Health Physics Pergamon Press 1975. Vol. 28 (June), pp. 693-697. Printed in Northern Ireland
OCCURRENCE OF STRONTIUM-90 IN LIVING CORAL FROM THE NORTHERN GULF OF MEXICO LAWRENCE L. GRIFFIN, JERRY K. MYERS, DAVID C. CARLYLE and ANDREW D. SUTTLE, JR. The Marine Biomedical Institute of the University of Texas Medical Branch at Galveston,
200 University Boulevard, Galveston, Texas 77550 (Received 12 Jub 1974; accepted 13 September 1974)
Abstract-Strontium-90 has been extracted from Montastna cauernosa coral which had been living on the West Flower Garden Bank submerged reef (lat 27'52.6' N, long 93'49.0' W) in the northern Gulf of Mexico in 1972. The calculated gOSractivity level in the sampled coral is 0.5 f 0.2 dislminlg of coral ash or 0.1 1 f 0.05 pCi/g wet coral. This level is larger than, but the same order of magnitude as, 90Sr radioactivity levels found in 1967 seawater samples from deeper locations in the Gulf of Mexico. It is thought that the composition of seawater in the Flower Garden vicinity is representative of water along the outer continental shelf of the Texas Gulf Coast, so that the existence of 90Sr in coral from the FIower Garden indicates a correlative low level presence of that nuclide in such water. INTRODUCTION
1967 the level of in waters of the Gulf of Mexico was generally the same order of magnibodies of seawater (DUURSMA, 1973) has ex- tude as the very lowest levels observed in seaperienced some contamination by fission product water from numerous sites in the Atlantic and radioisotopes. VDOVENKO et al. (1971, 1973) Pacific Oceans. I n addition the concentration appears to have been relatively constant have found strontium-90 at low levels in sea- of with depth to 1000 m in parts of the Gulf. The water samples collected in 1967, and results of et aZ. were collected the present study show that this nuclide has samples of VDOVENKO been accumulated in submerged coral growing approx 4 yr after cessation of large scale nuclear near the edge of the continentaI shelf. There weapons testing, and it is expected that the has been widespread atmospheric distribution concentration of 9%- should have decreased of the fission products of nuclear bomb testing somewhat since 1967 or may have remained (KURODA,1973; NOYCEet al., 1973) with a relatively constant. A slight decrease would be corresponding deposition of radionuclides to due to decay of the 28-yr half-life isotope and terrestrial and aquatic surfaces. The distribu- to its having been carried away by waters of the tion of OOSrdeposited in the Atlantic Ocean has Gulf Stream, a proposition which VDOVENKO in particular been well studied (BOWENet al., et al. (1971, 1973) consider to be suggested by 1968, 1969). Atmospheric deposition, land their depth studies on the Gulf Stream vicinity. run-off of bomb and nuclear reactor products Since fallout products of the recent Chinese and (POLIKARPOV,1966; BOWENand NOSHKIN,French tests have been distributed into the 1970) and the possible delivery of non-Gulf Northern hemisphere (KURODA,1973; NOYCE water by oceanic currents which flow into the et al., 1973), it also is likely that a continued Gulf of Mexico (see NAS 1971, Chap. 4) delivery into the Gulf of Mexico of 90Sr from represent potential sources of entry of artificial these and from fission reactor products et al. 1971, 1973; POLIKARPOV, radionuclides into the ecosphere of this body of (VDOVENKO, 1966) has been maintained. water. Very few studies of artificial radionuclide WEST FLOWER GARDEN BANK levels in the Gulf of Mexico have been reported The West Flower Garden Bank (lat 27'52.6' (DUURSMA, 1974; NAS, 1971). However, the results presented by VDOVENKO et al. (1971, N, long 93'49.0'W), a living member of the 1973) indicate that during the winter of 1966- West Indian coral reef community (EDWARDS,
THEGULF of Mexico like other non-oceanic
693
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OCCURRENCE OF STRONTIUM-90 IN LIVING CORAL
strontium from bone ash. Coral specimens are ashed a t 900°C for 4-6 hr and are ground to a powder. This ground ash is presumed to be roughly equivalent to bone which has been ashed a t 900°C and is processed through two precipitations from 70% nitric acid with nonradioactive strontium carrier added during each precipitation. For 10 g of coral ash, the 70% nitric acid volume utilized for the first separation is 400 ml, followed by 200 ml during the second separation. I n each instance the precipitate is collected by centrifugation. T h e second precipitate is dried to expel nitric acid, dissolved in water, and filtered to remove silicates; 10 % sodium carbonate solution is added to precipitate alkaline earth carbonates from the filtrate. The carbonate precipitate is collected by vacuum filtration, dried and mounted for beta counting on a stainless steel planchet. The beta counting of these samples has been performed on a Beckman Low Beta detector. This