Award: OCE-1433150

This grant funded a research cruise to collect samples that we are using to understand the relationship between climate, ocean circulation, ocean biology and atmospheric carbon dioxide. Specifically, we investigated why atmospheric CO2, which is highly correlated with climate, varied between the last glacial period and the modern preindustrial time. The improved understanding of the relationship between ocean overturning circulation and climate is societally important, because future changes in it are likely to have large effects, on global climate, such as changes in the climate of northwest Europe, the position of the Intertropical Convergence Zone (ITCZ) as well the uptake of anthropogenic carbon dioxide. We successfully achieved our primary goal of collecting sedimentary porewaters and making geochemical measurements of sufficient fidelity to prove the porewater?s utility in unraveling the ocean chemistry of bottom water during the Last Glacial Maximum. Using this data we have shown that the low atmospheric carbon dioxide of the last glacial maximum was not principally due to the global increase biological productivity (driven by iron fertilization) in the ocean as has been hypothesized. Alternatively, we argue that due to the shoaling of the of the deep waters which sink in the North Atlantic (NADW) and expansion of the waters that sink to depth in the Southern Ocean (AABW), there was an increase in nutrients in the AABW cell of the overturning circulation and a decrease in the NADW cell (a result of the AABW becoming a more efficient nutrient trap). This, combined with iron fertilization led to greater biological productivity in the Southern Ocean but no significant change in unused nutrients (since total nutrients were higher). As shown by Ito and Follow (2014), higher Southern Ocean productivity is expected to lead to the sequestering of carbon dioxide in the deep ocean via gas exchange disequilibrium. In summary, we argue, that the lower glacial carbon dioxide resulting from increased productivity in the Southern Ocean was the sequestration of disequilibrium carbon dioxide with the proximal cause being Fe-fertilization but that this did not increase the overall whole ocean efficiency of nutrient use. This project supported Ph.D. student, Kira Homola? materials collected during the cruise will be the focus of her thesis research. Eleven graduate students (from GSO, MIT/WHOI, TAMU, BU, Montana State, & Caltech) participated in the cruise as shipboard scientists. Three undergraduates from Indiana University of Pennsylvania also participated as shipboard scientists. Last Modified: 07/26/2018 Submitted by: Arthur J Spivack