Collaborative Research: US GEOTRACES GP17-OCE and GP17-ANT: Export and remineralization rates of bioactive and particle reactive trace elements using thorium-234

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The overarching goal of the international GEOTRACES Program is to “identify processes and quantify fluxes that control the distributions of key trace elements and isotopes in the ocean, and to establish the sensitivity of these distributions to environmental conditions.” These trace elements exist at exceeding low concentrations in the ocean, yet play a key role in the growth and distribution of marine organisms, in particular marine phytoplankton. In addition, many trace elements are important to study as pollutants, such as copper and lead, which at high levels can be harmful to marine life. Thus, studies of the trace metal sources and sinks are needed. To meet this goal, the project is designed to measure how quickly these trace elements are cycled through marine food webs, in particular as they are transported by marine snow, i.e. slowly settling marine particles that carry these trace elements and carbon to the deep sea. The work will be conducted aboard two upcoming US GEOTRACES expeditions to the South Pacific, Southern Ocean, and Amundsen Sea. The project will provide training for two postdoctoral investigators, one focused on sample analysis and one on the modeling. To carry out this study, this proposal will use the naturally occurring particle reactive radionuclide Thorium-234 (234Th, half-life = 24.1 d) to quantify variability in trace elements and isotopes scavenging, residence times, and particle export from the surface ocean and their attenuation into the deep sea. This project will sample across extreme biogeographical and trace elemental gradients that range from the clear waters of the low dust, low productivity South Pacific (GP17-OCE) to the “greenest” high productivity Amundsen Sea and its polynyas in the Southern Ocean (GP17-ANT). Assessing these spatial and vertical gradients requires 234Th sampling at every station (n=66 casts x 13 depths) and measuring the TEI/234Th ratio on particles (n=43 x 8 depths x 2 size classes). There results of this project will lead to an increased understanding of how the trace elements are modified by removal and regeneration associated with scavenging on to, and remineralization off of, sinking particles.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.