argon-methane proportional counter is lead-shielded with anti-coincidence and an automatic sample changer; it maintains a relatively constant background count rate of 2 counts/min. With several of the samples, counting has been initiated within a few days after separation of strontium, and 100 minute counting intervals have been continued for a week or more to demonstrate ingrowth of the daughter isotope (half-life 64 hr). The harder beta emissions of this isotope (Emax= 2.2 MeV) are detected in increasing number as the sample approaches -OoY secular equilibrium. Satisfactory consistency has been maintained METHODS in both radiological and chemical yields from Initial efforts aimed a t separating strontium samples processed from identical sources with from the calcium of one hundred or more grams equivalent handling and equal amounts of of coral employing 70 % HNO, precipitation added strontium carrier, although a t present the followed by ion exchange (similar to GREGORY, quantity of non-radioactive strontium accreted 1961) have been set aside. T h e present extrac- by the examined coral and the relative tion procedure employs fuming nitric acid and amount of calcium remaining in the final water to effect precipitations of the nitrates of precipitate have not been determined. The samples of ten or more grams of ashed coral dissolved nitrates of the second precipitation residue. The ash is suspended in water, and flame test strongly positive for the presence of fuming nitric acid is added to produce a strontium. I t appears that counting results 70% HNO, concentration. This method is a n recorded after these separations represent the adaptation of the Harley Method as reported activity of better than 90 % of the total strontium by MARTELL(1956) for separation of radio- (MARTELL,1956).
1971), lies 204 km south-southeast of Galveston, 'Texas and 18.5 km from the edge of the continental shelf, in a region removed from the Gulf Stream. Seawater in this region experiences minimal salinity variation (EDWARDS, 1971). The reef is composed of a variety of common coral species and is submerged at depths from 21 to 61 m. Currents at the depths of the reef are variable both with depth and time, although the prevailing current seems to be from the south-southeast with fluctuating velocities from zero to one knot (EDWARDS, 1971). Since several species of coral have been shown to accrete strontium a t a slightly higher level relative to calcium than the Sr/Ca ratio observed in seawater (ODUM,1951), it seems likely that coral growing on this reef in the years since the onset of man-made nuclear fission reactions should have incorporated skeletal 90Sr in detectable amounts. In order to determine OOSrlevels in such coral, a series of radiochemical extractions of strontium from coral of the West Flower Garden Bank have been initiated a t the Marinc Biomedical Institute in Galveston. These studies are part of a broader investigation of the distribution of longer-lived radionuclides in the northern Gulf of Mexico and of their incorporation into its marine environment. The coral used for these studies was collected by Institute divers during an expedition in the late summer of 1972. Large blocks of both living and dead coral were collccted a t depths of roughly 25 m and were transported to the laboratory for radiochemical analysis.
L. L. GRIFFIN, J. K. MYERS, D. C. CARLYLE and A. D. SUTTLE, JR. RESULTS AND DISCUSSION The results reported here are from extractions performed on six 10.0-g samples of mixed ash of a small protrusion of Montastrea cavernosa (EDWARDS, 1971) growing as part of a larger colony of the same species at the surface of one of the coral blocks a t time of recovery from the reef. Figure 1 shows the observed count rates above a background of 2.18 f 0.1 1 counts/min, plotted semilogarithmically with time for each of the six samples. T h e rate of increase of these counts is roughly consistent with the expected rate of ingrowth of Two of the samples were counted relatively soon after separation, and the other four somewhat later. I n order to determine the amount of ingrowth, the first two observations of each one of the youngest pair of samples have been averaged and this average over four observations is presumed to be the count rate after 200min counting. Likewise, observations for the older samples have been averaged over their last four counting intervals. The two average observations are
1m
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FIG. 1. A semilogarithmic plot with time of observed beta counting activity above background. T = 0 is the time of initial separation of strontium carbonate extracted from each of the 6 10.0-g portions of M . cavernosa ash. x and o represent the two earlier counted samples, and the solid symbols, the four older ones. The curve depicts an average activity calculated from the early and late observations.
