Determination of naturally occurring radionuclides in the Southern Ocean during GEOTRACES program

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Hanfland, C. and Friedrich, J. (2004): Determination of naturally occurring radionuclides in the Southern Ocean during GEOTRACES program , SCAR Open Science Conference, 26.-28.7.2004, Bremen, Germany. Terra Nostra Abstract Volume of XXVIII SCAR Meeting .
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The Southern Ocean is considered to play a key-role in the modern climate and is often quoted as an early-warning system for the worlds climate as a whole. Processes that govern the polar environment today are, however, far from being understood satisfactorily. Yet a solid knowledge of biogeochemical interactions serves as a basis for both reconstruction of the palaeoclimatic conditions as well as reliable predictions of future trends. In this respect, radionuclide studies can yield valuable additional information, their specific half-lives providing valuable time-relevant information and their difference in reactivity allowing for a wide range of applications from water mass characterization to particle studies.During the seventies, a global geochemical survey was carried out in order to study the three-dimensional distribution of various tracers in the worlds oceans (Geochemical Ocean Sections; GEOSECS). From the group of naturally occurring radionuclides, several isotopes were investigated. With better techniques available, concerning both sampling and analytical protocols, a new initiative has been started (GEOTRACES) which aims to determine the global distribution as well as to evaluate the oceanic sources, sinks and the internal cycling of selected tracers. The overall goal of this effort is a better understanding of global biogeochemical cycles.We present a set of stably dissolved (226Ra, 228Ra, 227Ac) and particle-reactive radionuclides (234Th, 210Po), all being short-lived products from the U-Pb and Th-Pb decay chains, that are especially valuable for oceanographic and biogeochemical investigations.The supply of the rather mobile elements to the water column is mostly by diffusion from sediments through decay from a particle-reactive parent while their distribution in the water column is governed by their respective half-lives. 228Ra is indicative for shelf water input whereas 227Ac is used as a tracer for deep waters. Hence, the combination of both allows for a better distinction of deep upwelling versus lateral input of water masses.Particle-reactive isotopes come into application in the study of particle dynamics, e.g. for the determination of the export production. 234Th and 210Po represent highly particle reactive radionuclides that are produced in situ in the water column at a known rate from rather conservative parents (234Th from 238U, 210Po via 210Pb from 226Ra). In productive areas of the ocean, both 234Th and 210Po are rapidly taken up by particles from surface water.Topics of interest within the Atlantic sector of the Southern Ocean include:- the tracking of water masses within the Agulhas Retroflection Area, a highly dynamic region with inter-ocean water exchange between the Indian and the Atlantic Ocean- a better understanding of the dynamics associated with the eastern Weddell Gyre, where upwelling North Atlantic Deep Water accounts for nutrient-rich surface waters- the study of biogeochemical processes around subantarctic islands, i.e. their potential of releasing micronutrients into the passing Antarctic Circumpolar Current and the fate of organic particles produced in the wake of these islandsWe will present first results connected with these questions and indicate further needs for future investigations.

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