Collaborative Research: GEOTRACES Atlantic Section: Trace Element Sources and Sinks Elucidated by Short-Lived Radium and Thorium Isotopes

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In this project, researchers at the Woods Hole Oceanographic Institution and the University of South Carolina will measure a suite of uranium/thorium series radionuclides on the first U.S.GEOTRACES cruise in the North Atlantic. The goal of GEOTRACES is to identify processes and quantify fluxes that control the distribution of trace elements and isotopes (TEIs) in the ocean, and to establish the sensitivity of these distributions to changing environmental conditions. Thorium and radium radioisotopes are well suited to study the sources and sinks of TEIs on time and space scales that are required to interpret lateral and vertical TEI distributions in the NOrth Atlantic basin. Accordingly, this project will employ the radium ?quartet? (224Ra, 223Ra, 228Ra and 226Ra) to quantify horizontal and vertical transport of dissolved TEIs, as well as the shorter lived thorium isotopes, 234Th and 228Th, used to quantify particle scavenging, vertical fluxes and remineralization rates of bioactive and/or particle reactive TEIs. A wide range of processes will be encountered along the proposed GEOTRACES N. Atlantic leg, and the short-lived U-Th series isotopes proposed herein will play a key role in the interpretation of observed TEI distributions. The utility of the Th and Ra tracers can be used to address two main key processes: boundary inputs of TEIs and TEI particle cycling including scavenging, vertical export and remineralization. To this end, the sampling plan includes: (1) detailed underway sampling of short-lived Ra isotopes (224Ra, 223Ra) at the western and eastern margins to quantify terrestrial inputs and off-shelf TEI transport; (2) upper ocean (0-1000 m) 234Th and 228Th profiling to capture export and remineralization patterns, (3) full water column 228Ra and 226Ra profiles to derive (a) submarine groundwater discharge, (b) midwater transport of TEIs from shelf/slope-basin interactions and (c) diapycnal mixing rates across the thermocline and benthic boundary layer, and (4) near bottom sampling to ascertain TEI sources and sinks associated with abyssal plain sediments and hydrothermal systems. Broader Impacts: The PIs have actively involved undergraduate and high school students in their prior NSF-funded projects, and this activity will continue in this project through the participation of an undergraduate intern. The anticipated timing of the GEOTRACES line is ideal for the student?s participation in the cruise as well as for several weeks back in the laboratory analyzing samples.