695
then considered in terms of the two average expected per cent ingrowths of activity. I t is assumed that the observed activities are the sum of a constant activity and a n ingrowth activity, where expected ingrowths a t time t a r e calculated using the relation (FRIEDLANDER et al., 1965) ~,(t= ) AY (1 - e-A').
A, is the activity of A:' is the activity expected to be observed a t secular equilibrium, and il is the disintegration constant of An activity of 2.44 & 0.73 counts/min per 10.00 g of coral ash is obtained as the average activity for 'JOY a t the time of secular equilibrium. This is accompanied by a n average constant activity of 1.50 & 0.70 counts/min per 10.0 g of ash. The curve in Fig. 1 has been constructed using the two calculated results to arrive at expected total activities. T h e data are somewhat weighted in favour of the last observations, but, since the standard deviation over all observations is 0.40 counts/min, it is felt that the two calculated activities reliably represent the observed data. The average total activity expected a t secular equilibrium is then 3.94 f 1.O 1 countslmin. This result is in good agreement with a n average activity of 3.2 f 1.7 countslmin per 10 g ash observed for six other samples from living coral and coral sand which have been processed through the same separation with slightly inequivalent intermediate handling and have been counted some 4 to 6 months after original isolation and mounting. The average constant activity for the M. cavernosa extractions agrees with the 9OSr activity which is predicted by the average 9OY secular activity. The estimated detection efficiency factor of the counting machine for 9OY is 0.5 & 0.1 which suggests a total or 9OSr activity in 4n- geometry of 5.0 & 1.8 dis/min originating in the carbonate precipitates in secular equilibrium. This number coupled with factor of 0.30 f 0.15 to account for detection efficiency and precipitate selfabsorption of the weak 90Sr beta (Emax= 0.546 MeV) leads to a n activity of 1.5 f 0.9 countslmin expected to be observed for that nuclide in the sample. This activity is in good agreement with the average observed constant activity of 1.5 & 0.7 counts/min per 10.00 g of coral ash.
696
OCCURRENCE OF STRONTIUM-90 I N LIVING CORAL
Results obtained from yttrium milking (MAR1956) of the six M. cavemasa strontium extractions after a standard 14-day ingrowth period also generally are in agreement with the BOY activity originally observed. Activities of the two milkings which have been counted within 27.5 hr of yttrium separation average 2.04 -f 0.76 countslmin and decay with time. Each activity has been least squares fit to a log decay; the mean half-life of the two decays is 71.7 f 27.6 hr which is in reasonable agreement with the 64-hr half-life of OoY. Remilks of the other four samples also display an appropriate decay. These results therefore further support the value of 2.4 & 0.7 counts/ min SOY activity observed from the alkaline earth carbonate precipitates recovered from 10.0 g of ash. I n view of the above data, it is concluded that a n average gOSractivity of 5 f 2 dis/min has been extracted from 10.0 g of M. cavernosa ash. If this ash were presumed to consist of roughly 100% calcium oxide, its 90Sr activity of 5 dislmin would correspond to a n activity of roughly 25 dislmin per 100 1. for the same amount of calcium extracted from seawater, based on a calcium concentration of 400 mg/l (NAS 1971). This would represent a n activity greater by a factor of approximately two to three than the average of the 1967 activities reported for Gulf of Mexico water by VDOVENKO et al. (1 971, 1974). The OOSractivity incorporated into coral, however, is a reflection both of the concentration and discrimination factors (POLIKARPOV, 1966) of the organism in its surrounding seawater and of the seawater activity level during the period of the coral’s growth and surface exposure to the water. Such factors have not been determined, but it is believed that the coral which has been sampled for this report had experienced most of its growth during the decade prior to its collection in 1972. It is interesting to note that the observed 9OSr activity for this coral can be considered to be of the same order of magnitude as the 1967 Gulf of Mexico seawater activities (VDOVENKO et al., 1971, 1973). An activity of 5 2 dislmin is equivalent to 0.22 -f 0.09 pCi/g of ash, or to 0.1 1 f 0.05 pCi/g of wet coral. Although this level of activity is low, its presence indicates that the TELL,
