Award: OCE-1232669

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. The goal of this effort was to measure naturally occurring isotopes of radium and thorium. By measuring their concentrations in the ocean, we can quantify rates of both horizontal (with radium) and vertical (with thorium) processes that impact trace element distributions in the ocean. This is important, as while the distribution of numerous trace elements will be mapped by other GEOTRACES scientists, their distribution cannot be properly interpreted without concurrent measurement of tracers capable of providing rates of internal elemental cycling processes and fluxes at boundaries and across interfaces. For radium, the idea is that radium is largely soluble with source at the sea floor and ocean margin sediments. Its transport via largely horizontal physical processes will lead to a decrease in its concentration from the source. Given the half-life of each of the four radium isotopes (days to >1,000 years), we can estimate time scales of horizontal mixing. Thorium isotopes in contrast are more particle-reactive, so found associated with marine particles that are both suspended and sinking. Their sources are the dissolved uranium isotopes. Any loss of thorium on sinking particles (a so called U/Th disequilibrium) can be used to quantify the flux of thorium on sinking particles. Using ratios of thorium to other trace elements on the same particles, we can then determine the sinking flux of trace elements. This particular GEOTRACES sampling took place between Peru and Tahiti in October/November 2013, resulting in the collection by this team of 1000?s of samples for the shorter-lived 234Th and 228Th as well as the radium "quartet" (224Ra, 223Ra, 228Ra, 226Ra). The cruise track was selected to include gradients in productivity (higher near shore), changes due to an extensive oxygen minimum zone (OMZ) and impacts due to hydrothermal activity. Our results have shown across these gradients, higher particle export near the margins and rapid remineralization (loss of sinking particles) just above the OMZ . In addition, the distribution of 228Ra displays high activities in the first 300 m up to approximately 115°W, suggesting lateral advection, and close to the bottom due to the radium diffusion from deep sediments. Radium isotopes are also being used to constrain the time scale of transport of the neutrally buoyant hydrothermal plume observed along the east Pacific Rise. 223Ra, 228Ra, and 226Ra are all elevated in the plume, while any hydrothermal signal of 224Ra is obscured by a large benthic source at this station. The ratio of 223Ra:226Ra has been used to estimate that the age of the plume at a station near the vents to be on the order of weeks, suggesting that the vent is likely 5-10 km away. The radium isotopes will also help determine the age of the plume farther away from the ridge axis, which will be valuable in determining the transport rates of hydrothermally influenced trace elements in the deep Pacific ocean. We note that this project supported three Ph.D. students, two from MIT-WHOI PhD program and one as a visiting student from France. Several talks at conferences and workshops have been given, peer reviewed publications produced as well as one patent for new marine sampling system for collection of trace metal clean particles and dissolved components. Finally all data are made available through the GEOTRACES program office and the Woods Hole BCO- Data Management Office. Last Modified: 08/29/2016 Submitted by: Ken O Buesseler