoll is 7.6:1.0. The ratio
of CodT /¢ 069 in an area 300 miles wide north to south and 200
to 500 miles west of Eniwetok was constant at a ratio of ap-
proximaitedsy dale:

The ratios of CoD!/zn©5 for the plankton stations are
shown in Figure 4b. The values for the five plankton samples
on which ton-exchange separations were made are shown cross-
hatched; the remeining CodT /zn®5 ratios for plankton were
calculated from the gamma spectra on total samples. The
ratios for Eniwetok and Bikini samples were taken from ion-
exchange separation data in an earlier report (16).

The ratio of Co57/zn©5 at both atolls was 0.06:1.0, indi-
cating a relatively high level of zn©5 with respect to Co’.
The same condition prevailed at stations 3, 4, 5, and 6, 200
miles south and west of Eniwetok Atoll. However, the ratio
north of Eniwetok and Bikini along the northern boundary of
the restricted zone was slightly higher (0.1:1.0) than that of

the atolls. The center of high Co?’ /zn©5 ratio (5.5:1.0) was

-~15-

observed at station 42 approximately 480 miles west and
slightly north of Eniwetok Atoll and did not coincide with
the center of greatest total radioactivity which occurred
approximately 110 miles due north of Eniwetok Atoll (21).
The C057 /zn®©5 ratios decreased with increasing distance in
all directions from station 42.

The ratios of Co>!/fe55 for the five plankton samples
are shown in Figure 4c. The ratios are similar to those of

Co57/zn©5. The values are as follows:

Cool Cool

Station 7n©5 Fe5
339 0.6 0.5
3) eS O26 0.4
43, 44,45,46 0.9 q-2
SUR Sil 5255s OZ O12

Bis 53559 Oo 0.1

-16-

Discussion

The retention or rejection of a given radioactive ele-
ment by the organisms in the contaminated area of the ocean
is determined by several factors, many of which also function
in the distribution of naturally occurring elements of the
sea. The effects of some of these factors are known; others
are not. A better understanding of the unknown effects may
be attained by observations on the accumulation of individual
radioactive elements from fallout within the trophic levels
of the contaminated biosphere. By observing which radioele-
ments are concentrated in the phytoplankton, the zooplankton,
and the nekton and relating these observations to (a) the
known characteristics of the fallout elements before and
after entry into the sea; (b) the half lives of the radio-
isotopes, the amounts produced at detonation, and their
distribution with respect to fallout particle size; and (c)
the known biological factors involved in the uptake and re-
tention of different elements, the eventual fate of the major
fallout radioisotopes in the sea may be determined.

In the waters of the open Pacific Ocean the naturally
occurring trace elements, zinc, cobalt, ruthenium, manganese,
cesium, strontium, and possibly zirconium and cerium, in the
water are present mainly in solution. The small amount of
naturally occurring iron occurs in the colloidal and particu-

late form. All of the above named elements, however, except

= 17-

cesium and strontium, when introduced into the sea from fall-
out, would be present most likely in the insoluble or par-
ticulate state. Indirect evidence (8, 20) and direct
observations (9) support this view. The indirect evidence is
based on geochemical studies in which the potential supplies
of the elements to the seas from weathering of igneous rocks
are compared with the amounts of the elements present in
solution in the sea. These values cannot be applied directly
to fallout analyses but they do provide the basis for an
estimate of the fraction of a given fallout element that would
remain ini solution jin ‘ther sear

The levels in the sea (ppm) of the naturally occurring
forms of those elements reported in the present work are shown
in Table 3. Also listed are the forms of the naturally occur-
ring elements in sea water, the percentage of the fallout ele-
ments in solution according to direct and indirect evidence,
the principal chemical state of the fallout in sea water, and
the average percentages of the fallout elements observed in
plankton samples collected about three months after fallout.

