Collaborative Research: U.S. GEOTRACES North Atlantic Section: Analysis of 230Th, 232Th and 231Pa

DESCRIPTION: In this project four collaborating labs -- Woods Hole Oceanographic Institution, University of Rhode Island, University of Minnesota and Lamont-Doherty -- will undertake measurements on the US GEOTRACES North Atlantic section of the dissolved and particulate concentrations of 230Th and 231Pa, two isotopes designated as "key" or critical to the success of the GEOTRACES program. In addition, they will measure 232Th concentrations and provide supporting analyses of particle composition. The proposed work fulfills scientific objectives defined in the U.S. GEOTRACES North Atlantic Implementation Plan.

These new 231Pa, 230Th and 232Th data are expected to lead to improved models of material exchange between the boundaries and interior Atlantic on a basin-wide scale. This project will permit the research team and the marine radiogeochemical community to test hypotheses regarding the relative importance of regional variations in circulation, particle composition, and particle flux that influence the redistribution of particle-associated elements, including carbon, trace metals and contaminants, between the continental shelf/slope regions and interior basins in the Atlantic. Furthermore, obtaining new water column 231Pa and 230Th data will allow testing of a key hypothesis: that 230Th is largely removed within the North Atlantic, whereas nearly half of the 231Pa produced in-situ is exported with North Atlantic Deep Water. Sedimentary 231Pa/230Th ratios have become the principal proxy used to evaluate past changes in Atlantic Meridional Overturning Circulation (AMOC). Completing the proposed work will characterize the importance of particle composition for 231Pa/230Th fractionation, and thereby constrain the potential bias in using 231Pa/230Th as a kinematic proxy for AMOC. These results, in turn, will lead to a more reliable assessment of the role of AMOC variability in past climate change.

Broader Impacts The hypotheses and objectives outlined in this proposal are central to the International GEOTRACES program, whose focus is on the global-ocean distribution of trace elements and isotopes in seawater. 231Pa and 230Th are designated key parameters in the GEOTRACES Science Plan as their measurement on all sections is deemed to be critical to the success of this international ocean science program. The proposed research will provide new opportunities for graduate student (LDEO) and post doc (UMinn) research experience, enhance ocean science interactions between several US institutions (LDEO, URI, UMinn, WHOI), foster collaboration with international GEOTRACES colleagues, and contribute to existing science education programs designed for teachers and the public.

OUTCOMES: LDEO has completed nearly all sample analysis, the University of Minnesota has completed 2/3 of their samples. WHOI has developed a new method to dissolve filters without using perchloric acid. One Journal article has been submitted.