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.
Seawater contains many chemical constituents beyond the most common salt, sodium chloride. Study of these elements allows us to examine many important features of the ocean including its capacity to store carbon and to transport heat. In this project four collaborating labs - Woods Hole Oceanographic Institution, University of Rhode Island, University of Minnesota and Lamont-Doherty Earth Observatory - undertook measurements on a set of samples collected during the first two cruises of the US GEOTRACES program in the North Atlantic (www.geotraces.org/). In this part of the overall GEOTRACES program we analysed radioactive elements including thorium (230Th) and protactinium (231Pa). Both of these isotopes are produced from the radioactive decay of uranium. Since uranium is evenly distributed in the ocean we can calculate how much of each isotope is being formed during any given period of time. In principle the rapid removal of thorium from seawater onto falling particles, coupled with this predictable formation rate allows us to test hypotheses regarding ocean circulation, particle composition, and particle flux that influence the redistribution of particle-associated elements, including carbon, trace metals and contaminants, within the Atlantic. The second isotope 231Pa is also rapidly removed fro seawater, but at a slightly lower rate such that the ratio of these two isotopes contain additional information on the rates of oceanic processes. Coupling these two analyses is proving important in understanding the modern ocean, as well as the ocean in the past. In addition, to our 231Pa and 230Th analyses we also measured the concentrations of thorium-232 across the Atlantic. This isotope of thorium enters the ocean very differently than 230Th and 231Pa. It is contained in rocks and clays on the continents and is carried out to the ocean by the winds as dust, and in the rivers as suspended material. It is extremely difficult to make accurate measurements of wind-blown dust– the oceans are vast, and dust storms and river output are variable both in space, time and intensity. It is important that we quantify these inputs accurately, however, since it is through dust fertilization that parts of the ocean are biologically productive. In this project we have generated some of the highest resolution analyses of 232Th in seawater to date. These data are giving us information about the average rate at which thorium has been added to the ocean from dust, rivers and seafloor sediments. The science that we carried out in this proposal is central to the International GEOTRACES program, whose focus is on the global-ocean distribution of trace elements and isotopes in seawater. Increasing this understanding is critical to predicting what may happen to the chemistry and biology of the oceans in a changing world. The proposed research has provided new opportunities for a graduate student a number of post doctoral researchers. It has also enhanced ocean science interactions between several US institutions and fostered collaboration with international GEOTRACES colleagues. Publications and databases produced as a result of this research: Anderson RF, MQ. Fleisher, Robinson LF, R. L Edwards, J A Hoff S. B Moran, M Rutgers van der Loeff, AL. Thomas, AL. Thomas, R Francois (2012) Intercal: Intercalibration of 230Th, 232Th, 231Pa and 10Be GEOTRACES Limnology and Oceanography Methods Auro, M.E., Robinson, L.F., Burke, A., Bradtmiller, L.I., Fleisher, M.Q., Anderson, R.F., 2012. Improvements to 232-thorium, 230-thorium, and 231-protactinium analysis in seawater arising from GEOTRACES intercalibration. Limnol. Oceanogr. Meth. 10, 464-474 GEOTRACERS Intermediate data product: www.geotraces.org/dp/idp2014
