Modeling the Ocean Distribution of Neodymium Isotopes: Testing the Bottom-Up Hypothesis

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Neodymium (Nd) isotopes have been proposed as a tracer of past ocean circulation; however, the processes that determine the distribution of Nd isotopes in the modern ocean are still not well understood. Previous work has assumed Nd is exchanged between the water column and sediments in shallow parts of the ocean near continents, but models applying this process have difficulties reproducing observed distributions of Nd isotopes. This study will use a computer model to test a new idea: the bottom-up hypothesis, which suggests input of Nd from deep sea sediments to the water column everywhere in the ocean, including the deep abyssal plains far away from continents. Simulations of the ocean during the last ice age will be performed to explore the physical and chemical impacts from changes in ocean circulation at this time. This study may transform our ability to reconstruct deep circulation in past oceans, which is important to better understand cycling of carbon and other biologically and climatically important elements. The PIs plan to publish a paper with the model description, user manual, and code, as well as hold webinars for existing user of the model and potentially new users from the GEOTRACES community. The project would include the participation of Dr. Du, an international collaborator. One graduate student and one undergraduate student would be supported and trained as part of this project. The students would be trained in analytical techniques and modeling. Neodymium (Nd) isotopes will be implemented in an established global three-dimensional model of ocean circulation and biogeochemistry. Structurally different model versions will be generated to explore the roles of scavenging, boundary exchange and benthic flux on dissolved and particulate Nd isotope distributions. New measurements of surface sediment Nd isotopes will fill gaps in the existing database, which will be used to model the isotopic composition of the benthic flux. Models will be comprehensively evaluated with GEOTRACES observations to test the hypothesis that a ubiquitous flux of Nd along the sea floor is the dominant source of Nd in the water column. Previous studies assume exchange only along upper ocean margins and conservative behavior in the interior, which suggests Nd isotopes are a water mass tracer without rate information. In contrast, the benthic flux hypothesis implies abyssal flow rate to be an important influence on Nd isotopes. This suggests that Nd isotope measurements from sediments could be used to infer rates of bottom water flow and the abyssal overturning circulation rate. Simulations of the Last Glacial Maximum (~20,000 years ago) ocean will test this idea and evaluate the ability to use Nd isotopes to reconstruct abyssal circulation.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.