Radionuclides: A Tool for Oceanography
Radionuclides: A Tool for Oceanography
Edited ByP. Geugueniat, J.C. Guary, R.J. Pentreath
Edition 1st Edition
First Published 1989
eBook Published 2 September 2003
Pub. location London
Pages 484 pages
eBook ISBN 9780429080838
SubjectsEnvironment & Agriculture
Geugueniat, P. (Ed.), Guary, J. (Ed.), Pentreath, R. (Ed.). (1989). Radionuclides: A Tool for Oceanography. London: Routledge, https://doi.org/10.1201/9781482286489
Proceedings of an International Symposium jointly organized by the 'Societe Francaise pour L'Energie Nucleaire' SFEN and 'L'Institut National des Techs. de la Mer' INTECHMER - CNAM at Cherbourg, France, 1-5 June 1987.
TABLE OF CONTENTS
ByR. j Pentreath
DETERMINATION OF CARBON-14 BY ACCELERATOR MASS-SPECTROMETRY: OCEANOGRAPHIC APPLICATIONS. J. C. Duplessy, E. Bard, M. Arnold and P. Maurice Centre des Faibles Radioactivites Laboratoire mixte CNRS-CEA 91198 Gif-sur-Yvette Cedex FRANCE ABSTRACT Accelerator mass-spectrometry (AMS) enables the determination of 14C/12C ratios with only one mg of carbon, i.e. with samples 1000 times smaller than those required for conventional radiocarbon g-counting. For sediment dating, radiocarbon ages can thus be measured on monospecific samples of 1000-2000 foraminifera picked by hand. This enables an age to be assigned to the same sample which is to be used in the oxygen isotope measurement or to the dominant species which drive sea surface paleotemperature estimates. For studying the oceanic circulation within the main thermocline and the carbon cycle, 14C measurements can be obtained with water samples of only 100 ml, without major chemical treatment on board. INTRODUCTION Among cosmonuclides, 14 C with its half-life of 5,730 years has been widely-used for sediment dating and to explore the global ocean circulation in relation with the oceanic part of the carbon cycle. Until recent years, the 14C/12C ratio was only measured by the conventional g-counting method, which provides an accurracy up to 4 per mil but requires samples as large as 1-5g of carbon. This relatively large size of the samples was the cause of several difficulties in either sample collection or in interpretation of the geochemical significance of the analytical result. The development of AMS in recent years permits now to characterise 14C atoms by their mass and to count the 14C/ 1-7C ratio in samples as small as 1 mg of carbon with an accuracy of 1% for modern activities. The accuracy of this measurement should be improved in the near future. This dramatic reduction in the size of the samples required to make one analysis results in a burst of new applications. We shall describe in this paper some major advances in oceanography connected with the determination of 14 C in sediment and water samples using the Tandetron of Gif sur Yvette, a small tandem accelerator devoted to the measurement of cosmonuclides for geochemical studies in Earth's sciences. The routine procedure for AMS measurements has been described by Arnold et al. (1).
DEGLACIAL WARMING OF THE NORTH ATLANTIC Over the last million years, the Earth's climate experienced major changes, with alternation of glacial and interglacial periods. These climatic variations are recorded in the variations of the fossil planktonic foraminiferal shells, which accumulate on the sea floor : On the one hand, as the composition of the fauna in ocean surface water depends mainly on the temperature, the composition of the fossil fauna in a deep sea core may be used to estimate the past sea surface temperature (2). On the other hand, continental ice-volume changes may be reconstructed by oxygen isotopic analysis of the same foraminiferal shells (3). The timing of the deglaciation has important consequences relating to mechanisms of climate change. However, details of the climatic evolution of the ocean and of the continental ice-volume during the last glacial to interglacial transition are not recorded in most sediment cores because of the short duration of this event. Althrough the sediment material is deposited at a rate on the order of a few centimeters per thousand years, a major problem encountered when attempting to record such a rapid climatic change in deep sea records is created by the natural process of bioturbation: The activity of benthic organisms disturbs the original stratification of the deposited shells and mixes the upper few centimeters of sediment. As a result, the variations of the paleontological and isotopic compositions in the sedimentological record are both smoothed and disturbed. Radiocarbon ages can be measured by A. M. S. on monospecific samples of 1,000-2,000 foraminifera picked by hand. This technique offers several advantages: First, the foraminiferal samples can be much more pure than those used for the classical method of 8-counting, which required samples so large that usually the carbonate fraction larger than 60 µ was analyzed. The presence within the North Atlantic sediment of ice-rafted carbonate, or of wind-, river- and current-transported carbonates (derived from old continental rocks), biased classical 14C ages towards older values. Second, as stable isotopes are also measured on monospecific samples, this enables an age to be assigned to the same sample which is to be used in the a180 measurement (4). A simple deconvolution model can then be used to date precisely the internal stages of the deglaciation and to measure the volocity of the climatic changes in the North Atlantic Ocean (5). Fig. 1 displays the oxygen isotope record measured in the foraminifer Globigerina bulloides, the 14 C ages measured in the same species and the sea surface temperature (S.S.T.) estimates in core SU 81-18 (37°46' N, 10°11' W). The sedimentation rate during the deglaciation is so high in this core (35 cm/kyr), that the bioturbation smoothing is negligible. The deglaciation (defined on the isotopic record) began about 14,500 years ago. The melting of continental ice first resulted in aS.S.T. drop of 6°C for both summer and winter S.S.T. The end of the first phase of the deglaciation, after 12,500 ± 150 B.P., was marked by a sharp temperature rise with a mean rate of roughly 4°C per century. This warm phase was followed by a dramatic cooling, well-known as Younger Dryas cold event (from 11,000 ± 170 B.P. to 10,400 ± 130 B.P.). The readvance of the cold surface water corresponds to a temperature drop of 0,5 to 1°C per century. The final S.S.T. warming, which led to modern conditions, was less abrupt than the first one with a mean rate of temperature rise close to 1°C per century until 9,360 ± 130 B.P.. Fig 2A displays the oxygen isotope records of both G. bulloides and N. pachyderma (left coiling) in core CH 73-139 C (54°38' N, 16°21' W). The interpretation of the isotopic date is more difficult than in core SU 81-18, because the bioturbational disturbance is not negligible. For example, the D180 values of G.bulloides exhibit
their first decrease at a depth 30 cm below that of N. pachyderma.. A.M.S. 14C dates demonstrate that this 30 cm lead of the G. bulloides record over the N. pachyderma record is an artifact of bioturbation. The inconsistancy between the two isotopic records roughly disappears when both records are deconvolved and plotted with respect to the '4 C ages measured on the same foraminiferal species (Fig. 2B). This time scale places the beginning of the deglaciation between 15,000 yr B.P. and 14,500 yr B.P. and basically fits that of core SU 81-18. The abundances of fourteen foraminiferal species have been deconvolved with the same bioturbation parameters which successfully explain the discrepancies between the isotopic records. The transfer function has been applied to these restored foraminiferal data in order to generate an unmixed S.S.T. record (Fig. 2C), which exhibits the same trends as the raw S.S.T. estimates but emphasizes the large amplitude of the temperature changes betwen 15,000 yr B.P. and 10,000 yr B.P.. The comparison of the paleoclimatic records of core CH 73-139 C and SU 81-18 may be used to estimate the rate of climatic change and the kinematics of the retreat of the cold water mass in the high latitude North Atlantic. The polar front is defined as the steep thermal gradient which separate the warm Gulf Stream waters from the cold polar water mass. The CLIMAP reconstruction (6) shows that core SU 81-18 is located in the southern part of the polar front during the last glacial maximum. Assuming that the passage of the polar front across the core will constitute the dominant thermal event within the general deglacial warming, A.M.S. 14C ages of G. bulloides indicate that its retreat from the latitude of Portugal is dated at about 12,500 yr B.P. (7). A.M.S. 14 C ages measured on G. bulloides between the levels 130 and 160 cm in core CH 73-139 C demonstrate that those levels correspond in fact to the dating of a single "pulse-like" abundance maximum for this species, of which shells have been bioturbated between 130 and 160 cm. The four radiocarbon ages are not statistically different and correspond to a mean value of 11,540 ± 170 yr B.P., which dates the invasion of warmer waters in the North Atlantic