Of the radioactive fallout elements shown, only two, cesium
and strontium, would occur normally in soluble form in the sea.
The radioisotopes of these elements were found in the least
amount in plankton. In contrast, the radioactive fission:
products with the lowest solubilities, Zr95 and Cel44, were

present in plankton in the greatest amount (20.2, 8.6 per cent)

-18-

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=16=

for this group of radioelements. This observation is in
agreement with the findings of the Taney survey (10) in
which the major fission products in plankton were found to
be Celt4_ppl4t4 | However, & gamma spectrum curve made on
these plankton samples November 1957 by the present author
revealed an appreciable amount of CoS0, zr95-Nb95 were not
found in the Taney samples because of the short half life of
Zr95 (65d).

In the case of the non-fission-product radioelements,
zinc, cobalt, and iron were present in the plankton (based
on an average of the five samples) in approximately equal
amounts at about 24 per cent each and manganese at a level
of less than one per cent of the other three elements. All
of these elements, from fallout in the sea, would exist
initially in the insoluble form.

Limited data from both control experiments (4) and
field observations (21) suggest that the major route for up-
take of fallout radioisotopes by marine animals is through
the ingestion of radioactive particles. Chipman (5) noted
also that, in addition to the filter feeding animals, two
species of algae were able to concentrate Cel44 several
thousand times over the levels in the water although the
radioisotope was in the particulate form. The present work
is in agreement with the postulate that the major source of
radioactive elements from fallout to marine zooplankton is

through the uptake of particulate material. The uptake may

2a02

involve either adsorption of the particle onto the organism
or ingestion of particulate material. The latter would
include both food particles and non-living particulate
detritus.

Several other factors, however, also control the uptake
of specific radioelements. In the case of strontium and
cesium chemical competition plays a major role. Naturally
occurring potassium, the competitor to cesium, is present in
sea water at a high level of about 380 ppm. In comparison
to this level of occurrence, Cs137 would be present in very
small amounts, even in an area of heavy fallout. Because of
the limited ability of animals to differentiate between po-
tassium and cesium, uptake of the latter would be low.

Natural calcium occurs at a level of about 400 ppm in
sea water and exhibits chemical competition to strontium with
regard to uptake by marine organisms. In the case of these
elements, however, the uptake is not directly proportional to
occurrence in the water; rather strontium is discriminated
against with reference to and in the presence of naturally oc-
curring calcium by factors of approximately 3 in calcareous
algae and foraminiferans, 2 in arthropods, 2 to 7 in molluscs,
2.5 in bryozoans (25) and 3 to 10 in marine fishes (19).

In laboratory experiments utilizing radioactive strontium
Chipman (2) observed that Artemia larvae reached a steady
state in the level of radiostrontium about 0.7 that of sea

water and Burroughs, Townsley, and Hiatt (1) reported a value

=\}

Of GboOULIEORS) tanta Shishi (Tilapia). In the case of fish bone
and scales, however, the concentration factors may be higher.
In the croaker (Micropogon undulatus) concentration factors
for strontium in vertebrae and scales were 2.5 and 2.1 re-
spectively those of sea water.

In addition to the above mentioned factors, isotope
dilution by stable strontium would result in reduced uptake
of $r99 by marine organisms. Stable strontium is present in
the sea at a level 6 to 1300 times that of the naturally oc-
curring forms of the other elements represented in fallout
(Table 3, column 2). Thus the discrimination against radio-
strontium as a consequence of the presence of stable stron-
tium in sea water would be 6 to 1300 times that to which
the other radioelements would be subjected because of the
presence of their stable counterparts.

Further discrimination against the uptake of radiostront ium
in these organisms may be caused by the scavenging action of
calcite formed from coral aragonite in fallout material. In a
simple experiment by the author, pulverized coral was sprinkled
onto and allowed to settle through sea water contaminated with
gr39 Clo. Approximately 11 per cent of the radionuclide was
removed from solution the first hour. No reduction in activity
was noted in a parallel control experiment. Suito, Takiyama
and Uyeda (22) reported that the ashes from the March 1, 1954
weapons test at Bikini which fell on the No. 5 Pukuryu Maru

consisted of white granules of calcite approximately

=20—

390 p in diameter (mostly 190-400 p). Identification of

the chemical form was made by electron and X-ray dif-

fraction techniques. Calculations of Miyoshi (18) indicate
that particles of calcite 400 p in diameter would settle

from the surface to 200 meters in 26 minutes and those 100 p
in diameter in 7 hours. Thus the mechanism for rapid
scavenging of Sr°97?9 tn fallout introduced into the sea
would be provided by calcite settling through the thermocline.