fission product OOSr has been incorporated into the biota of an underwater coral reef in a relatively isolated location. The relatively constant salinity of the water and the reef’s apparent isolation from major currents indicates that such water has been a n average representative of seawater along the outer continental shelf of the Texas Gulf Coast. Accretion of OOSr by West Flower Garden Bank coral therefore reflects a concomitant activity level of the nuclide throughout this region. I n view of the likelihood of increased run-off of fission products as the number of nuclear reactor sites utilized for energy production in the continental United States increases (Anon, 1974), it seems that information concerning the present activity of these products in the Gulf of Mexico may represent baseline data. The 9OSr activities reported here suggest that such studies in Gulf of Mexico coral be continued and expanded in order to establish more precisely the baseline for these alkaline earth accreting organisms relative to the surrounding seawater. I n addition, studies of the aquatic activity levels of fission products should be conducted at various other sites and depths in the Gulf and a t the mouths of important estuaries which flow into the Gulf so that sources of the contaminating radioactivity and its distribution pattern throughout the Gulf of Mexico may likewise be established. Acknowledgements-This work has been supported by the Robert A. Welch Foundatiori. We very gratefully acknowledge the assistance and suggestions of Dr. R. S. CLARK and the cooperation of members of the Health Physics group of the Lyndon B. Johnson Manned Spacecraft Center of the National Aeronautics and Space Administration, where the beta counting was performed. We are indebted to Mr. G. MONTIN and Mr. J. 0. COVINCTON of the Flower Gardens Ocean Research Center of the Marine Biomedical Institute for useful information concerning the coral and the nature of the seawater at the reef and are indebted to all FGORC members who participated in the collection of the coral. We also wish to thank Dr. F. H. TULEY for assistance during the initial phases of this investigation. REFERENCES
Anonymous, 1974, Nuc. Znd. 21, 22. BOWENV. T., NOSHKIN V. E. and SUCIIIARA T. T., 1968, USAEC HASL-197, 1-2.
L. L. GRIFFIN, J. K. MYERS, D. C. CARLYLE and A. D. SUTTLE, JR. BOWENV. T., NOSHKINV. E., VOLCHOK H. L. and T. T., 1969, Science, N.Y. 169, 825. SUGAHARA BOWEN V. T. and NOSHKIN V. E., 1970, USAEC HASL-217, 1-1 19. DUURSMA E. K., 1974, in Oceanogr. Mar. Biol. Ann. Rev. 10, (Edited by BARNESH.) (New York: Hafner) EDWARDS G. S., 1971, Geology of the West Flower Garden Bunk (College Station, Texas: Texas A & M University Publication TAMU-SG-71-215). J. W. and MILLER J. M., FRIEDLANDER G., KENNEDY 1964, Nuclear and Radiochemistry (2nd Ed.) (New York: Wiley). GREGORY L. P., 1961, Analyt. Chem. 33,971. KURODA P. K., 1973, USAEC ORO-2529-36. MARTELL E. A., 1956, The Chicago Sunshine Method, AECU-3262 (Oak Ridge, Tenn.: USAEC Technical Information Service). National Academy of Sciences, (NAS), 1971,
.
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Radioactivity in the Marine Environment (Washington: National Academy of Sciences Publishing Office). J. D., NOYCEJ. R., MOORE0. T., SHERWOOD P. K., 1973, DANIEL P. R., BECKJ. N. and KURODA Health Phys. 25, 109. ODUM H. T., 1951, Publs Inst. mar. Sci. Uniu. Tex. 4,38. POLIKARPOV G. C., 1966, Radioecology of Aquatic Organisms (New York: Reinhold). VDOVENKO V. M., KOLESNIKOV A. G., SPITSYN V. I., BERNOVSKAYA R. N., GEDEONOV L. I., GROMOV V. V., IVANOVA L. M., NELEPO B. A., TIKHIMOROV V. N. and TRUSOV A. G., Atomn. energ. 31, 409 (available in translation from Washington: Plenum Publishing Corp.). VDOVENKO V. M., GEDEONOV L. I., GRITCHENKO A. G., IVANOVA L. M., KOLESNIKOV A. G. and NELEPOB. A., 1973, Proc. Symp. Hydrogeochem. Biogeochem Vol. I-Hydrogeochemistry, p. 337 (Washington: Clarke Co.).