at 55°N. We therefore assume that this event marks the passage of the polar front at the location of core CH-73-139 C. We thus conclude that the North Atlantic polar front began its retreat at about 12,500 yr B.P. at the latitude of Portugal and reached the latitude of Ireland almost 1,000 years later, retreating with a mean velocity of about 2 km/year. Recently however, Broecker (pers. comm.) measured by A.M.S. an age of about 12,500 yr B.P. for the first warming phase in a core close to core CH 73-139 C. This discrepancy with our estimate may be due either to an artifact of bioturbation in Broecker's core or to a small sedimentation hiatus in core CH 73-139 C. The retreat velocity of the polar front that we determined is therefore a minimal value. The cooling associated with the Younger Dryas cold event and the warming which followed have been synchroneous (± 400 years) between 37°N and 55°N, since we do not observe any significant age difference for the temperature variations recorded in both cores. This favours theories explaining the Younger Dryas by instabilities within the climatic system itself, e.g. a catastrophic influx of polar ice into the North Atlantic. The high velocity of the advance and the retreat of cold waters suggests that these temperature changes had affected only the most surficial waters and not the whole hydrologic structure of the North Atlantic ocean. PENETRATION OF BOMB RADIOCARBON IN THE OCEAN Under natural conditions, the distribution of 14 C in the deep ocean is influenced by many processes. Bottom water formation in the Norwegian Sea and the Weddell
Sea provides a direct input of surface-water 14C . Additional input of 14C to the deep sea occurs by transport along isopycnal surfaces, by lateral and vertical mixing in the main thermocline of the ocean, by dissolution of carbonate skeletons, and by oxidation of organic material from sinking particles. As compared to other oceanographic markers, 14C adds a measure of time because it decays at the rate of 1% every 80 years. As a consequence, the distribution of 14C in the abyssal waters of the world ocean has been used to estimate the replacement times for Pacific, Indian and Atlantic ocean deep waters (8). The steady state of the carbon cycle was broken in the late fifties and in the sixties with the large scale testing of nuclear weapons. The 14C activity of the atmosphere in the northern hemisphere increased by about 100% and has been decreasing since the end of the nuclear tests in the atmosphere. This atmospheric 14C activity decrease is due to the homogeneization of the troposphere and to the penetration of CO2 into other carbon reservoirs, mainly the ocean and the terrestrial biosphere. The bomb produced 14C is therefore a good tracer to quantify the oceanic invasion by the atmospheric CO2 excess resulting from human activities. The conventional radiocarbon 13-counting requires the sampling of about 250 litres of sea water and chemical extraction of CO2 on board the oceanographic vessel. The same 14 C measurements can be done with 100 ml water samples poisoned with 1 ml of saturated HgC1 2 solution (9). This operation is designed to supress isotope fractionation due to respiration and photosynthesis of microorganisms, which might affect the 13C and 14 C content of the water sample. In the laboratory, the total dissolved CO2 is extracted from the sea water in a vacuum line in which the 100 ml aliquot of sea water is acidified and flushed with pure helium gas. The evolving CO 2 is trapped in liquid nitrogen and the target is prepared by catalytic reduction of this CO2 on iron powder (10). As an example of the potential of measuring oceanic 14 C by A.M.S., we present here three profiles performed in the Indian Ocean during the INDIGO-2 cruise on board of the M/S Marion Dufresne. The radiocarbon activities (Fig. 3) are expressed by means of the 6. 14 C scale (11). Stations 32 and 45 have the same locations as stations 424 and 420 of the GEOSECS expedition (12) and therefore will enable a comparison of the two sets of 6. 14C data. The three profiles exhibit the presence of bomb 14 C in the upper part of the water column, until a depth of about 1000 m where 0 14 C values are equal to the GEOSECS values and represent the so-called pre-bomb level. Following Broecker et al. (13), we have estimated the water column inventories of bomb produced 14C by integrating the area between the observed A 14 C curve and the reconstructed 6. 14C curve versus depth for pre-nuclear time. At station 28, the INDIGO integrated amount of 14C ( 1140 is 15.9 109 atoms/cm 2 , in agreement with GEOSECS data obtained at the same latitude but not at the same location (Fig. 4). At station 32, the INDIGO El lIC is 7.8 10 9 atoms/cm2 , identical with that measured at GEOSECS station 424 (114 = 7.7 10 9 atoms/cm 2 ) although the two profiles exhibit significant differences. By contrast, at equatorial station 45, the INDIGO 1 14C is 10.3 109 atoms/cm2 , twice the value measured at the same location during the GEOSECS expedition (1, 14C = 4.8 109 atoms/cm2. The column inventories for bomb radiocarbon at the stations performed during the GEOSECS expedition showed pronounced minimal values in the low latitudes (Fig. 4). Oceanic models have showed that a sizable fraction of the bomb-14C that entered the tropical ocean has been transported to the adjacent temperate zones and that the low-latitude deficit enables an equatorial upwelling component to be
calculated (13,14). The three INDIGO stations selected in the narrow belt between 0°S and 20°S showed a steep 114C gradient at the time of the GEOSECS survey (1978). At the time of the INDIGO cruise (1986), the 1 14C value at the equatorial station 45 has roughly doubled during the last 8 years. Salinity and dissolved oxygen profiles demonstrate that the most likely explanation for the reduction of the equatorial deficit in the Indian Ocean is the advection of thermocline water called Banda Sea Water flowing from the east. This low-salinity water, which is rich in dissolved oxygen and in Tritium (15), is due to a throughflow from the North Pacific into the Indian Ocean, which results in its relatively high 14C content. We thus conclude that A.M.S. can be used successfully to trace the penetration of bomb produced 14 C into the ocean. This new technique should leach to the introduction at 14 C measurements into the WOCE program devoted to the understanding of the dynamics of the global ocean. REFERENCES 1. Arnold M., Bard, E., Maurice, P., and Duplessy, J. C., 14C dating with the Gif sur Yvette Tandetron accelerator : Status report. Nuclear Instrument and Methods K1.987), P. 120-123. 2. Imbrie, J. and Kipp, N. G., A new micropaleontological method for quantitative paleoclimatology : Application to a late Pleistocene Carribean core. In the Late Cenozoic Glacial Ages, ed. K.K. Turrebian (1971), p. 71-181. 3. Shackleton, N. J. and Opdyke, N. D., Oxygen isotope and paleomagnetic strati-graphy of equatorial pacific core V 28-238: oxygen isotope temperatures and ice volume on a 105 and 106 year scale. Quaternary Res., 1973, 3, 39-55. 4. Duplessy, J. C., Arnold, M., Maurice, P., Bard, E., Duprat, J. and Moyes, J., Direct dating of the oxygen isotope record of the last deglaciation by 14C Accelerator mass spectrometry. Nature, 1986, 320, 350-352. 5. Bard, E., Arnold, M., Duprat, J., Moyes, J., and Duplessy, J. C., Reconstruction of the last deglaciation: deconvolved records of a18 0 profiles, micropaleon-tological variations and accelerator mass spectrometric 14C dating. Climate Dynamics, 1987,1, 101-112. 6. CLIMAP Project Members, Seasonal reconstructions of the Earth's surface at the last glacial maximum. Geol. Soc. Am. Map Series MC-36 (1981). 7. Duplessy, J. C., Bard, E., Arnold, M., and Maurice,P., A.M.S. 14 C- chronology of the deglacial warming of the North Atlantic Ocean. Nuclear Sciences and Methods, (1987), p. 223-227. 8. Stuiver, M., Quay, P. D., and Ostlund, H. G., Abyssal water carbon-14 distribution and the age of the world oceans. Science, 1983, 219 849-851. 9. Bard, E., Arnold, M., Maurice, P. and Duplessy, J.C., Measurements of bomb radiocarbon in the ocean by means of accelerator mass-spectrometry : Technical aspects. Nuclear Instrument and Methods (1987), 297-301.
10. Vogel, J. S., Southon, J. R., Nelson, D. E. and Brown, T. A., Performance of catalytically condensend carbon for use in accelerator mass spectrometry. Nuclear Instrument and Methods, 1984, 233 289-293. 11. Stuiver, M. and Pollach H. A., Discussion : Reporting of 14C data. Radiocarbon 1977,19 355-363. 12. Stuiver, M. and Ostlund H. G., GEOSECS Indian Ocean and Mediterranean Radiocarbon. Radiocarbon, 1983, 1-19. 13. Broecker, W. S., Peng, T. H., Ostlund, H. G. and Stuiver M., The distribution of bomb radiocarbon in the ocean. I. Geophys. Res., 1985, 90 (C4), 6953-6970. 14 Wunsch, C., An estimate of the upwelling rate in the Equatorial Atlantic based on the distribution of bomb radiocarbon and quasi-geostrophic dynamics. J. Geophys. Res., 1984,
7971-7978. 15. Fine, R. A., Direct evidence using tritium date for throughflow from the Pacific into the Indian Ocean. Nature, 1985, 315, 478480.
