Award: OCE-0927064

Trace elements such as iron, zinc and cobalt serve as essential nutrients that influence the fertility of marine ecosystems. Trace elements such as lead, mercury and copper also occur as contaminants in the ocean due to human activities. Finally, trace elements, and their isotopes, serve as tracers for processes that we cannot observe directly, such as ocean circulation and the delivery of freshwater to the ocean via submarine groundwater discharge. The international GEOTRACES program was designed to investigate the marine biogeochemical cycles of trace elements and their isotopes. In other words, investigators seek to determine the sources of trace elements at ocean boundaries, including the interfaces with the atmosphere, the coast, and the sea bed, as well as to establish the processes that remove trace elements from the ocean, including active biological uptake by organisms and passive sorption to particles that settle from the sea surface into the abyss. This project supported our work in the context of the U.S. GEOTRACES study of trace elements and their isotopes in the North Atlantic Ocean. Our project uses naturally occurring radionuclides to determine locations of trace element supply and removal, while using radioactive disequilibria between soluble uranium isotopes and their insoluble decay products (thorium and protactinium) to determine the rates of those processes. We found evidence for: 1) Boundary scavenging - enhanced removal of trace elements in regions of high biological productivity and particle flux off the coast of NW Africa. 2) Enhanced scavenging and removal within the extended hydrothermal plume emanating from the Mid Atlantic Ridge. 3) Enhanced scavenging and removal within benthic nepheloid layers, where resuspended particles provide abundant mineral surfacess for adsorption and removal of trace elements. 4) The predominant scavenging (removal) of dissolved Th and Pa by sorption to lithogenic minerals and to calcium carbonate. Particulate organic matter has an order of magnitude affinity for sorption of these phases than we found for lithogenic and carbonate particles. Opal was not an important particulate phase removing these tracers simply because of its very low abundance in the tropical North Atlantic. We are also interpreting the combined distributions of Th-232 and Th-230 to provide estimates of: 1) The rate of supply of trace elements from dissolution of mineral aerosols in surface waters, and 2) The rate of supply of trace elements by exchange with sediments at ocean margins. Results from our collaborative project were included in eGEOTRACES, an electronic atlas of selected static sections and 3-D animations created to visualize preliminary results from the GEOTRACES program. eGEOTRACES is publicly available at http://egeotraces.org. Last Modified: 06/13/2015 Submitted by: Robert F Anderson