Co-precipitation would provide another scavenging mecha-
nism for radiostrontium in the fallout area. When calcium
carbonate is precipitated in sea water strontium is co-pre-
cipitated and carried down with the calcium. This technique
is commonly used in radiochemical separations on sea water.
The amount of strontium removed from solution in the sea by
the precipitation of calcium derived from oxidized and hydro-
lyzed coral is not known but probably is not low.

Strontium, co-precipitated with calcium carbonate into
the particulate form, then would become available to filter
feeding organisms. However, a limited fraction of the parti-
cles would settle out of reach of the biosphere into deep
water. A rapid turnover of strontium has been observed in
all marine invertebrates and fishes studied up to now (1, 3).
Thus, ingested radioactive strontium in the particulate form
would be continually recycled to the sea in solution so that
in time it would come to equilibrium with the naturally oc-

curring non-radioactive form.

-23-

Three of the radioactive non-fission-product elements,
zinc, cobalt, and iron, contributed an average of 71 per
cent of the total activity in the plankton samples. This
observation is in contrast to those on terrestrial samples
in which these isotopes were absent or, at most, present in
trace amounts only (16). Thus, either a concentrating
mechanism must function in the sea for the non-fission-
product elements, making them available to the organisms,
or an exclusion mechanism must operate on land. The ex-
tremely low levels at which these isotopes occur in dirt
samples near the target area suggest that concentration oc-
curs in the sea.

In the case of these radioelements the previously dis-
cussed factors that control uptake by marine organisms, with
the possible exception of scavenging action by calcium hy-
droxide or calcite, would tend to cause increased uptake.
Thus, these elements in fallout probably occur in the sea in
particulate form, are not subject to chemical competition by
similar elements or to appreciable isotope dilution by their
stable counterparts, nor would co-precipitation occur in the
strict sense of the word. However, negatively charged cobalt,
menganese, and zinc would tend to precipitate with iron into
a finely divided form, exhibiting only a limited tendency to
sink below the thermocline and thus remain available to the
plankton in the mixing layer.

The isotopes Mn54, Fe55, Co57, Co58, co, ana zn®5 are

transition elements and have characteristics of variable

-?h-

valence, easy oxidation and reduction, and a marked tendency
to form complexes. Except for the elements carbon, nitrogen,
and phosphorus, the transition elements are concentrated to
the greatest degree by marine plants and animals over the
levels in the water (23826)8 These are the elements that form
the most stable complexes with organic chelating materials
(17,14,27) and it is most likely by this mechanism that the
heavy metals zinc, copper, nickel, cobalt, iron, and manga-
nese are concentrated in the marine biosphere. If surface
binding of these metals to organisms is a major factor for
their concentration then & concurrently important factor is
that of the extent of the available organic adsorptive surface.
Plankton in the sea, especially phytoplankton, provide the
greatest surface area with respect to protoplasmic volume and,
in addition, probably provide a greater volume of living ma-
terial than any other of the marine organisms. Thus, the major
initial concentration of radioactive zinc, cobalt, iron, and
menganese from the environment probably occurs in the phyto-
plankton.

In addition to the surface adsorption of non-fission-
product radioisotopes onto phytoplankton and zooplankton, the
orocess of direct uptake and assimilation may well be of im-
portance in these organisms. The roles of these non-fission
elements in the physiology of planktonic organisms are not
known but in higher animal forms, at least, they are of major

imvortance.