The two methods of estimating sediment accumulation rates agree extremely well, allowing us to use the constant Al flux model to calculate accumulation rate changes down core in 154-18. These apparent rates and the age of each sample horizon are shown in Figure 5. From Table 2 and Figure 4 it is apparent that the accumulation rates gradually increase towards the ridge crest, but in 154-18 achieve maximum rates of 20 cm/ky. Comparison with the other figures reveal that high accumulation rates result in lower calcite and Th(ex)-230 and Pa(ex)-231 contents, but higher Fe/Al (and Mn/A1) ratios. Therefore the hydrothermal pulses interpreted from Figure 2 have a duration of approximately 1000 years (Fig. 4) during the Holocene. RI (wp.x)
mass containing the dissolved nuclides from the east (16). At the ocean margin off Baja California the horizontal transport of Th-230 and Pa-231 along isopycnals (2,6) supplies the excess Th-230 and Pa-231 which is also removed with little fractionation by Mn cycling and by the high alumino-silicate flux. We would like to thank the Captain and crew of the R/V Thomas G. Thomson for their efforts, and Dr J.W. Murray for allowing us to participate on his cruises. Analytical expertise and stamina was provided by Frances Lindsay and Mike Saunders. GBS acknowledges NERC support on grant GR3/6175. Its 1. Anderson, R.F., Bacon, M.P. and Brewer, P.G., Removal of Th-230 and Pa-231 from the open ocean. Earth Planet. Sci. Lett., 1983a, 62. 7-23. 2. Anderson, R.F., Bacon, M.P. and Brewer, P.G., Removal of Th-230 and Pa-231 at ocean margins. Earth Planet. Sci. Lett., 1983b, 66. 73-90. 3. Shimmield, G.B., Murray, J.W., Thomson, J., Bacon, M.P., Anderson, R.F. and Price, N.B., The distribution and behaviour of Th-230 and Pa-231 at an ocean margin, Baja California, Mexico. Geochim. Cosmochim. Acta, 1986, 50. 2499-2507. 4. Yang, H-S., Nozaki, Y., Sakai, H. and Masuda, A., The distribution of Th-230 and Pa-231 in the deep-sea surface sediments of the Pacific Ocean. Geochim. Cosmochim. Acta, 1986, 50. 81-89. 5. Nozaki, Y., Horibe, Y. and Tsubota, H., The water column distributions of thorium isotopes in the western North Pacific. Earth Planet. Sci. Lett., 1981, 54. 203-216. 6. Nozaki, Y. and Nakanishi, T., Pa-231 and Th-230 profiles in the open ocean water column. Deep-Sea Res., 1985, 32. 1209-1220. 7. Kadko, D., A detailed study of some uranium series nuclides at an abyssal hill area near the East Pacific Rise at 8 °45'N. Earth Planet. Sci. Lett., 1980, 51. 115-131. 8. Mangini, A. and Sonntag, C., Pa-231 dating of deep-sea cores via Th-227 counting. Earth Planet. Sci. Lett., 1977, 37. 251-256. 9. Lyle, M.W. and Dymond J., Metal accumulation rates in the southeast Pacific - errors introduced from assumed bulk densities. Earth Planet. Sci. Lett., 1976, 30. 164-168. 10. V eeh, H., Depostion of uranium from the ocean. Earth Planet. Sci. Lett., 1967, 3. 145-150.
11. Rydell, H., Kraemur, T., Bostrom, K. and Joensuu, O., Post-depositional injections of uranium-rich solutions into East Pacific Rise sediments. Mar. Geol., 1974, 17. 151-164. 12. L alou, C. and Brichet, E., Anomalously high uranium contents in the sediment under Galapagos hydrothermal mounds. Nature, 1980, 284. 251-253. 13. S himmield, G.B. and Price, N.B., The behaviour of molybdenum and manganese during early sediment diagenesis - offshore Baja California, Mexico. Mar. Chem., 1986, 19. 261-280. 14. S awlan, J.J. and Murray, J.W., Trace metal remobilisation in the interstitial waters of red clay and hemipelagic sediments. Earth Planet. Sol. Lett., 1983, 64. 213-230. 15.L upton, J.E. and Craig, H., A major helium-3 source at 15 °S on the Past Pacific Rise. Science, 1981, 214. 13-18. 16.K linkhammer, G. and Hudson, A., Dispersal patterns for hydrothermal plumes in the South Pacific using manganese as a tracer. Earth Planet. Sci. Lett., 1986, 79. 241-249. 17.B ostrom, K. and Peterson, M.N.A., The origin of aluminium-poor ferromanganoan sediments in areas of high heat flow on the East Pacific Rise. Mar. Geol., 1969, 1. 427-447. 18. D ymond, J., Geochemistry of Nazca plate surface sediments: An evaluation of hydrothermal, biogenic, detrital and hydrogenous sources. Geol. Soc. Amer. Memoir, 1981, 154. 133-173. 19.K rishnaswami, S., Authigenic transition elements in Pacific pelagic clays. Geochim. Cosmochim. Acta, 1976, 40. 425-434. 20. K adko, D., Th-230, Ra-226 and Rn-222 in abyssal sediments. Earth Planet. Sci. Lett., 1980, 49. 360-380. 21. M oore, W.S., Ku, T.L., Macdougall, J.D., Burns, V.M., Burns, R., Dymond, J., Lyle, M.W. and Piper, D.Z., Fluxes of metals to a manganese nodule: radiochemical, chemical, structural, and mineralogical studies. Earth Planet. Sci. Lett., 1981, 32. 151-171. 22. R uhlin, D.E. and Owen, R.M., The rare earth element geochemistry of hydrothermal sediments from the East Pacific Rise: Examination of a seawater scavenging mechanism. Geochim. Cosmochim. Acta, 1986, 50. 393-400.
EFFECT OF NATURAL COLLOIDAL MATTER ON THE EQUILIBRIUM ADSORPTION OF THORIUM IN SEAWATER INTRODUCTION
Sherry E. H. Niven and Robert M. Moore Department of Oceanography Dalhousie University
of 24.1 days) allows liquid scintillation counting of its /3-decay at low molar concentra-tions (detection limit of 10-17 M). Consequently, in these experiments, the partitioning of 234 Th among the dissolved, colloidal, and particulate fractions could be measured in reasonable sample volumes (100 ml aliquots from 2 1 bottles) without increasing the natural concentration of Th (10 -15m 234 Th spikes were added to ultrafiltered coastal seawater with a 232Th concentration of approximately 10-13M). Experiments were carried out under constant natural conditions of pH, salinity, and temperature; and variable concentrations of dissolved organic matter (DOM), colloidal, and particulate matter. Two liter polyethylene bottles were acid cleaned and soaked with ultrafiltered, photo-oxidized seawater for one week prior to use. Replicate bottles were then filled with fresh ultrafiltered seawater (with known DOM concentration) and colloidal and/or particulate matter was added from stock suspensions. Suspensions were equilibrated for 24 hours at 4°C before being spiked with 234Th tracer. Acid stabilized Th stock solution (100 pl) was added to give initial sample activities of 2-4x 104dpm
Uncertainties in the results due to ultrafiltration and counting errors are 3% for the particulate fraction and 6% for the dissolved and colloidal fractions. Materials Coastal seawater from Dalhousie University's Aquatron system was photo-oxidized and ultrafiltered (nominal molecular weight cut-off of 1, 000) to prepare batch seawater free of organic material and particles (both colloids and > 0.2 pm). This seawater was used as a control and to prepare samples with specific concentrations of dissolved organic matter (DOM), colloids, and particles. Th was prepared by extraction from uranyl nitrate using a procedure based on
as a quench monitor. The activity of the 234Th quench standard was determined to 1.6% accuracy by counting its 63 and 93 KeV 7-rays on a Ge(Li)-detector calibrated with Amersham Mixed Standard 1000. Background for the liquid scintillation counter was 38 cpm. Exudates from batch cultures of the diatom as dissolved (< 1, 000 NMW) organic matter in the experiments.
