Deep North Atlantic Salinity, Density and Pre-formed Nitrate during the Last Glacial Maximum

University Of Rhode Island, Kingston RI

Investigators

Abstract

Increasing atmospheric CO2 is recognized as one of the world's most pressing environmental challenges. Many details of this challenge require further scientific understanding, including the feedbacks between ocean circulation, atmospheric CO2 and climate. It is generally accepted that CO2 transfer between the ocean and atmosphere has driven the co-variation of CO2 and climate over the past 800,000 years. However, there is no consensus on what drives these transfers principally because there is a lack of data to test proposed mechanisms. The proposed project is directly aimed at understanding the relationship between ocean circulation and climate, particularly atmospheric CO2, during the Last Glacial Maximum (LGM, ~18,000 years ago). The improved understanding of the relationship between ocean circulation and climate is societally important because future changes in the circulation are likely to have large effects, such as changes in the climate of northwest Europe, the position of the Intertropical Convergence Zone (ITCZ) and the uptake of anthropogenic CO2 into the ocean. Scientific understanding of the controls of ocean response to climate change will inform the societal response to anthropogenic CO2. Additionally, this project will support full participation and leadership development of a female Ph.D. student in science. More specifically, the work focuses on laboratory analysis of sedimentary pore fluids collected during a recent research expedition in the North Atlantic, and interpretation of the data. This analytical and interpretative work is aimed at addressing the following hypotheses: - During the Last Glacial Maximum (LGM), deep-ocean density stratification in the North Atlantic was dominated by salinity variation rather than temperature variation. - During the LGM, North Atlantic deep water was a mixture of water originating in the North Atlantic and water originating in the Southern Ocean. - The present relationship between d18O and water density existed during the LGM, enabling use of benthic carbonate d18O to infer density. - The relative balance of the key nitrate (NO3-) removal processes was similar to that of today, as was the average NO3- concentration in deep water. - Deep water in the LGM North Atlantic was dominated by low preformed nutrient water. - A significant fraction of atmospheric pCO2 reduction during the LGM was due to the low pre- formed nutrients in the Atlantic. The analytical work includes, high precision determination of chloride concentrations by titration, the development of a new density based method of paleo-salinity determination and the isotopic analysis (O and N isotopes) of nitrate. This data will be used to constrain inverse diffusion models to infer bottom water properties of the LGM western North Atlantic.