Award: OCE-0925158

This project is part of a larger program called "GEOTRACES" (http://www.geotraces.org), which seeks to identify processes that control the distribution of trace elements in the ocean. As the first U.S. contribution to the international GEOTRACES program, the research cruise focused on the North Atlantic Ocean between western Europe and the eastern United States. Our contribution to this effort was to measure a suite of naturally occurring radionuclides, the isotopes of radium and thorium. The main value of these radionuclides is that they can provide rates of important ocean processes. Radium isotopes can serve as indicators of trace element inputs at ocean boundaries (e.g. land-ocean and hydrothermal vent-deep sea). Thorium isotopes can be used to quantify trace element removal from the ocean associated with marine particle transport processes. We made several key contributions to the program through our thorium and radium isotope measurements made in parallel with the major GEOTRACES trace elements. Near the sea floor, both at the ocean margin and over the abyssal plain, we found that benthic nepheloid layers (BNLs; particle-rich water masses) were more prevalent and vertically extensive than previously thought and had a measurable impact on the distribution of sediment-associated trace elements. For elements that are particle seeking, BNLs could represent an important removal process from the deep ocean and along ocean margins. For trace elements that are mobilized from particle surfaces and sediment pore spaces, BNLs could be an important oceanic source term. In the upper ocean, we used thorium and radium to construct a budget for the trace element iron along the West African margin. We learned that ocean currents could supply more iron (a key nutrient for plankton growth) to the surface waters of the eastern Atlantic Ocean than dust, which is commonly believed to be the major source of iron to the open ocean. We also found that sinking particles carried a large fraction of the iron supply to the deep ocean, away from the sunlit upper ocean where phytoplankton can utilize it. Lastly, we note that this project supported one Ph.D. student and one postdoctoral investigator. We also contributed to international capacity building by hosting a Ph.D. student from China in our labs for one year. He also participated in the 2010 cruise from Portugal to Cape Verde. Our modifications to a commercially available oceanographic sampling system led to a pending patent, which we expect will be licensed and internationally marketed by a local company. Last Modified: 05/29/2013 Submitted by: Matthew A Charette