Project: U.S. GEOTRACES Pacific Section: Analysis of 230Th, 232Th and 231Pa

The International GEOTRACES program is a multi-national effort to survey (by a series of cross-ocean sections) and understand the global-ocean distribution of trace elements and isotopes in seawater. The GEOTRACES Science Plan has designated the natural radioisotopes 231Pa and 230Th to be key parameters inasmuch as their measurement on all sections is deemed to be critical to the success of entire international ocean science program

In this project, a research team from the Lamont- Doherty Earth Observatory of Columbia University and the University of Minnesota - Twin Cities will undertake measurement of the dissolved and particulate concentrations of 230Th and 231Pa on the US GEOTRACES Peru-Tahiti section, one of the American contributions to the global GEOTRACES effort. Additionally, they will measure dissolved and particulate 232Th concentrations and analyze a limited number of aerosol samples, aerosol leachates, and surface sediments for these radionuclides. These data will be used to: 1) Quantify the rates of boundary scavenging of 231Pa and 230Th associated with the biologically productive Peru upwelling system, 2) Quantify the rates of bottom scavenging of 231Pa and 230Th thought to be associated with resuspended sediments in nepheloid layers, 3) Quantify the uptake of dissolved Th and Pa by metalliferous particles associated with the hydrothermal plume emanating from the East Pacific Rise as well as by authigenic particles formed within the oxygen minimum zone, 4) Quantify the supply of lithogenic 232Th from margin sediments by the combined study of 232Th and 230Th, and 5) Quantify the supply of lithogenic 232Th from aerosols by the combined study of 232Th and 230Th.

Through collaboration with other US GEOTRACES investigators, the team expects that this work will provide unprecedented constraints on the processes that supply trace elements to the ocean as well as the processes that remove them.