A high-resolution study of Thorium isotopes in relation to the cycling of other trace elements in the Southern Ocean

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Natural radionuclides are powerful tracers for the rate of transport processes in the ocean. In the framework of GEOTRACES, a large international study on the cycling of trace elements and isotopes in the world ocean with extensive activities in the International Polar Year, we propose to determine the high-resolution distribution of Th isotopes, 231Pa and 227Ac along the Zero meridian and in the Drake Passage. The sampling has taken place in coordination with sampling of other trace elements by an international team of geochemists on an expedition with RV Polarstern. This joint sampling allows us to directly apply the information on particle dynamics (aggregation, disaggregation and particle sinking rates) and terrigenous input that we will obtain from the distribution of thorium isotopes and 231Pa, to the transport of other tracers. Similarly, we will be able to confront the results on water mass ventilation and upwelling, as we will derive from 230Th/231Pa and 227Ac distributions, with hydrographic data and the conclusions drawn from other tracers described in parallel proposals (Nd/Hf isotopes; freons and He isotopes). The data will be interpreted along with other tracer data in (inverse) GCM models. We expect that this approach will improve our ability to use a set of tracers as more reliable proxies for past ocean climate.