Linking high latitude nutrient supply to export productivity and nitrogen cycling in the tropical Pacific across the Plio-Pleistocene

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Recent work has demonstrated the importance of nutrient supply in driving export productivity events in the eastern equatorial Pacific and stimulating denitrification downstream. However, this view is limited to the last 30,000 years. On orbital timescales, variations likely relate to changes in overturning rate in the high latitude Southern Ocean. On million-year timescales, it is hypothesized that the shoaling of the ventilated thermocline, due to global cooling, strengthened the connection between high and low latitude biogeochemistry. This study, led by a researcher from the University of Rhode Island, seeks to understand how variations in nutrient supply, export productivity, oxygen demand, and water column denitification relate to large scale changes in ocean circulation on long timescales. Longer records are essential in order to evaluate the significance of high latitude nutrient supply in driving low latitude biogeochemical shifts, and will provide insight into both the cyclic changes occurring on orbital timescales and longer timescale shifts. The study focuses on two key questions: 1) What is the role of nutrient supply from high latitudes in driving documented episodes of high productivity in the eastern equatorial Pacific?, and 2) What is the role of enhanced export and oxygen demand in modulating water column denitrification in the eastern tropical Pacific? The study builds on available downcore data and add nitrogen isotope and export productivity records from the Southern Ocean and tropical Pacific across the last 3 million years. Records from a series of sites across the tropical Pacific will result in a stacked record of denitrification changes and an estimate of nutrient supply changes in the eastern equatorial Pacific. The Southern Ocean records will be used to estimate preformed nutrient concentrations at the origin of mode waters. The new records will be incorporated into a larger international collaboration that aims to develop a coherent picture of how climate-driven circulation changes in the Plio- Pleistocene impact the global cycling of nitrogen and carbon. This project also represents a first step toward developing a complete Cenozoic history of nutrient cycling and export productivity in the eastern equatorial Pacific.Broader impacts include education and training of a graduate student, public outreach activities, and international collaboration with a French scientist.