ByPhaeodactylum tricornutum were used Phaeodactylum tricor-
(supplied courtesy of Cabot Corp., Boston, MA), which consists predominantly of 7-A1203 (16). Alon has previously been used as a model oxide surface in studies of adsorption of organic matter and some trace metals onto particle surfaces (16,17). Alon particles are reported to be nonporous and approximately spherical with a specific surface area of 120 m2 g-1 (17). Scanning electron microscopy showed that the Alon particles provided for this study consisted of particles with a range of diameters from 0.04 - 0.5 pm. Consequently, to ensure addition of only > 0.2 pm particles in the experiments, a cleaned Alon suspension (17) was fractionated by a series of filtrations. Alon was added to experiments to give particle concentrations typical of near-shore surface waters (0.1 - 5.0 mg 1-1). Particle concentration was monitored by light scattering measurements using a fluorometer. After 25 days, no significant change in scattering was measured in the photo-oxidized seawater samples. A decrease in scattering of < 16% was observed with time in the 1.0 mg C1-1 samples however. This decrease may be due to dissolution of the alumina particles or to the attachment of particles to the walls of the bottles. RESULTS To determine the effect of colloidal matter on the equilibrium partitioning of Th, 234Th spikes were first added to Alon suspensions that had been prepared in colloid-free seawater (ultrafiltered through a 1, 000 NMW filter). Figure 1 shows the partitioning results for Alon suspensions (0 - 5.0 mg 1-i) in colloid-free seawater with dissolved organic matter (DOM) concentrations of 0.1 mg C1-1 (Figure la) and 1 mg C1-1 (Figure ib). Separation of the samples into dissolved (< 1, 000 NMW), colloidal (1, 000 NMW-0.2 pm), and particulate (> 0.2 pm) fractions showed that with increasing Alon concentration, colloidal Th (The) decreased concurrently with an increase in particulate Th (Thy ). Alon concentration had little effect on the percentage of Th in the dissolved fraction (Thd). The colloidal fraction of 234 Th was found, by sequential filtrations, to pass ultra-filters with nominal molecular weight cut-offs of > 5, 000 and to be retained by the YM2 ultrafilter (1, 000 NMW). Under the conditions of Figure 1 (i. e. colloid-free seawa-ter), hydrolysis complexes and organic complexes are the only species of Th present in solution (13). Therefore, the colloidal 234 Th measured in the photo-oxidized seawater samples (Figure la) was assumed to consist of Th hydroxide complexes. The colloidal Th in the 1.0 mg C1-1 DOM samples (Figure lb) was assumed to be predominantly bound in hydroxides as well since < 5% of the total sample 14C was detected in the colloidal fraction. This concentration of Phaeodactylum tricornutum exudates (< 0.05 mg C1-1 ) would complex < 3% of the total Th in the sample (7). Amicon YM2 ul-trafilters have been reported to retain species with molecular weights much lower than their nominal molecular weight cut-off of 1, 000 (down to 200 MW) and to have a 50% retention at 380 MW (18). Therefore, these hydroxide complexes may be Th(OH)4, the dominant species of the Th in seawater (13), or polynuclear hydroxide complexes. The dissolved Th species in Figure 1 are probably organic complexes or hydrolysis complexes not retained by the YM2 ultrafilter. Phaeodactylum tricornutum exudates have been found to form strong complexes with Th; at 1.0 mg C 1 -1, 47% of total Th is bound in dissolved organic complexes (7). From a study of Th adsorption by metal oxides in seawater, Hunter et al. (19)
2 3 Alumina Concentration (mg/I)
of 234Th associated with the particulate fraction. Consequently, the Kd values calcu-lated for the samples (Table 2) show an inverse correlation with colloid concentration. Aggregation of the colloidal matter into filterable (> 0.2 pm) particles, as indicated by C measurements, was < 7%.
14c, were found not to be adsorbed by the Alon. The Kd values calculated for the Figure la samples ([DOM]=0.1 mg C 1 -1) were constant for Alon concentrations from 0.1 to 5.0 mg 1 -1 (see Table 1). Kd values for unreplicated samples are not included in Table 1 because of the unknown error in the measurements. The variability in the of the 1.0 mg C samples (Figure lb) may
metals from seawater; however, information on the sorption capacity of natural colloidal matter and on the aggregation/disaggregation processes between the particle classes is still needed. CONCLUSIONS Th adsorption by alumina particles in seawater was found to be predominantly of Th hydrolysis complexes that are retained by a 1, 000 nominal molecular weight ultra-filter. Competing solution complexation of Th by Phaeodactylum tricornutum exudates and sorption by colloidal matter decreased Th adsorption by the alumina. The sorption of metals by colloidal matter (pre-existing in the experimental solution or added with particles) must be considered in laboratory determinations of Kd values. Field data on the concentration, composition, and aggregation reactions of natu-rally occurring colloidal matter are required to determine the effect of colloid adsorption on the marine geochemistry of trace metals. REFERENCES 1. Balistrieri, L., Brewer, P.G. and Murray, J.W., Scavenging residence times of trace metals and surface chemistry of sinking particles in the deep ocean. Deep-Sea Res., 1981, 28A. 101-121. 2. Whitfield, M. and Turner, D.R., The role of particles in regulating the composition of seawater. In Aquatic Surface Chemistry: Chemical Processes at the Particle-Water Interface, ed. W. Stumm, Wiley- Interscience, New York, 1987, pp. 457-493. 3. Morel, F.M.M. and Gschwend, P.M., The role of colloids in the partitioning of solutes in natural waters. In Aquatic Surface Chemistry: Chemical Processes at the Particle-Water Interface, ed. W. Stumm, Wiley-Interscience, New York, 1987, pp. 405-422. 4. Li, Y.-H., Burkhardt, L., Buchholtz, M., O'Hara, P. and Santschi, P.H., Partition of radiotracers between suspended particles and seawater. Geochim. Cosmochim. Acta, 1984, 48. 2011-2019. 5. Higgo, J.J.W. and Rees, L.V.C., Adsorption of actinides by marine sediment: Effect of the sediment/seawater ratio on the measured distribution ratio. Environ. Sci. Technol., 1986, 20. 483-490. 6. Gschwend, P.M. and Wu, S.-c., On the constancy of sediment-water partition co-efficients of hydrophobic organic pollutants. Environ. Sci. Technol., 1985, 19. 90-96. 7. Niven, S.E.H., The partitioning of thorium among dissolved, colloidal, and partic-ulate fractions in seawater. Ph.D. dissertation, Dalhousie University, 1988. 8. Broecker, W.S., Kaufman, A. and Trier, R.M., The residence time of thorium in surface sea water and its implications regarding the fate of reactive pollutants. Earth Planet. Sci. Lett., 1973, 20. 35-44.
9. Santschi, P.H., Adler, D., Amdurer, M., Li, Y.-H. and Bell, J.J., Thorium iso-topes as analogues for "particle-reactive" pollutants in coastal marine environ-ments. Earth Planet. Sci. Lett., 1980, E. 327-335. 10. Cospito, M. and Rigali, L., Determination of thorium in natural waters after ex-traction with Aliquat-336. Anal. Chim. Acta, 1979, 106. 385-388. 11. Moore, R.M. and Hunter, K.A., Thorium adsorption in the ocean: reversibility and distribution amongst particle sizes. Geochim. Cosmochim. Acta, 1985, 49. 2253-2257. 12. Anderson, R.F., The marine geochemistry of thorium and protactinium. Ph.D. dissertation, M.I.T./W.H.O.I., 1981, 40-43. 13. Langmuir, D. and Herman, J.S., The mobility of thorium in natural waters at low temperatures. Geochim. Cosmochim. Acta, 1980, 44. 1753-1766. 14. Wilson, M.A., Gillam, A.H. and Collin, P.J., Analysis of the structure of dissolved marine humic substances and their phytoplanktonic precursors by 1H and "C nu-clear magnetic resonance. Chem. Geol., 1983, 4Q. 187-201. 15. Gershey, R.M., MacKinnon, M.D., Williams, P.J. le B and Moore, R.M., Compari-son of three oxidation methods used for the analysis of the dissolved organic carbon in seawater. Mar. Chem., 1979, 7. 289-306. 16. Davis, J.A., Complexation of trace metals by adsorbed natural organic matter. Geochim. Cosmochim. Acta, 1984, 48. 679-691. 17. Davis, J.A., Adsorption of natural dissolved organic matter at the oxide/water interface. Geochim. Cosmochim. Acta, 1982, 44g. 2381-2393. 18. Wilander, A., A study on the fractionation of organic matter in natural waters by ultrafiltration techniques. Hydrologie, 1971, 34. 190-200. 19. Hunter, K.A., Hawke, D.J., and Choo, L.K., Equilibrium adsorption of thorium by metal oxides in marine electrolytes. Geochim. Cosmochim. Acta, 1988, 52. 627-636. 20. Kim, Chemical behaviour of transuranic elements in natural aquatic systems. In Handbook on the Physics and Chemistry of the Actinides. Vol. 4, eds. A. J. Freeman and C. Keller, Elsevier Science Publishers B. V., Amsterdam, 1986, pp. 413-456. 21. Tsunogai, S. and Minagawa, M., Settling model for the removal of insoluble chem-ical elements in seawater. Geochem. J., 1978, 12. 47-56.