OCCURRENCE OF STRONTIUM-90 IN LIVING CORAL FROM THE NORTHERN GULF OF MEXICO LAWRENCE L. GRIFFIN, JERRY K. MYERS, DAVID C. CARLYLE and ANDREW D. SUTTLE, JR. The Marine Biomedical Institute of the University of Texas Medical Branch at Galveston,
200 University Boulevard, Galveston, Texas 77550 (Received 12 Jub 1974; accepted 13 September 1974)
Abstract-Strontium-90 has been extracted from Montastna cauernosa coral which had been living on the West Flower Garden Bank submerged reef (lat 27'52.6' N, long 93'49.0' W) in the northern Gulf of Mexico in 1972. The calculated gOSractivity level in the sampled coral is 0.5 f 0.2 dislminlg of coral ash or 0.1 1 f 0.05 pCi/g wet coral. This level is larger than, but the same order of magnitude as, 90Sr radioactivity levels found in 1967 seawater samples from deeper locations in the Gulf of Mexico. It is thought that the composition of seawater in the Flower Garden vicinity is representative of water along the outer continental shelf of the Texas Gulf Coast, so that the existence of 90Sr in coral from the FIower Garden indicates a correlative low level presence of that nuclide in such water. INTRODUCTION
1967 the level of in waters of the Gulf of Mexico was generally the same order of magnibodies of seawater (DUURSMA, 1973) has ex- tude as the very lowest levels observed in seaperienced some contamination by fission product water from numerous sites in the Atlantic and radioisotopes. VDOVENKO et al. (1971, 1973) Pacific Oceans. I n addition the concentration appears to have been relatively constant have found strontium-90 at low levels in sea- of with depth to 1000 m in parts of the Gulf. The water samples collected in 1967, and results of et aZ. were collected the present study show that this nuclide has samples of VDOVENKO been accumulated in submerged coral growing approx 4 yr after cessation of large scale nuclear near the edge of the continentaI shelf. There weapons testing, and it is expected that the has been widespread atmospheric distribution concentration of 9%- should have decreased of the fission products of nuclear bomb testing somewhat since 1967 or may have remained (KURODA,1973; NOYCEet al., 1973) with a relatively constant. A slight decrease would be corresponding deposition of radionuclides to due to decay of the 28-yr half-life isotope and terrestrial and aquatic surfaces. The distribu- to its having been carried away by waters of the tion of OOSrdeposited in the Atlantic Ocean has Gulf Stream, a proposition which VDOVENKO in particular been well studied (BOWENet al., et al. (1971, 1973) consider to be suggested by 1968, 1969). Atmospheric deposition, land their depth studies on the Gulf Stream vicinity. run-off of bomb and nuclear reactor products Since fallout products of the recent Chinese and (POLIKARPOV,1966; BOWENand NOSHKIN,French tests have been distributed into the 1970) and the possible delivery of non-Gulf Northern hemisphere (KURODA,1973; NOYCE water by oceanic currents which flow into the et al., 1973), it also is likely that a continued Gulf of Mexico (see NAS 1971, Chap. 4) delivery into the Gulf of Mexico of 90Sr from represent potential sources of entry of artificial these and from fission reactor products et al. 1971, 1973; POLIKARPOV, radionuclides into the ecosphere of this body of (VDOVENKO, 1966) has been maintained. water. Very few studies of artificial radionuclide WEST FLOWER GARDEN BANK levels in the Gulf of Mexico have been reported The West Flower Garden Bank (lat 27'52.6' (DUURSMA, 1974; NAS, 1971). However, the results presented by VDOVENKO et al. (1971, N, long 93'49.0'W), a living member of the 1973) indicate that during the winter of 1966- West Indian coral reef community (EDWARDS,
THEGULF of Mexico like other non-oceanic
693
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OCCURRENCE OF STRONTIUM-90 IN LIVING CORAL
strontium from bone ash. Coral specimens are ashed a t 900°C for 4-6 hr and are ground to a powder. This ground ash is presumed to be roughly equivalent to bone which has been ashed a t 900°C and is processed through two precipitations from 70% nitric acid with nonradioactive strontium carrier added during each precipitation. For 10 g of coral ash, the 70% nitric acid volume utilized for the first separation is 400 ml, followed by 200 ml during the second separation. I n each instance the precipitate is collected by centrifugation. T h e second precipitate is dried to expel nitric acid, dissolved in water, and filtered to remove silicates; 10 % sodium carbonate solution is added to precipitate alkaline earth carbonates from the filtrate. The carbonate precipitate is collected by vacuum filtration, dried and mounted for beta counting on a stainless steel planchet. The beta counting of these samples has been performed on a Beckman Low Beta detector. This argon-methane proportional counter is