Subsurface warming as a trigger for Heinrich events

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Few discoveries have galvanized the interest of the paleoclimate community more than Heinrich events, which represent the episodic discharge of icebergs from the Hudson Strait Ice Stream of the Laurentide Ice Sheet to the North Atlantic Ocean during late-Pleistocene glaciations. Although commonly attributed to internal ice-sheet instabilities, their occurrence at the culmination of a large reduction in the Atlantic meridional overturning circulation (AMOC) suggests a possible trigger by climate. Models suggest that ocean responses to an AMOC reduction might trigger Heinrich events through warming of intermediate-depth (subsurface) waters, destabilizing grounding lines and ice shelves with attendant ice-stream surging. Evidence for subsurface warming remains widely debated, however, with an alternative hypothesis involving brine formation to explain geochemical signals recorded at intermediate water depths. In this project, a team of geologists, paleoceanographers, and geochemists from Oregon State University will evaluate the hypothesis that Heinrich events were triggered by subsurface warming following a strong reduction in the AMOC. Using a variety or geochemical proxies measured on benthic foraminifera from several intermediate-depth sites in the western and central subpolar North Atlantic, they will establish the timing and amplitude of warming relative to Heinrich events, with particular emphasis on Heinrich event 1. The outcome of this study will be resolution of the otherwise enigmatic climatic context of Heinrich events. This work also extends beyond an explanation of the geologic record by providing a demonstration of the potential for a future response of land ice to changing ocean circulation and temperature, an issue of great concern in projections of future sea-level rise. This work thus has direct societal relevance by developing an improved understanding of the response of ice sheets to current and possible future changes in ocean circulation and temperature, which are among the greatest causes for concern in projections of land-ice loss and consequent sea-level rise. This project will also provide a unique environment for mentoring and training young scientists. The proposal will train a Ph.D. student and recruit an undergraduate student from an underrepresented group through the Increasing Diversity in Earth Science program.