COG3

"Thorium and protactinium in the North Pacific Ocean: tracers of biogeochemistry and lithogenic inputs"

Abstract: The particle reactive radionuclides, Th-232, Th-230 and Pa-231, help trace the sources and sinks of trace elements in the ocean. Adsorption to sinking particulate matter, or scavenging, is the main removal term for these metals as well as many other insoluble elements. An important source term for lithogenic elements, such Th-232 or the micro-nutrient Fe, is dissolution of aerosol dust. Both of these processes can be quantified by taking advantage of natural radioactive disequilibria in seawater. New measurements from the North Pacific Ocean are used to show that the scavenging process varies in intensity on the gyre-scale across the subtropical-subarctic front. Fluxes of Th-232 derived from dust dissolution are also calculated and show elevated delivery of lithogenic elements to the surface ocean underlying the Asian dust plume. Some questions about the biogeochemical cycle of Th and Pa are also discussed in the context of the GEOTRACES program, a global survey of trace elements and isotopes in the ocean.