-25-

The probability that the non-fission radioisotopes would
be present in particulate form in the sea was discussed
earlier. Particles of non-fission-product material either
adsorbed onto or assimilated into the phytoplankton or ex-
isting as free perticulate matter in the water would be avail-
able to the invertebrate filter feeders. Thus the second
concentration stage for non-fission radioisotopes would occur
in the zooplankton through the filter feeders. Of course
factors such as (a) average life spans of the phytoplankton
and zooplankton organisms (b) average biological half lives
of the different radioactive elements within the zooplankton,
and (c) species differences between filter feeders regarding
choice of food, the assimilation of the different radioele-
ments and retention of the radioelements would influence the
levels of the non-fission radioactive elements in the zoo-
plankton. These factors are not known.

There is little probability, however, that the above
mentioned factors or any other biologicel effects are re-
sponsible for the variations in ratio between Co>’ and C060
found in plankton samples from the different stations. For
the heavier elements at least, organisms have little or no
ability to differentiate between isotopes of a given element.
Thus, the variation in ratio of Co! to 6000 probably reflects
the variation in level of occurrence of these two isotopes in

the fallout material in different geographical locations.

-26-

The variation in the ratios Co27/zn65 and Co?! /Fe55 in
plankton samples with change in collection site may also be
due to differences in levels of the three isotopes in the
fallout material. However, the possibilities should not be
overlooked that it may be due to preferential scavenging of
zine by calcite and calcium hydroxide in the area of intense
fallout or to biological effects. Data that would resolve
these points are not évailable and an effort should be made
to obtain the necessary information in any future operation.

Evidence that the differences in C057 /zn65 ratios are
real and not due to techniques of separation and analysis
is found in the gamma spectrum curves on whole plankton
samples (Fig. 2). Inspection of the curves from the three
samples shows clearly the differences between the levels of
zn©5 and both of the cobalt isotopes, Co57 and C0®9, in the

whole sample before separations and analyses were performed.

1. During July 1957, chemical separations by lton-exchange
resin columns were made on five grouped plankton samples taken
in and west of the Eniwetok Proving Ground in September 1956.
Also, radiochemical separations for Sr90-y90 were made. The
radioactive isotopes in the separated fractions were identi-

fied by decay rates, maximum beta energies and gamma energies.

2. The distribution pattern of CoD? and Zn©5 in plankton
samples not subjected to ion-exchange separation was deter-

mined by analysis of gamma spectrum curves.

3. The predominant fission product radioisotopes found were
Zr95-Nb?>, Celt4_ ppl 44 and in one sample, Rul06_Ry106 ,

Fission products accounted for 22 to 44 per cent of the total
radioactivity at the collection date. Sr90-y¥90 was not found
in the ion-exchange separations nor in fuming nitric acid pre-

cipitations made on duplicate aliquots.

4, The non-fission radioisotopes Fe>>, Cool, CoD8 , and zn©5
contributed 55 to 77 per cent of the total radioactivity, and

Co©° ang Mn54 accounted for a total of 0.8 to 1.6 per cent.

5. Co57 and Co60 were present at Bikini Atoll in a 1:1 ratio
and at Eniwetok in a 5.3:1 ratio. These ratios were re-
flected in plankton samples collected in the vicinity of the

two atolls. The ratio of Co57/Co®° tn an area 300 miles wide

north to south and 200 to 500 miles west of Eniwetok was

= e=

ClomSicanic railed ely,

6. The ratio of Co57/zn65 at Bikini and Eniwetok Atolls was
0.06:1.0. Similar ratios occurred 200 miles south and west
of Eniwetok Atoll. Slightly higher ratios occurred to the
north. The center of high Co57/zn65 ratio (5.5:1.0) was
found approximately 480 miles west and slightly north of
Eniwetok Atoll and did not coincide with the center of high
total radioactivity in plankton, which was located 110 miles

north of Eniwetok Atoll.

7. The changes in ratio of Co>//Fe°5 with change in geo-

graphical location were similar to those of Co57/zn®5.

lJ.

a.

=2G-=

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