Millennial-scale atmospheric CO2 variability during the last deglaciation: Testing the biological pump hypothesis using upper ocean carbon isotope records

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The largest increase in atmospheric CO2 in the recent geologic past occurred during the last deglaciation (from 20,000 to 10,000 years ago). As ice sheets in the Northern Hemisphere melted, atmospheric CO2 levels rose by ~30%. Despite extensive research, the underlying cause of the CO2 change has yet to be fully explained. Recent ice core results show that the CO2 rise can be traced to an isotopically light carbon source (i.e. carbon with a low 13C/12C ratio). One possible cause is reduced marine photosynthesis, which would result in the accumulation of light carbon in the surface ocean and atmosphere. Less photosynthesis would also reduce the amount of low 13C/12C carbon that falls from the surface ocean to intermediate depths in the form of organic matter. Thus, reduced marine biological productivity should yield opposing carbon isotopic signals at surface and intermediate depths, with the surface ocean becoming isotopically lighter (lower 13C/12C) and intermediate depths becoming heavier (higher 13C/12C). The results of this work could transform our understanding of the climate system by constraining the source of carbon isotopic signals and therefore the driver of atmospheric CO2 variability during the last deglaciation. The broader educational impact of the proposed work includes the participation of a graduate student and an undergraduate at the University of Connecticut.The primary goal of this project is to use data from multiple locations to assess changes in marine photosynthesis during the last deglaciation. The research team will use high resolution sediment cores from five different locations, including the Southwest Atlantic, the Southeast Atlantic, the Southern Ocean, and Southwest Pacific, and the eastern tropical Pacific. Each core was retrieved from intermediate (~1000 m) depths that should be sensitive to the amount of organic matter falling through the water column. The team will assess variations in photosynthesis by analyzing the carbon isotopic ratio in the fossils of benthic (sea floor dwelling) and planktonic (surface ocean dwelling) organisms. The team will also analyze the ratio of boron to calcium in the benthic fossils to infer overall changes in carbon concentration at intermediate water depths.