Flux 1 2 3 4 5 6 1 2 3 4 5 1 2 3 % 6 0 0 1 0 0 0
SPATIAL AND TEMPORAL VARIATIONS IN THE LEVELS OF CESIUM IN THE NORTH WESTERN MEDITERRANEAN SEAWATER (1985-1986) Calmet D., Fernandez J.M., Maunier P., Baron Y. Commissariat i l'Energie Atomique Institut de Protection et de Surete Nucleaire Departement d'Etudes et de Recherches en Securite Service d'Etudes et de Recherches sur l'Environnement Station Marine de TOULON BP n*330 83507 LA SEYNE sur MER France ABSTRACT The explosion at the Chernobyl 4 reactor led to the discharge into the atmosphere of 137Cs, the fallout was clearly discernible in the surface waters of the north western part of the Mediterranean coast, although there was no significant health hazard. This radioelement was used as a tracer to plot the stratification and dispersal of water from the Rhone at local and regional levels. It appears to cross the Golfe du Lion along a north east - south west axis. Vertical salinity and temperature profiles plotted with a CTD probe enabled a distinction to be made between the various masses of water present in the Golfe du Lion. There is a difference in the levels of 137Cs activity in the surface and deep water, as measured at reference points. In September 1986, the surface water only showed traces to a depth of 50 m, whereas the whole of the continental shelf water showed traces in December 1986. Outside the continental shelf at the same period, there was no noticeable increase in the 137Cs levels in water at greater depths. INTRODUCTION Since 1977, the Commissariat & l'Energie Atomique's Environmental Studies and Research Service at the TOULON Marine Station has been working on the processes involved in the dispersal of natural or anthropogenic radioelements in the Mediterranean sea, as part of a general research programme (RADMED programme , from RADioactivity and MEDiterranean sea). Samples of seawater are taken annually from the north west Mediterranean basin. Until the first three months of 1986, the results were largely used to monitor the changes in l 37Cs isotopes levels in the water from atmospheric fallout as a result of weapons testing between 1955 and 1979, in the northern hemisphere (1). Unlike the latter type of fallout, which is referred to as global, the explosion at the Chernobyl reactor on 26 April 1986 led to the release of radioelements at
a specific point in time, with a fraction of the fallout reaching the eastern part of the north-west Mediterranean basin (2). Chernobyl fallout deposits assesment were led from aerosols and soils measurements and from atmospheric models results. These cesium latest deposits, which can be pinpointed to a specific time and place, can therefore be regarded as tracer elements, with a specific 137Cs/134 Cs close to 2.0, which can be plotted to determine the dynamics of the waters of the north-west Mediterranean basin. The radioelements released by the Chernobyl accident reached the marine environment via two routes: a direct route, whereby the atmospheric fallout fell to surface of the sea, and an indirect route via the rivers, after surface deposits on the soil had been washed away by rainwater. On the French section of the Mediterranean coast, two rivers of unequal importance flow out to sea: the Var and the Rhone. However, the waters of the RhOne carry not only the radioelements arising from the Chernobyl accident but also those associated with the discharge of low-level radioactive liquid effluents from various nuclear plants such as cesium isotopes, with a 137Cs/134Cs ratio greater than 4. The monitoring of radioelements arising from atmospheric fallout on the surface of the sea enables seasonal vertical changes to be detected, at regional level, in the various bodies of water whereas the monitoring of radioelements discharged by rivers leads to a definition of the limits of geographical areas under the influence of the deposits of coastal rivers. EQUIPMENT AND METHOD Radioactive tracers have been monitored during 7 oceanographic cruises in collaboration with research teams from the CNRS and the University. The cruises enabled us to cover a network of sampling points all over the French Mediterranean coast. After establishing the coastal area by means of 25 sampling points in the course of the CORSE86 (86.09.23-86.10.06), LITTORAL86A (86.02.03-10, 86.04.03-11, 86.04.21-29) and LITTORAL86B (86.11.05-13) cruises, efforts were concentrated in particular on the area reached by the waters of the Rhone, as defined by 15 sampling points during the DYPOLO1 (86.08.28-86.09.07) and DYPOLO2 (86.11.19-30) cruises, and the area of the Golfe du Lion covered by 24 sampling points as part of the PELAGOLIONO1 (86.09.02-11) and PELAGOLIONO2 (86.12.04-12) cruises. Over the network of sampling points, volumes of water between 90 and 120 litres were systematically pumped from a depth of 0.5m. At some points selected on the basis of results of vertical contours of salinity, temperature or turbidity obtained in real time with a bathymeter, the various bodies of water identified between the surface and 1700 m were sampled using 30 1 Niskin bottles.
All of the seawater samples were filtered (0.45 gm) in order to remove radioelements associated with the particles, the quantity of which varied according to the area studied, especially in the Rh6ne estuary area where cesium isotopes are mainly linked to suspended matter. Gamma-emitting radioelements were measured by radiochemical methods described elsewhere (3,4). RESULTS AND DISCUSSION The results of our research are presented according to two areas: the coastal fringe subjected to river pollution, and the area defined by the continental shelf of the Golfe du Lion, where the river deposits are diluted. The coastal fringe The results of seawater sampling conducted between February and April 1986 (LITTORAL86A), at less than 2 nautical miles from the coast, reveal the presence of a single artificial radioelement: 137 Cs. 134 Cs was not detected. Levels in the region of 1 mBq/1 are typical of the eastern part of the coast whereas the west has slightly higher readings in the region of 4 mBq/1 emphasing the action of the waters of the Rh6ne in this part of the coast (Figure 1A) where 134Cs could also be detected ( 137Cs/134 Cs between 4 to 9). In November 1986 (LITTORAL86B), 137 Cs activity levels were higher all along the coast, varying from 3 to 9 mBq/1 with a 13/Cs/ 134 Cs from 2.3 to 4.0 , eliminating the differences between east and west as found in previous surveys (Figure 1B). Thus the Chernobyl fallout is clearly discernible, especially in the eastern part of the French Mediterranean coast. These observations are in line with those results obtained from atmospheric and land sampling (2). During these two cruises, the coastal area directly under the influence of water from the Rhone appears to have a higher 137 Cs content. A detailed study of the area in August, September and November 1986 during the DYPOL 01 and 02 cruises confirmed the presence of a decreasing spatial gradient of 137 Cs levels from the estuary towards the open sea (Figure 2). The Rhone water area of dilution corresponds to a situation classically described as a fundamental current flow, under the influence of the Ligurian-Provenal current (5). The vertical distribution of 137 Cs at 3 levels defined as a function of turbidity contours, at 3 sampling points distributed along a radial perpendicular to the coast, reveals the predominance of 137 Cs in the surface waters (Figure 3). The 3 sampling points display a marked discontinuity between the surface level of activity and the intermediate and deep levels. The discontinuity corresponds to the surface dispersal of the waters of the Rhone, which are less dense than the receiving seawater.
Local spatial heterogeneity, both horizontal and vertical, is accompanied by daily temporal variations which we detected at sampling point 5. Between 28.08.86 and 03.09.86, the 137Cs concentration of the surface waters, under the permanent influence of Rhone deposits, fluctuated between 4 and 24 mBq/1 (Figure 4). This spread of values reflects both the variations in deposits over the period of time in question and the variable rates at which freshwater and seawater mix. The Golfe du Lion In September 1986, levels of 137Cs in the surface waters of the continental shelf of the Golfe du Lion varied from 1.5 to 6.5 mBq/1 with an average of 3.0 mBq/1 and a 137Cs/ 134Cs between 2.5 to 5.0. These figures were much lower than those obtained at sampling points at the eastern boundary of the gulf, on average 12 mBq/1 (4.5 mBq/1 of 134Cs), but slightly higher than those at the southern boundary which were equal to 2.5 mBq/1 (1.0 mBq/1 of 134Cs) on average (Figure 5A). The decreasing spatial gradient from east to west seems to correspond to a stock of cesium carried by waters in the Ligurian-Provenal geostrophic current coming from the north-west Mediterranean basin. The absence of major rainfall prior to the dates of the sampling definitely restricted the amount of soil washed away into the Rhone and thus the quantity of 137 Cs the latter was carrying. The levels recorded in the Gulf must therefore have been chiefly due to direct atmospheric fallout onto the surface waters in the eastern part of the north-west basin. At the time of the sampling operation in December 1986 (Figure 5B), the mean 137Cs and 134Cs activities of the surface waters of the continental shelf were higher: 7.0 mBq/1, for activity levels between 6.0 and 8.0 mBq/1 of 137Cs and 1.0 to 2.5 mBq/1 for 134Cs. A central strip running from north east to south west, joining the Rhone delta to the Cote Vermeil, had the highest 137 Cs readings which were constantly in the region of 8.0 mBq/1 (Figure 5B). These findings are to be compared with those of ALAIN (6), MINAS (7) and TOURNIER (8). The higher volume of deposits in the waters of the Rhone after the autumn rains and the increase of low level industrial releases made it possible to track the Rh6ne water across the gulf. Sampling points at the east and south boundaries of the continental layer of surface water characterised by high 137Cs levels and appearance of 134 Cs reflects the gradual spread of these elements towards the sea bed.
lf recorded the lowest values, between 2 and 6 mBq/1 of As regards sampling points on the Golfe du Lion continental shelf, the vertical distribution of 137Cs and 134Cs readings gave the highest values from the surface to a depth of 70 m in September 1986 and to a depth of 200 m in December 1986. The increase over a period of time in the depth of the
Outside the continental shelf, at sampling points located at depths of 1700 m, 137 Cs was not detected in the deepest bodies of water (Figure 6). The dispersal of dissolved 17Cs therefore seems to be relatively slow as, 8 months after the introduction of 137 Cs into the surface waters, the fraction containing dissolved 137 Cs had not noticeably increased in the deepest layers.