lead-shielded with anti-coincidence and an automatic sample changer; it maintains a relatively constant background count rate of 2 counts/min. With several of the samples, counting has been initiated within a few days after separation of strontium, and 100 minute counting intervals have been continued for a week or more to demonstrate ingrowth of the daughter isotope (half-life 64 hr). The harder beta emissions of this isotope (Emax= 2.2 MeV) are detected in increasing number as the sample approaches -OoY secular equilibrium. Satisfactory consistency has been maintained METHODS in both radiological and chemical yields from Initial efforts aimed a t separating strontium samples processed from identical sources with from the calcium of one hundred or more grams equivalent handling and equal amounts of of coral employing 70 % HNO, precipitation added strontium carrier, although a t present the followed by ion exchange (similar to GREGORY, quantity of non-radioactive strontium accreted 1961) have been set aside. T h e present extrac- by the examined coral and the relative tion procedure employs fuming nitric acid and amount of calcium remaining in the final water to effect precipitations of the nitrates of precipitate have not been determined. The samples of ten or more grams of ashed coral dissolved nitrates of the second precipitation residue. The ash is suspended in water, and flame test strongly positive for the presence of fuming nitric acid is added to produce a strontium. I t appears that counting results 70% HNO, concentration. This method is a n recorded after these separations represent the adaptation of the Harley Method as reported activity of better than 90 % of the total strontium by MARTELL(1956) for separation of radio- (MARTELL,1956).
1971), lies 204 km south-southeast of Galveston, 'Texas and 18.5 km from the edge of the continental shelf, in a region removed from the Gulf Stream. Seawater in this region experiences minimal salinity variation (EDWARDS, 1971). The reef is composed of a variety of common coral species and is submerged at depths from 21 to 61 m. Currents at the depths of the reef are variable both with depth and time, although the prevailing current seems to be from the south-southeast with fluctuating velocities from zero to one knot (EDWARDS, 1971). Since several species of coral have been shown to accrete strontium a t a slightly higher level relative to calcium than the Sr/Ca ratio observed in seawater (ODUM,1951), it seems likely that coral growing on this reef in the years since the onset of man-made nuclear fission reactions should have incorporated skeletal 90Sr in detectable amounts. In order to determine OOSrlevels in such coral, a series of radiochemical extractions of strontium from coral of the West Flower Garden Bank have been initiated a t the Marinc Biomedical Institute in Galveston. These studies are part of a broader investigation of the distribution of longer-lived radionuclides in the northern Gulf of Mexico and of their incorporation into its marine environment. The coral used for these studies was collected by Institute divers during an expedition in the late summer of 1972. Large blocks of both living and dead coral were collccted a t depths of roughly 25 m and were transported to the laboratory for radiochemical analysis.
L. L. GRIFFIN, J. K. MYERS, D. C. CARLYLE and A. D. SUTTLE, JR. RESULTS AND DISCUSSION The results reported here are from extractions performed on six 10.0-g samples of mixed ash of a small protrusion of Montastrea cavernosa (EDWARDS, 1971) growing as part of a larger colony of the same species at the surface of one of the coral blocks a t time of recovery from the reef. Figure 1 shows the observed count rates above a background of 2.18 f 0.1 1 counts/min, plotted semilogarithmically with time for each of the six samples. T h e rate of increase of these counts is roughly consistent with the expected rate of ingrowth of Two of the samples were counted relatively soon after separation, and the other four somewhat later. I n order to determine the amount of ingrowth, the first two observations of each one of the youngest pair of samples have been averaged and this average over four observations is presumed to be the count rate after 200min counting. Likewise, observations for the older samples have been averaged over their last four counting intervals. The two average observations are
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FIG. 1. A semilogarithmic plot with time of observed beta counting activity above background. T = 0 is the time of initial separation of strontium carbonate extracted from each of the 6 10.0-g portions of M . cavernosa ash. x and o represent the two earlier counted samples, and the solid symbols, the four older ones. The curve depicts an average activity calculated from the early and late observations.
