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.
One of the main goals in oceanography is to understand why the ocean has its particular chemical makeup and how various elements enter and leave the ocean. In this project, we study how a particular group of elements is transported within and removed from the ocean. As it turns out, most of the oceans have the same amont of uranium regardless of depth or geographic location. Because of the constant uranium concentration, a number of elements are produced from uranium in the ocean at constant and known rates. These include varieties of the elements thorium (thorium-230) and protactinium (protactinium-231). In this study, we analyzed the concentrations of these varieties of these elements in seawater collected from across the north Atlantic Ocean from Europe and north Africa to the East Coast of the U.S. Because they are produced at a known rate, our measurements give us information about how they are transported from one portion of the ocean to another and how they are removed from the ocean. In addition, we measured concentrations of another variety of the element thorium (thorium-232). This variety of thorium is contained within dust blown onto the ocean from the continents (for example from the Sahara desert) and within sediments such as small clay particles transported to the ocean in rivers. Measurements of this variety of thorium gave us information about the rate at which thorium has been added to the ocean from dust and sediments. In order to carry out the project, we needed to refine our skills in measuring these elements because their concentrations in sea water are vanishingly small. For example, imagine you collected a bucket of sea water and wanted to concentrate the protactinium in that bucket to high levels. Suppose you took one millionth of that bucket of water, which contained every atom of protactinium in that bucket. You would not be anywhere close to having a concentrated solution of protactinium. You would need to take one millionth of that one millionth (containing all the original protactinium), then one millionth of that one millionth (containing all the protactinium) before you would have a concentrate of the protactinium contained in the original bucket. In the course of this project, we refined our measurement skills to do just that. We were then able to carry out the project successfully. Once we refined our measurement skills and measured hundreds of sea water samples, we were rewarded by a remarkable picture of the concentrations of the varieties of these elements across the Atlantic. Illustrated in the accompanying diagram is one of these pictures (for thorium-232). Just from a visual perusal of the diagram, one can see the main outcomes/conclusions of our work. High concentrations near the surface, particularly off of Africa are indicative of input from dust blown over the ocean by winds. High concentrations along the seafloor are from sediments on the ocean bottom brought up into the deep ocean by currents along the seafloor. Low concentrations above the Mid-Atlantic Ridge (the high point in the seafloor in the middle of the picture) are caused by small particles that incorporate thorium within them, then settle to the sea floor. These particles form when water from submarine hot springs (along the Mid-Atlantic Ridge) mix with seawater. Low concentrations in deep waters along the African coast are caused by sinking particles that carry thorium on their surfaces, effectively removing it from the ocean and burying it in the accumulating sediments of the sea floor. This picture and similar ones for other elements tell us a lot about the cycles of pertinent elements in the ocean, just at first glance. However, these measurements will merit careful attention and further analysis in coming years. They will further our understanding of how the ocean works.
