Award: OCE-1231211

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 margins. 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. This particular GEOTRACES sampling took place between Peru and Tahiti in October/November 2013 to characterize the distributions of 228Th and the radium "quartet" (224Ra, 223Ra, 228Ra, 226Ra) through the water column. 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. We can compare our results for 226Ra with results from the GEOSECS program made 40 years earlier. The close agreement for stations located near each other provides a direct inter-comparison with this earlier study. Our results for 228Ra show that its distribution displays high activities in the upper 300 m from the coast to approximately 115°W, suggesting rapid lateral advection. High activities close to the bottom are due to 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. The isotopes 223Ra, 228Ra, and 226Ra are all elevated in the plume, but any hydrothermal signal of 224Ra is obscured by a large benthic source at this station. The activity ratio of 223Ra:226Ra shows that the age of the plume at a station near the vents is 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. Several talks at conferences and workshops have been given and peer reviewed publications produced, Finally all data are made available through the GEOTRACES program office and the Woods Hole BCO- Data Management Office. Last Modified: 12/02/2016 Submitted by: Willard S Moore