695
then considered in terms of the two average expected per cent ingrowths of activity. I t is assumed that the observed activities are the sum of a constant activity and a n ingrowth activity, where expected ingrowths a t time t a r e calculated using the relation (FRIEDLANDER et al., 1965) ~,(t= ) AY (1 - e-A').
A, is the activity of A:' is the activity expected to be observed a t secular equilibrium, and il is the disintegration constant of An activity of 2.44 & 0.73 counts/min per 10.00 g of coral ash is obtained as the average activity for 'JOY a t the time of secular equilibrium. This is accompanied by a n average constant activity of 1.50 & 0.70 counts/min per 10.0 g of ash. The curve in Fig. 1 has been constructed using the two calculated results to arrive at expected total activities. T h e data are somewhat weighted in favour of the last observations, but, since the standard deviation over all observations is 0.40 counts/min, it is felt that the two calculated activities reliably represent the observed data. The average total activity expected a t secular equilibrium is then 3.94 f 1.O 1 countslmin. This result is in good agreement with a n average activity of 3.2 f 1.7 countslmin per 10 g ash observed for six other samples from living coral and coral sand which have been processed through the same separation with slightly inequivalent intermediate handling and have been counted some 4 to 6 months after original isolation and mounting. The average constant activity for the M. cavernosa extractions agrees with the 9OSr activity which is predicted by the average 9OY secular activity. The estimated detection efficiency factor of the counting machine for 9OY is 0.5 & 0.1 which suggests a total or 9OSr activity in 4n- geometry of 5.0 & 1.8 dis/min originating in the carbonate precipitates in secular equilibrium. This number coupled with factor of 0.30 f 0.15 to account for detection efficiency and precipitate selfabsorption of the weak 90Sr beta (Emax= 0.546 MeV) leads to a n activity of 1.5 f 0.9 countslmin expected to be observed for that nuclide in the sample. This activity is in good agreement with the average observed constant activity of 1.5 & 0.7 counts/min per 10.00 g of coral ash.
696
OCCURRENCE OF STRONTIUM-90 I N LIVING CORAL
Results obtained from yttrium milking (MAR1956) of the six M. cavemasa strontium extractions after a standard 14-day ingrowth period also generally are in agreement with the BOY activity originally observed. Activities of the two milkings which have been counted within 27.5 hr of yttrium separation average 2.04 -f 0.76 countslmin and decay with time. Each activity has been least squares fit to a log decay; the mean half-life of the two decays is 71.7 f 27.6 hr which is in reasonable agreement with the 64-hr half-life of OoY. Remilks of the other four samples also display an appropriate decay. These results therefore further support the value of 2.4 & 0.7 counts/ min SOY activity observed from the alkaline earth carbonate precipitates recovered from 10.0 g of ash. I n view of the above data, it is concluded that a n average gOSractivity of 5 f 2 dis/min has been extracted from 10.0 g of M. cavernosa ash. If this ash were presumed to consist of roughly 100% calcium oxide, its 90Sr activity of 5 dislmin would correspond to a n activity of roughly 25 dislmin per 100 1. for the same amount of calcium extracted from seawater, based on a calcium concentration of 400 mg/l (NAS 1971). This would represent a n activity greater by a factor of approximately two to three than the average of the 1967 activities reported for Gulf of Mexico water by VDOVENKO et al. (1 971, 1974). The OOSractivity incorporated into coral, however, is a reflection both of the concentration and discrimination factors (POLIKARPOV, 1966) of the organism in its surrounding seawater and of the seawater activity level during the period of the coral’s growth and surface exposure to the water. Such factors have not been determined, but it is believed that the coral which has been sampled for this report had experienced most of its growth during the decade prior to its collection in 1972. It is interesting to note that the observed 9OSr activity for this coral can be considered to be of the same order of magnitude as the 1967 Gulf of Mexico seawater activities (VDOVENKO et al., 1971, 1973). An activity of 5 2 dislmin is equivalent to 0.22 -f 0.09 pCi/g of ash, or to 0.1 1 f 0.05 pCi/g of wet coral. Although this level of activity is low, its presence indicates that the TELL,