A study of the area under the direct influence of deposits from the waters of the RhOne showed major local stratification together with a 137Cs surface dilution gradient from the coast towards the open sea. Sampling of Rhone water enabled us to produce a map of how the water was dispersed in the Golfe du Lion, which it crosses along a north east - south west axis. Atmospheric and earth contamination provided a reservoir of cesium in the surface waters, vertical penetration being progressive. The layer of water nearest the surface was in fact contaminated to a depth of 50 m in September 1986, and 200 m in December after the seasonal thermocline has disappeared. We would like to express our special thanks to Mr. G. JACQUES and Mr J.C.ALOISI, who headed the PELAGOLION and DYPOL cruises, and to all members of crews of the R.V. Catherine LAURENCE, Le NOROIT and Le SUROIT. REFERENCES 1. United Nations, Sources and effects of ionising radiation, UNSCEAR,1984, 781. 2. Commissariat i 1'Energie Atomique, Institut de Protection et de Surete Nucleaire, L'accident de Tchernobyl, Rappport IPSN 2/86 rev. 3, Octobre 1986, 163. 3. Foulquier L., Philippot J.C., Baudin-Jaulent Y., Metrologie de l'environnement. Echantillonnage et preparation d'organismes d'eau douce. Mesure des radionucleides emetteurs gamma. Rapport CEA/R-5164, 1982. 4. Calmet D., Synthese radioecologique des differents compartiments de l'environnement marin du Cotentin. These de Doctorat d'Etat Aix-Marseille II, 1986, 254. 5. Blanc F., Leveau M., Plancton et eutrophie: aire d'epandage rhodanienne et Golfe de Fos (Traitement mathematique des donnees). These de Doctorat d'Etat, Universite d'Aix-Marseille II, 1973, 681. 6. Allain C., Topographie dynamique et courants generaux dans le bassin occidental de la Mediterrande (Golfe du Lion, Mer Catalane, Mer d'Alboran et ses abords, secteur de la Corse), Rev. Tray. Inst. Peches Marit., 24, 1960, 121-145. 7. Minas H.J., Recherche sur la production organique primaire dans le bassin mediterranden nord-occidental. Rapport avec les phenomenes hydrologiques. These de Doctorat d'Etat, Universite d'Aix-Marseille II, 1968, 227. 8. Tournier H., Hydrologic saisonniere du Golfe du Lion en hiver. Rev. Tray. Inst. Peches Marit., 33, 1969, 265-300.
THE CONTAMINATION OF THE NORTH SEA AND BALTIC SEA BY THE CHERNOBYL FALLOUT H. Nies, Ch. Wedekind Deutsches Hydrographisches Institut Bernhard-Nocht-Str. 78 D-2000 Hamburg 4 Federal Republic of Germany ABSTRACT The German Hydrographic Institute (DHI) investigated the Cher-nobyl Fallout in the North- and the Baltic Sea. After the radioactive cloud has reached the southern North Sea at 3 May and the western Baltic at 5 May, a great deal of different radionuc-lides could be determined in the marine environment, such as Cs134, Cs 137, Ru 103, Ru 106, Zr/Nb 95, Ba/La 140, 1131, Te/I 132. Nuclides with lower activity levels have been Sr 90, Pu 239/240, Pu 238, Am 241, Cm 242, Mo/Tc 99, Ce 141, Ce144, and Ag 110m. During several cruises to the North Sea and to the Baltic Sea the levels of the contamination by this Fallout could be detected. The Chernobyl Fallout is discernable from other sources of artificial radioactivity in the North Sea and the Baltic sea byits typical Cs134/137 activity ratio. The relatively fast water movement along the coast of Belgium, the Netherlands, and Germany renewed the Chernobyl contaminated water of the southern North Sea within 4 months after the accident by uncontaminated water from the Channel. However, in the German Bight itself Chernobyl Fallout is still to be identify in December 1986, which is due to runoff of the river Elbe. During an internationally co-ordinated monitoring programme in October/November 1986 the different levels of the Chernobyl Fallout could be determined in the entire Baltic Sea. The highest contamination levels were measured in the southern part of the Bothnian Sea with up to 800 mBq/1 Cs 137. In the western Baltic and in the Baltic Proper the contamination of the Sea is relatively low with about 40 mBq/1 Cs 137. In the Baltic Proper the Chernobyl contamination has not yet reached the water below the halocline. However, in the Bothnian Bay an almost homogenious mixing down the water column can be established. INTRODUCTION Prior to the nuclear reactor accident at Chernobyl the concen-trations of artificial radionuclides in the North Sea were mainly influenced by the discharges from the nuclear fuel reprocessing plants at La Hague (France) and Sellafield Works (United Kingdom). In the Baltic Sea, these radionuclides were transported only to a slight extent by particular weather and hydrographic conditions, so that the radioactive inventory of the Baltic Sea was determined by nuclear weapon Fallout of the sixties. The Chernobyl accident has considerably increased and changed the inventory of artificial radionuclides in the marine environment of the North Sea and the Baltic Sea. In the following
paper, the essential results of the measurements of Chernobyl Fallout nuclides, carried out by the Deutsches Hydrographisches Institut in 1986, are to be presented and discussed. RESULTS AND DISCUSSION The input of the Fallout from Chernobyl into the Sea took place via the atmosphere, whereby a large-scale and time synoptic monitoring became necessary. After the accident had become known, the DHI therefore took water samples immediately as a precaution at several positions in the North Sea and the Baltic Sea, in order to investigate them as soon as possible for radioactive contamination. Aerosol investigations carried out in Hamburg simultaneously provided data about the time of the arrival of the radioactive cloud and about its nuclide composition. The nuclide pattern was ascertained by Gamma spectroscopy. Hereby, the relative activity of the most important nuclides in the analyzed aerosols, in relation to Cs 137 on the 5th/6th May, are given in the following table: Nuclide Rel. activity Half-life Cs 137 1.00 30 years Cs 134 0.52 754 days Ru 103 2.00 39.3 days Ru 106 0.75 368 days Ba/La 140 0.67 12.8 days I 131 6.00 8.0 days Te/I 132 2.30 3.2 days Some further nuclides, such as Sr90, Pu239/240, Pu238, Am241, Np239, Cm 242, Te 129m, Mo/Tc 99, Cs 136, Ce 141, Ce/Pr 144, Zr/Nb 95, Ag 110m, were detectable in lesser concentration in different samples from the region of the sea. The DHI's radiological measurement network (positions of the measuring network station are given in Fig 1.) delivered the first information about radioactive precipitation into the sea at 3 May at the station Light Vessel "Borkumriff" at 21.00 hrs. after the set in of thunder showers. The input into the southern North Sea occurred from west to east direction by the movement of a thunder front. The input into the western Baltic was detected at the 5 May at station Fehmarnbelt. The input into the Baltic Proper was registered at 29 May by the travelling station RV GAUSS, which was in the area south of Gotland. At that time no direct input by rain of the Fallout into the water took place in this area. It merely concerned precipitation from aerosols and fog.
