A high-resolution study of Thorium isotopes in relation to the cycling of other trace elements in the Southern Ocean
Final Report Abstract
Natural radionuclides are powerful tracers for the study of transport and scavenging (rates, processes) in the ocean. The distribution of many trace elements was measured in high resolution during the Zero and Drake expedition (RV Polarstern, February-April 2008), but the interpretation of these distributions in terms of biogeochemical cycles requires information on particle dynamics. Therefore, in the framework of GEOTRACES, a large international study on the cycling of trace elements and isotopes in the world ocean, we determined the high-resolution distribution of Th isotopes and 231Pa in seawater and sizefractionated particulate samples in parallel to other trace elements (e.g. Nd, Hf, Al, Mn and Fe) along the Zero Meridian and in the Drake Passage. The major objectives of our project were: 1) to study 230Th and 234Th scavenging and Th/Pa fractionation in the water column, 2) to combine the long-lived 232Th with radiogenic isotopes to quantify the supply rate and source of terrigenous particles and 3) to confront 231Pa/230Th distribution with the source provenance signal derived from Nd and Hf isotopes (IFM-Geomar, Kiel). Thereby, within AWI, we developed an analytical protocol for the joint analysis of the radionuclides, Hf and Nd from the same (large-volume) water samples. This joint sampling allowed us to directly apply the information on particle dynamics (adsorption/desorption and particle sinking rates) and terrigenous inputs that we obtained from the distribution of thorium isotopes and 231Pa, to the transport of other tracers (Nd, Hf, Fe, Mn and Al). The results for the Drake Passage are already gathered in a publication. Using both dissolved and size-fractionated particulate 230Th and 234Th data, with a reversible-scavenging model in the mid-depth clear water, we estimated the apparent settling, adsorption and desorption rates that are consistent with the very few published data from other regions of the Southern Ocean. This study pointed out a difference in scavenging between the zones North and the South of the Southern ACC Front (SACCF) at Drake Passage. Comparison of our 230Th and 232Th data in dissolved and particulate phases with Mn and Al data (NIOZ, The Netherlands) and CFC (IUP-Bremen) in the bottom waters close to the Antarctic Peninsula indicated the significant lateral inputs of 230Th-rich deep water from the Weddell Sea and probable sediment re-suspension due to the deep water/topography interactions. The size-fractionated study showed that most of the particulate thorium and protactinium are generally associated with fine suspended particles. The study of the fractionation factor FTh/Pa in the water column showed a North-South decrease that is consistent with what was previously observed further downstream in the southern part of the Atlantic Ocean. We attributed this southward decrease of the fractionation between Th and Pa to the high influence of opal abundance toward the Antarctic. We confronted the results on water mass circulation and upwelling from the 230Th/231Pa distributions with hydrographic data and the conclusions drawn from other tracers described in parallel projects from the same bundle (Nd/Hf isotopes; CFC and He isotopes). The total 230Thxs vertical distributions closely reflect the influence of reversible scavenging and upwelling of deep water from North to South in the studied circumpolar area. The Pa distribution together with the 3He/4He ratio data (IUP-Bremen) at Drake Passage pointed out the different origins of the water masses from the ACC. Finally, this new radionuclide dataset will flow into numerical models of ocean circulation and biogeochemical cycling as are planned in the GEOTRACES1 program to help improving our understanding of the biogeochemical cycles of particle-reactive elements in the ocean.