fission product OOSr has been incorporated into the biota of an underwater coral reef in a relatively isolated location. The relatively constant salinity of the water and the reef’s apparent isolation from major currents indicates that such water has been a n average representative of seawater along the outer continental shelf of the Texas Gulf Coast. Accretion of OOSr by West Flower Garden Bank coral therefore reflects a concomitant activity level of the nuclide throughout this region. I n view of the likelihood of increased run-off of fission products as the number of nuclear reactor sites utilized for energy production in the continental United States increases (Anon, 1974), it seems that information concerning the present activity of these products in the Gulf of Mexico may represent baseline data. The 9OSr activities reported here suggest that such studies in Gulf of Mexico coral be continued and expanded in order to establish more precisely the baseline for these alkaline earth accreting organisms relative to the surrounding seawater. I n addition, studies of the aquatic activity levels of fission products should be conducted at various other sites and depths in the Gulf and a t the mouths of important estuaries which flow into the Gulf so that sources of the contaminating radioactivity and its distribution pattern throughout the Gulf of Mexico may likewise be established. Acknowledgements-This work has been supported by the Robert A. Welch Foundatiori. We very gratefully acknowledge the assistance and suggestions of Dr. R. S. CLARK and the cooperation of members of the Health Physics group of the Lyndon B. Johnson Manned Spacecraft Center of the National Aeronautics and Space Administration, where the beta counting was performed. We are indebted to Mr. G. MONTIN and Mr. J. 0. COVINCTON of the Flower Gardens Ocean Research Center of the Marine Biomedical Institute for useful information concerning the coral and the nature of the seawater at the reef and are indebted to all FGORC members who participated in the collection of the coral. We also wish to thank Dr. F. H. TULEY for assistance during the initial phases of this investigation. REFERENCES
Anonymous, 1974, Nuc. Znd. 21, 22. BOWENV. T., NOSHKIN V. E. and SUCIIIARA T. T., 1968, USAEC HASL-197, 1-2.
L. L. GRIFFIN, J. K. MYERS, D. C. CARLYLE and A. D. SUTTLE, JR. BOWENV. T., NOSHKINV. E., VOLCHOK H. L. and T. T., 1969, Science, N.Y. 169, 825. SUGAHARA BOWEN V. T. and NOSHKIN V. E., 1970, USAEC HASL-217, 1-1 19. DUURSMA E. K., 1974, in Oceanogr. Mar. Biol. Ann. Rev. 10, (Edited by BARNESH.) (New York: Hafner) EDWARDS G. S., 1971, Geology of the West Flower Garden Bunk (College Station, Texas: Texas A & M University Publication TAMU-SG-71-215). J. W. and MILLER J. M., FRIEDLANDER G., KENNEDY 1964, Nuclear and Radiochemistry (2nd Ed.) (New York: Wiley). GREGORY L. P., 1961, Analyt. Chem. 33,971. KURODA P. K., 1973, USAEC ORO-2529-36. MARTELL E. A., 1956, The Chicago Sunshine Method, AECU-3262 (Oak Ridge, Tenn.: USAEC Technical Information Service). National Academy of Sciences, (NAS), 1971,
.
697
Radioactivity in the Marine Environment (Washington: National Academy of Sciences Publishing Office). J. D., NOYCEJ. R., MOORE0. T., SHERWOOD P. K., 1973, DANIEL P. R., BECKJ. N. and KURODA Health Phys. 25, 109. ODUM H. T., 1951, Publs Inst. mar. Sci. Uniu. Tex. 4,38. POLIKARPOV G. C., 1966, Radioecology of Aquatic Organisms (New York: Reinhold). VDOVENKO V. M., KOLESNIKOV A. G., SPITSYN V. I., BERNOVSKAYA R. N., GEDEONOV L. I., GROMOV V. V., IVANOVA L. M., NELEPO B. A., TIKHIMOROV V. N. and TRUSOV A. G., Atomn. energ. 31, 409 (available in translation from Washington: Plenum Publishing Corp.). VDOVENKO V. M., GEDEONOV L. I., GRITCHENKO A. G., IVANOVA L. M., KOLESNIKOV A. G. and NELEPOB. A., 1973, Proc. Symp. Hydrogeochem. Biogeochem Vol. I-Hydrogeochemistry, p. 337 (Washington: Clarke Co.).