oceanographic measurements. In the southern part of the North Sea the mixed layer reached down to the sea floor and in the northern part of the North Sea down to a depth of 40 to 60 meters. In the vicinity of the south Norwegian coast a thickness of 100 meters can be assessed. Proceeding from that, one can calculate the radioactive inventory of the surface layer areally and represent it as areal deposition. Fig. 6 shows the Cs 137 distribution pattern of the Chernobyl Fallout deposition in kBq/ms resulting from the mean radionuclide ratio Cs134/Cs137. Ostsee SchleimOndung
1986 I 1987 Fig. 3 The Cs 137 and Sr 90 surface activity concentration at the position Schleimiinde in the western Baltic Sea
...... I , *
in the Northeast Atlantic. REFERENCES (1) Review of the continued suietability of the dumpsite for radioactive waste in the North-East Atlantic, Nuclear Energy Agency / OECD, Paris 1985 (2) Kupferman, S.L., Becker, G.A., Simmons, W.F., Schauer, U. Marietta, M.G., Nies, H., Nature, Vol. 319, 474-477 (1986) °W 10°
Table 2. Values of A and B in Eqv. 1 from liquid discharges of various radionuclides to the sea from the nuclear repro-cessing plants Sellafield and La Hague. La Hague (Table 1) Sellafield (Aarkrog et al. 1983-85) Radionuclide ABA 60 Co 1.02 2100 0.84 216 90Sr -- 1.12 7040 99Tc 0.67 18800 0.62 46000 106R u 0.63 315 - -137Cs --0.94 23000 239,240p u 0.61 2.71.04 1520 The equations were based on samples collected out to a di-stance of approximately 1000 km from the source. The back-grounds (from Sellafield and global fallout) of 90 Sr and 137Cs in the English Channel do not make it possible to calculate meaningful distance equations for these radionuclides for La Hague. In the calculation of the distance relations we normalized all seaweed data to Fucus vesiculosus. We used the following ratios for 99 Tc (calculated from Table 1): Ascophyllum/ves.: 1.41; serratus/ves.: 0.64; spiralis/ves.: 0.55 and pelvetia/ves. = 0.81. In a similar way the other radionuclides studied were normalized to Fucus vesiculosus. The distance relations may vary with time. If the discharges of a nuclide increase from year to year, A in Table 2 will increase, and if the annual discharges show a de-creasing tendency A will decrease. It is however remarkable that the A-values in Table 2 are so similar; they all lie between 0.6 and 1.1. This shows that the coastal currents in the North Sea region run over relatively long distance without any appreciable mixing with the open sea. From Table 2 we may estimate the discharges from La Hague rela-tive to those from Sellafield. We assume that the Fucus samples collected at the Channel coast in the first half of 1985 repre-sented La Hague releases from 1983 and 1984, and the samples from the Scottish coastline in June 1982 were related to Sella-field discharges in 1980-81. If we divide the distance inte-grals: 1000 2Tx.B.xAdx for La Hague (H) with those from Sellafield(S) 1 we find for 99 Tc: H/S = 0.30, for 60 Co H/S = 3.3 and for
239,240pu H/S = 0.024. The annual mean discharges in 1980-81 from Sellafield (BNFL 1979-1986)[ 3] were: 31 TBq 99 Tc, 0.76 TBq 60 Co, and 17.5 TBq 239,240 Pu. We would thus predict the annual mean discharges from La Hague in 1983-84 to 9 TBq 99 Tc, 2.5 TBq 60Co, and 0.4 TBq 239,240 Pu. We have no complete data from 1983-84 from La Hague, but for 1983 Patti et al. (1984) re-ported 11.7 TBq 99 Tc and for 1982 Calmet & Guegeniat (1985) reported annual discharges of 3.1 TBq "Co and 0.12 TBq 239,240 Pu. If we assume that the discharges from La Hague in the later years were relatively constant, we may conclude that the above estimates from Fucus measurements seem reason-able. CONCLUSION The radionuclide concentrations in seaweed along the continen-tal side of the English Channel are inversely proportional to the distance (in km) or square root of the distance from La Hague. Similar distance relations have been observed earlier in British coastal water for liquid discharges from Sellafield These relations may be applied for estimates of the annual discharges from the reprocessing plants in Western Europe. REFERENCES 1. Aarkrog A., Dahlgaard H., Hallstadius L., Hansen H. and Holm E. (1983). Nature 304, 49-51. 2. Aarkrog A. et al. (1983-1985). Risen Reports, Nos. 488, 509, 510, 527 and 528. Rises National Laboratory, Roskilde, Denmark. 3. BNFL (1979-1986). Annual report on radioactive discharges and monitoring of the environment 1978-1985. British Nuclear Fuels Limited, Warrington, Cheshire, U.K. 4. Calmet D. & Guegueniat P. (1985), in: IAEA-TECDOC-329 pp. 111-144 International Atomic Energy Agency, Vienna. 5. Casso S.A. & Livingson H.D. (1984). WHO-84-40. Woods Hole Oceanographic Inst. 6. Harley J.H. (editor) (1972). HASL-300. Environmental measurements Laboratory, New York. 7. Holm E., Rioseco J., & Garcia-Leon M. (1984). Nucl. Instr. and Meth. in Phys. Res. 223, 204-207. 8. Jefferies D.F., Steele A.K. and Preston A. (1982). Deep-Sea Res. 29, 713-738. 9. Kautsky H. (1973). Deutschen Hydrographischen Zeitshrift, 26, 242-246. Livingston
H.D., Bowen V.T. & Kupferman S.L. (1982). J. mar. Res. 40, 1227-1258. Patti F., Masson M., Vergnaud G. & Jeanmaire L. (1984), in: Technetium in the Environment (Desmet G & Myttenaere C. editors) pp. 37-51. Elsevier Applied Science Publishers, London & New York.
CHEMICAL PARTITIONING OF PLUTONIUM AND AMERICIUM IN SEDIMENTS FROM THE THULE REGION (GREENLAND) E. Holm, J. Gastaud, B. Oregioni IAEA International Laboratory of Marine Radioactivity Oceanographic Museum Monaco and A. Aarkrog, H. Dahlgaard Risa National Laboratory Roskilde Denmark and J.N. Smith Fisheries and Oceans Institute of Oceanography Dartmouth Canada ABSTRACT Chemical partitioning of plutonium and americium was studied by sequential leaching of sediments collected at about 200 m depth in 1968 and 1984 in the vicinity of the point of impact of an accidental loss of a nuclear device near Thule, Greenland, in January 1968. The fractions separated and determined were exchangeable, bound to carbonates, bound to Fe/Mn oxides, bound to organic matter-sulfides and residual. Radiochemical determinations of total samples showed a considerable inhomogeneity due to the presence of hot particles. These hot particles were associated with the residual fraction. The distribution of plutonium between the different phases did not change significantly between 1968 and 1984. Americium showed a greater tendancy to be associated with the exchangeable, carbonate and Fe/Mn oxide fractions than did plutonium. Americium built up in situ from the decay of 241 Pu is mainly found in the residual fractions.
INTRODUCTION Since the accidental loss of a nuclear device near Thule, Greenland, in 1968, several scientific expeditions have taken place in the area for the collection of sediment, water and biota samples, and the results were published (1-5). In 1984, the estimated inventories of 239+240p u an d 241 Am in the sediments derived from the accident were 1 TBq and 0.1 TBq respectively (4). Once a chemical species has been incorporated into a sediment, its ultimate fate depends on a number of very complex factors. Some elements can be considered to be irreversibly incorporated into a sedimentary component, whilst others may undergo post-depositional remineralization and take part in various bio-geochemical reactions. In order to predict the fate of transuranics or even make the most primitive evaluations of this kind it is necessary to understand something of the mechanisms by which these elements are incorporated into the various sedimentary phases. It is then not sufficient to carry out chemical analysis on the total samples. The partitioning of transuranics between the various sedimentary components can be studied by physical or chemical separation and subsequent analysis of individual components. The results will depend not only on the nature of the depositional environment but also on the source terms. The chemical and physical partitioning as well as the isotopic composition of the transuranium elements will differ if the source is in form of liquid release from a nuclear fuel reprocessing plant, global fallout from nuclear weapon tests, a nuclear device accident or fallout from a reactor accident such as the recent Chernobyl disaster. This paper presents the chemical partitioning of plutonium and americium between exchangeable, carbonate bound, Fe/Mn oxide bound, organic-sulfide bound and residual fractions in sediments collected in 1968 and 1984 contamined by the accidental loss of a nuclear device. MATERIAL AND METHODS During the expeditions a HAPS corer (6) was used for sediment collection. The sampling area was 145 cm 2 . The cores were cut in 3 cm depth sections to a total depth of 12 cm (or 15 cm when available). Four cores collected close to the point of impact were studied (Table 1). These samples were dried at 100°C, sieved and the fraction <60 pm kept
for analysis. Table 1. Sediments subject to this study Core No. Position Depth (m) Distance from point of impact (km) 1968 203 0.4
