Collaborative Research: A porewater perspective on benthic sources of neodymium to the North Atlantic

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Documenting deep ocean circulation changes is important to understanding past climate variability. Neodymium (Nd) isotopes, recorded in deep ocean sediments, have been used to reconstruct past ocean circulation. However, several recent studies suggest that some Nd is released into seawater from the underlying sediment, which would complicate the ability to use Nd isotopes to accurately track deep water in the ocean. This project will investigate Nd in seafloor sediments and near bottom water in the Northwest Atlantic. The study region is a location where North Atlantic Deep Water is formed. Broader impacts include mentoring support to allow two early-career scientists to serve as co-chief scientists for the field program. There is also support for high school students from an institution with a high percentage of students that are underrepresented in STEM, and undergraduates from a minority-serving institution.The interpretive framework of Nd isotopes as a paleo circulation proxy relies on deep ocean signals being driven by mixing between water masses with little influence from benthic sources of Nd to the ocean. It also requires that Nd isotopes are recorded accurately in sediments and endmember composition is independently known through time. Recent studies have challenged this framework, suggesting that benthic fluxes from ocean sediments may exert a primary control on deep water isotopic composition. However, the limited global coverage of direct benthic flux measurements (from only 10 published locations primarily in the North Pacific) prevents a complete understanding of the marine Nd cycle. This project will (i) quantify the fluxes of Nd to bottom water from sediments, (ii) examine how those fluxes affect the isotopic composition of North Atlantic Deep Water, (iii) investigate the role of diagenetic reactions in setting pore water rare earth element geochemistry, and (iv) determine what isotopic signals are recorded in marine archives. The field component of this project will occupy sites along two depth transects in the Labrador Sea, and it will involve collecting sediment and pore water from short cores, corresponding filtered bottom water, and deep-sea particles, in order to fully describe the system. The study location will allow for investigation of the role that regional geology and water depth play in setting the benthic flux of Nd to seawater. Altogether, this project will be one of the most thorough investigations to date of the controls on benthic Nd flux, the impact of benthic fluxes on the Nd isotopic composition of North Atlantic Deep Water, the sedimentary environmental conditions that facilitate Nd release, and the impact of these processes on authigenic Nd isotope records.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.