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Decoupling of 226Ra and Si in surface waters of the Atlantic Sector of the Southern Ocean

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Hanfland, C. and Rutgers v. d. Loeff, M. (2001): Decoupling of 226Ra and Si in surface waters of the Atlantic Sector of the Southern Ocean , Geochimica et cosmochimica acta, 66(S1), A307. .
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226Ra (half-life 1600 years) has been used as a tracer for ocean circulation and is rated a bio-intermediate element due to its participation in the biogeochemical cycle. Based on the similarity of vertical water column profiles of 226Ra and Si, it has been hypothezised that siliceous tests act as a main carrier phase for 226Ra (e.g. Ku et al. 1970). However, direct evidence for incorporation of 226Ra by silicate-forming plankton is very scanty. Measurements on different plankton species as well as controlled tank experiments yield ambiguous results.Here we present a high-resolution N-S-transect for 226Ra and Si in surface waters of the Antarctic Circumpolar Current (ACC) and the Weddell Gyre (WG) at 20° E. Both elements show a general increase from north to south with the highest values in the region where upwelling Circumpolar Deep Water enters the WG. 226Ra approximately doubles from 8 dpm/100kg to 17 dpm/100kg. Si concentrations reach maximum values of 70 µmol/l south of the Polar Front (PF). The relationship between both parameters is rather weak. The high resolution sampling shows that local variations in the Si concentrations are not mirrored by changes in the 226Ra activity. Particulate uptake of 226Ra continues north of the PF after the near depletion of Si, indicating a decoupling between diatom productivity and the 226Ra distribution in the Southern Ocean.We propose that acantharians, SrSO4-forming radiolarians, might act as a major carrier phase for radium in the upper ocean. They have recently been attributed an important role in the biogeochemistry of Ba (Bernstein et al. 1998). Based on the above observations and the geochemical similarities of Ra and Ba, it seems likely that the biogeochemistry of radium in the upper ocean is equally affected by acantharians as Ba is. For the open ACC, acantharian abundances can reach 30 000 individuals per m3. The continuing depletion of 226Ra after the exhaustion of Si north of the PF could indeed be linked to the increasing importance acantharians gain in the marine productivity cycle in subantarcic and subtropical waters.Bernstein, R.E., Byrne, R.H. and Schijf, J. (1998): Acantharians: a missing link in the oceanic biogeochemistry of barium. Deep-Sea Research I, 45, 491-505.Ku, T.-L., Li, Y.H., Mathieu, G.G. and Wong, H.K. (1970): Radium in the Indian-Antarctic Ocean south of Australia. Journal of Geophysical Research, 75, 5286-5292.

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