The radioanalytical procedure for plutonium and americium has been described elsewhere (7). Cores 53 (1968) and 84.01377 (1984) were selected for the sequential leaching experements. This choice was made on the basis fo their similar 239+240 Pu activities (850 mBq g -1 ) in the surface (0-3 cm) sections. The samples analyzed in this work had a dry weight of 1 g and it has been shown that there is no relation between frequency of hot samples and sample size (4). The chemical partitioning studies of plutonium and americium were performed by sequential leaching. The fractions separated and determined were: Fraction 1: Exchangeable (1M CH3COONH4, 20 ml, 2h shaking) Fraction 2: Bound to carbonates (1M CH3COOH, 20 ml, 4 h shaking) Fraction 3: Bound to Fe/Mn oxides (0.1 M NH2OH,HC1 in 25% CH3COOH, 20 ml, overnight shaking) Fraction 4: Bound to organic matter - sulfides (H202 pH=2, 10 ml, 85°C, 6h shaking + 1M CH3COONH4, 15 ml, lh shaking) Fraction 5: Residual (conc. HNO3, HC104, HF) The sequential leaching experiment was done twice on each sample from 1984 and five times on the sample from 1968. The activities were corrected to the date of collection. In this context for the build up of 241 Am during sample storage, we used a 241pu/239+240 Pu activity ratio of 3.3 + 0.4 previously determined in 1968 (4). RESULTS AND DISCUSSION There are a great variety of chemical techniques which have been designed to establish the distribution of non-residual elements among the components of a sediment (8). The suitability, relative advantages and inconveniences of these methods have been reviewed at the International Laboratory of Marine Radioactivity in Monaco, and a standard method has been adopted after experimentation to check reproducibility. The method used in this work has been used previously to study transuranic geochemical partitioning in deep-sea and near-shore sediments (9-10). In order to reduce the effect on trace metal or transuranic concentrations which result from grain size difference, it is common practice to exclude the coarser sediment fraction by sieving. In most techniques, the fraction which has a grain size of < 60 pm is used for analysis. Little work has been carried out on the distribution of transuranics in sediments with regard to grain sizes characteristics, but for heavy metals at grain sizes>60 pm, presence of trace metal-rich heavy minerals or large-sized trace metal-poor minerals might affect the results. When performing. this work we encountered several difficulties such as the inhomegeneity of the samples and that the chemical partitioning of fallout plutonium and americium is not known. The fallout background is estimated to be 23 Bq m-2 (5) which is very small compared to the levels (30000 Bq m-2 ) found in this study. The influence on the results from such plutonium and americium is therefore not likely to be significant. we have not taken into consideration any alteration of chemical distribution during storage although this might have occured in practice
(11). Americium-241 ww partly delivered at the accident, partly grown up from the decay of 241Pu in situ on the sea floor and also during storage in the laboratory. The fact that all results for americium had to be corrected to the date of collection render these results and the interpretation of data more uncertain. Shortly after the accident detailed nuclear track autography and microscopic studies of melted crust samples were conducted (12). The studies showed the plutonium to be in form of oxide particles with a very wide size distribution with a median diameter of 2ium. The particles
were associated with or adhering to particles and pieces of inert debris of all kinds of material (glass, metal, plastic, rubber, etc.). The subsequent cleaning of the crash site was very effective and it was estimated that only 350 g of plutonium of the 3150 g released were trapped in the ice. Sedimentation studies of melted ice cores showed that 85 to 95% of the debris and associated plutonium oxide sank immediately (12) when contaminated ice was transferred into the sea and broke up in June-July 1968. . It is obvious that such delivery of plutonium and americium to the sea floor is quite different to other sources such as global fallout, releases from nuclear fuel reprocessing plants, etc. In the Thule case the elements have not undergone any interaction in the atmosphere and a very limited one, if any, in the water column. The interactions and resulting chemical partitioning have taken place in the sediments. Consequently the observed activity ratio sediment/water for plutonium at the point of impacts is in the order of 10 10 (4), while at areas in a similar marine environment where fallout plutonium has been transferred to the seafloor by particle interaction in the water column this ratio is in the order of 104 - 105 (13). The isotopic ratios 238p n/239+240pn and 2411311/239+240pn are different depending on source, being much higher in the releases from nuclear fuel reprocessing facilities and in fallout from the Chernobyl accident than in global fallout from nuclear detonation tests and these, in turn, are higher than those for weapon grade plutonium. The 238pn/239+240 Pu activity ratio was 0.016+0.01 (n=39, 1 SE) and the 241pn/239+240 Pu had earlier been determined to 3.3+0.4 (n=6, 1 SE) at the time of the accident (January 1968) (4). The 2445/239+240pu activity ratio was determined in this study to 0.098+0.005 (n=29, 1SE) in 1984. From the available data we can now calculate the decay of 238 Pu and 241Pu relative to 239+ 240Pu and also the build up of 241 Am as shown in Fig. 1. We then estimate that the 241239+240 Pu activity ratio was about 0.037 at the time of the accident. Supposing that there was no 241Am present after the initial plutonium separation we observe that the plutonium was fabricated in 1962 + 1 year. Furthermore, we find that when the sample collection took place in August 1968 and August 1984, about 8% and 62% respectively of the americium had been formed in situ. The integrated activity area per unit (Bq m -2 ) was studied in the three sediment cores collected in 1984 and given in Table 1 as a function of dry mass depth (kg m -2 ). These cores represent different distances from the point of impact and accordingly different activity levels. The results are given in Fig. 2. We cannot observe any difference in the distribution pattern between the three cores or between americium and plutonium in these total activity analysis. The vertical distribution of americium in 1979 was also similar to that of plutonium, consequently showing unchanged americium/plutonium ratio with depth. A downward displacement of activity as well as a horizontal translocation of activity with time have been observed (4). It was suggested that the presence of plutonium and americium in the deeper layers of sediment is due to bioturbation and that physico-chemical mechanisms are less important (3). The integrated activity area per unit for the different chemical fractions of sediment core No. 84.01377 is shown in Fig. 3 as a function
Table 2. The 241 239/240 Pu activity ratio for different chemical fractions in surface sediment (0-3 cm) collected in 1968 and 1984. 241Am/239+240 Pu activity ratio Fraction 1968 1984 Exchangeable 0.20 + 0.04 1.21 + 0.30 Bound to carbonates 1.43 T 0.09 1.13 T 0.10 T 0.02 T 0.23 T
Bound to Fe/Mn oxides 0.28 + 0.08 0.20 Bound to organic matter-sulfides 0.28 0.12 + 0.05 Residual 0.026 + 0.001 0.10 0.03 weighted mean 0.040 + 0.007 0.098+ 0.005 (6;18) (n=29)
REFERENCES 1. Aarkrog, A., Radioecological investigations of plutonium in an arctic marine environment. Health Physics, 1971, 20, 31-47. 2. Aarkrog, A., Environmental behaviour of plutonium accidentally released at Thule, Greenland. Health Physics, 1977, 32, 271-84. 3. Aarkrog, A., A re-examination of plutonium at Thule, Greenland, in 1970. In Radioecology applied to the protection of man and his environment, EUR 4800, 1971, pp. 1213-19. 4. Aarkrog, A., Dahlgaard, H., Nilsson, K. and Holm, E., Further studies of plutonium and americium at Thule, Greenland. Health Physic, 1984, 46, 29-44. 5. Aarkrog, A, Boelskifte, S., Buch, E., Christensen, G.C., Dahlgaard, H., Hallstadius, L., Hansen, H. and Holm, E., Environmental Radioactivity in the North Atlantic Region. The Faroe Islands and Greenland included. 1984. Riso National Laboratory, Roskilde, Denmark, Report Riso-R-528, December 1985. 6. Kanneworff, E. and Nicolaisen, W., The HAPS, a Frame-supported Bottom Corer. Ophelia, 1973, 10, 119-29. 7. Ballestra, S. and Fukai, R. An improved radiochemical procedure for low-level measurements of americium in environmental matrices. Talanta, 1983, 30, 45-48. 8. Chester, R. and Aston, S.R., The partitioning of trace metals and transuranics in sediments. In Techniques for Identifying Transuranic Speciation in Aquatic Environments, International Atomic Energy Agency, Vienna, STI/PUB/613, 1981, 173-93. 9. Aston, S.R., Gastaud, J., Oregioni, B. and Parsi, P., Observations on the adsorbtion and geochemical association of technetium, neptunium, plutonium, americium and californium with a deep-sea sediment. In Behaviour of long-lived radionuclides in the marine environment, Commission of the European Communities, EUR 9214, 179-87. 10. Aston, S.R. and Stanners, D.A., Observation on the deposition, mobility and chemical associations of plutonium in intertidal sediments. In Techniques for Identifying Transuranic speciation in Aquatic Environments, International Atomic Energy Agency, Vienna, STI/PUB/613, 1981, 203-17. 11. Thomson, E.A., Luoma, S.N., Cain, D.J. and Johansson, C., The effect of sample storage on the extraction of Cu, Zn, Fe, Mn and organic material from oxidized estuarine sediments. Water, Air and Soil Pollution, 1980, 14, 215-33. 12. Langham, W.H., In Project Crested Ice, eds. G.E. Torres, H.B. Tracy and O.R. Smith, USAF Nuclear Safety, 1970, 65, 36-41.
13. Holm, E., Persson, B.R.R., Halistadius, L., Aarkrog, A. and Dahlgaard, H., Radiocesium and transuranium elements in the Greenland and Barents Seas, Oceanologica Acta, 1983, 6, 457-62. 14. Edgington, D.N., Characterization of transuranic elements at environmental levels. In Techniques for Identifying Transuranic speciation in Aquatic Environments, International Atomic Energy Agency, Vienna, STI/PUB/613, 1981, 3-25. 15. Edgington, D.N., Alberts, J.J.. Wahlgren, M.A., Karttunen, J.O. and Reeve, C.A. Plutonium and americium in Lake Michigan sediments. In Transuranium Nuclides in the Environment, International Atomic Energy Agency, Vienna, STI/PUB/410, 1976, 493-516.