tritium in seawater samples was analysed mass spectrometrically at the uni-versity of Bremen using a 3He ingrowth method. 3H concentration is given in tritium units (1 tu is the isotopic ratio of one 3H atom to 1018 protium (1H) atoms, equiva-lent to 118 mBq/l of water). the 3He ingrowth method can measure 3H levels down to 0.01 TU (precision at 1 σ is around 0.005 TU). 137cs was concentrated in seawater samples by adsorption onto aMp (ammonium molybdophosphate) using a method described in detail elsewhere [20]. 137cs activities were determined by low level γ–spectrometry with high efficiency HPGe detectors in MRI, Kanazawa University, university of Bratislava and Iaea-Mel [20]. Intercomparison exercises and regular analysis of Certified Reference Material IAEA–381 (Irish Sea water [21]) were or-ganized between participating laboratories to ensure quality and management of high precision data. recent analytical developments in analysis of very low concentrations of 137cs in underground laboratories allowed us to determine 137cs concentrations in relatively small seawater samples (10 l) with high sensitivity and precision, had not
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been possible before. thanks to these new developments it was possible to reach a high 137cs data density, which allowed us to draw a detailed picture of the spatial and depth distribution of 137cs in the Indian ocean.
3. results and dIscussIon
the distribution of global fallout tritium and 137cs in seawater of the south In-dian ocean along 20 °s is presented in fig. 1. the elevated tritium and 137cs levels in surface waters are well visible, especially in the case of 137cs, where the data density is better. the highest surface tritium levels (0.7 tu) were detected at around 40°e, 60°e and 100°e and the minimum at around 80°e. the tritium transect is thus copy-ing the cross-section of the subtropical gyre [17]. the central part of the gyre, outside of the main streams, should have the lowest tritium concentrations, which is con-firmed by observation of lower tritium values (0.5 TU) at around 80°E. Surface water
FIG. 1. 137Cs (top) and 3H (bottom) in in the water column of the South Indian Ocean along 20°S. Elevated 137Cs and 3H levels are well visible in surface and subsurface waters.
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samples collected in 1998 as a part of the Indian ocean transect [22] also showed higher 3H levels around 30°s, comparable with those presented in fig. 1. also Woce tritium data (1995) shows surface maxima at around 110°e (1.2 tu) and 63°e (1.1 tu), and the surface minimum at 75°e (1 tu).
the 137cs transect along the 20°s latitude also clearly depicts a presence of two cores of higher 137cs concentrations at 100°e and 60°e. the subsurface (100–200 m water depth) maximum of 137cs concentration near 100 °e is the highest level ob-served in southern hemisphere seawater, and of the same level as the maximum 137cs concentration observed in North Pacific surface water [23], as well as in the western subtropical Pacific Ocean [24]. As expected, the intrusion depth of 137cs is deeper in the eastern south Indian ocean stations and shallower in the western stations, in agreement with the main circulation patterns.
the elevated tritium and 137cslevels found in the south Indian ocean must be therefore due to transport of water masses from the western subtropical Pacific Ocean via the Indonesian seas to the south Indian ocean. recent oceanographic measure-ments [25] suggest that aproximately 107 m3/s of water flow from the Pacific Ocean into the Indian ocean through the Indonesian seas. the experimental results have recently been confirmed by modelling exercises carried out using an oceanic general circulation model [26].
the observed tritium and 137cssurface maxima in fig. 1 correspond to the main stream of the Indonesian throughflow to the south Indian Ocean subtropical gyre, which is situated at 20°s and 40°s. due to the circulation of tritium and 137cs in the gyre, and their physical and biogeochemical properties (half-lives over 10 years, both stay in seawater in a dissolved form), elevated concentrations of these radionu-clides are maintained in the subtropical gyre. the gyre acts therefore as a reservoir, accumulating radionuclides in the region on a time scale of several decades. as there is only a weak transport of water masses labelled with 137cs from the south Indian ocean via the agulhas current to the south atlantic ocean [27], the south Indian ocean subtropical gyre acts as a reservoir of dissolved pollutants transported from the Pacific and Indian Oceans, as well as from the Mediterranean Sea [17].
a major contamination of ocean by pollutants has occurred in the northern hemisphere, in which most human activities take place. the obtained results there-fore have important implications for the protection of the marine environment against contamination from land based sources, as well as for climate change studies.
4. conclusIons
the distribution of global fallout tritium and 137cs in the seawater of the south Indian Ocean, along 20 °S, about five decades after its main injection from atmos-pheric nuclear weapons tests on the ocean surface, is presented and discussed. the el-evated tritium and 137cs levels observed at the ocean surface indicate a transport of
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water masses from the subtropical West Pacific via the Indonesian Seas to the Indian ocean. although the differences in tritium concentrations alongside 20 °s are smaller than in the case of 137cs, the tritium transect has similar features to the 137cs transect.
the subtropical gyre in the south Indian ocean has been maintaining higher tritium and 137cs levels in surface and subsurface waters in the gyre. the obtained results have implications for the protection of the marine environment against con-tamination from land based sources, as well as for climate change studies.
ACkNOWLEDGEmENTs
the authors thank the captain and the crew of r/V Mirai for assistance during sampling. this work was supported by a grant from the Ministry of education, cul-ture, sports, science and technology (MeXt) of Japan, and by a grant-in-aid for science research (KaKenHI 18310017). ppp acknowledges support provided by the eu research & development operational program funded by the erdf (project no. 26240220004). the International atomic energy agency is grateful to the gov-ernment of the principality of Monaco for support provided to its Marine environ-ment laboratories.
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