Collaborative Research: U.S. GEOTRACES Arctic Section: Thorium-230, Thorium-232, and Protactinium-231 tracers of trace element supply and removal.

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In support of the 2015 U.S.GEOTRACES Arctic expedition, this project will focus on the fate and distributions of naturally-occurring radioisotopes in the Arctic Ocean. Such information is useful for understanding why other chemical substances, both natural and man-made, occur where they do in the ocean. Like other national initiatives involved in the International GEOTRACES Program, the goals of this U.S. Arctic expedition are to identify processes and quantify fluxes that control the distributions of key trace elements and isotopes (TEI) in the ocean, and to establish the sensitivity of these distributions to changing environmental conditions. Working at sea alongside a multi-institutional team of ocean trace element experts, investigators on this project will define regions of unusually high rates of TEI removal, anticipated to be located near basin margins and near the sea floor. By combining their measurements of naturally-occurring thorium and protactinium with TEI data collected by other participating investigators, they expect to be able to translate these rates into information that can be applied to other TEIs. Like most other participating investigators, this group will include graduate students as part of the research team and will participate in a variety of public educational outreach activities for Alaskan communities.This study will undertake measurements of the dissolved and particulate concentrations of 230Th and 231Pa, two isotopes designated as key or critical to the success of the GEOTRACES program. Additionally the team will measure dissolved and particulate 232Th concentrations and analyze a limited number of aerosol samples, aerosol leachates, sea ice, melt pond water and surface sediments for these radionuclides. The work plan will be broken down into five tasks geared to: (1) determine the rates of boundary scavenging of 231Pa and 230Th associated with the particle-rich waters near the southern margin of the Canada Basin; (2) determine the rates of bottom scavenging of 231Pa and 230Th associated with nepheloid layers that are prevalent in the Arctic Ocean; (3) assess the contribution to scavenging in the Canada basin by MnO2-coated particles, formed during early diagenesis in organic-rich sediments surrounding the Arctic Ocean; (4) determine the rate of supply of lithogenic 232Th from margin sediments using information derived from 230Th; and (5) determine the rate of supply of lithogenic 232Th from sea ice, including the aerosols and ice-rafted sediments that they transport, by the combined study of 232Th and 230Th. The proposed work fulfills core scientific objectives defined in the U.S. GEOTRACES Arctic Implementation Plan.