Trace element cycling and fluxes in the South Indian Ocean – a contribution to GEOTRACES

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We propose to apply isotopes of radium (Ra), thorium and helium (He) to determine in detail the sources, fluxes and sinks of trace elements (TEs: Cd, Co, Cu, Fe, Mn, Mo, Ni, V, Zn) in the water column along a 23°S zonal section in the South Indian Ocean (SIO), one of the least studied ocean regions on earth. The application of isotopes of radium (224Ra, 223Ra, 228Ra 226Ra,), thorium (234Th 232Th,), helium (3He, 4He) will facilitate the study of the biogeochemical cycles of TEs, and given that some TEs act as micronutrients, allow an assessment of their interactions with surface ocean productivity and the carbon and nitrogen cycles. Combining measurements of TEs with Ra and 234Th isotopes as tracers for horizontal and vertical fluxes, 232Th as a tracer for dust inputs, and He isotopes as a tracer for hydrothermal inputs, we will investigate and quantify the supply pathways of TEs to the SIO from the atmosphere, continent (mainly the Zambezi River), sediments (on continental slopes), and ocean crust (hydrothermalism of Indian Ocean Ridge). The study will be based on samples collected during FS Sonne cruise SO-276 scheduled for July – August 2020 in the SIO from South Africa to Australia. Our investigations will be part of the internationally coordinated program GEOTRACES and will contribute to the Second Indian Ocean Expedition Program (IIOE-2). The outcome of this study will have global significance for understanding the chemical environment in which ecosystems operate.