Barium/calcium ratios in non-spinose planktic foraminifera: a novel proxy for reconstructing paleo-productivity

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This project will test a novel hypothesis that deeper dwelling planktic foraminiferal barium-to-calcium ratios record surface-ocean productivity. The research will quantify the link between the barium geochemistry of deeper-dwelling planktic foraminifera (N. dutertrei and P. obliquiloculata) and established paleoproductivity proxies in marine sediments. Because export productivity is directly linked to carbon sequestration from the atmosphere and varies with climate cycles such as the El Nino-Southern Oscillation, it is a key parameter required to establish carbon cycle variability in the past ocean. A planktic foraminiferal derived productivity proxy will be particularly useful in areas where other productivity proxies are less reliable or compromised, such as in areas with low sediment accumulation rates and/or low preservation of biogenic minerals, or in coastal settings where sediments become anoxic and organic carbon accumulation rates can be biased by terrestrial sources. The broader impacts of this research include improved utility of foraminiferal calcite based climate reconstructions and the establishment of a new proxy for export productivity. The project will provide support for an early career female researcher, a graduate student, and two undergraduate students. The principal investigator (PI) will utilize her position as a Science Communication Fellow at the Oregon Museum of Science and Industry to provide public outreach.Laboratory culture experiments confirm that N. dutertrei record seawater Ba/Ca ratios during calcification. Yet, published records show that Ba/Ca is unusually high in fossil specimens of the deeper dwelling planktic foraminifera species, including N. dutertrei. The Ba/Ca ratio in fossil specimens far exceeds that which can be explained by seawater Ba/Ca ratios alone and may be due to calcification in microenvironments enriched in Ba, such as marine snow. The PI will evaluate the hypothesis that Ba/Ca in deep dwelling taxa is related to export production from surface waters by generating downcore Ba/Ca data in a suite of sediment cores that span a productivity gradient across the equatorial Pacific, compare the Ba/Ca records to existing paleo-productivity data spanning the time interval from the last glacial maximum to present from the same cores, and assess the fidelity of the proxy by completing reconstructions in well-preserved and poorly-preserved sediments. While preliminary data indicate that fossil foraminifer Ba/Ca ratios are lowered by dissolution, these data suggest that Ba/Ca ratios still reflect productivity even in the samples most heavily affected by dissolution. In addition to generating downcore Ba/Ca records, the PI will generate Mg/Ca records and stable isotope records (oxygen and carbon) from the same samples. The Mg/Ca data for N. dutertrei will be used to test a newly developed Mg-based temperature calibration for N. dutertrei. This research seeks to expand the utility of the paleoceanographically important, but underutilized, non-spinose planktic foraminifera species Neogloboquadrina dutertrei and Pulleniatina obliquiloculata and develop a new foraminifer-based proxy for productivity. High-resolution analytical techniques (laser ablation-ICP-MS, isotope ratio mass spectrometry, and solution based-ICP-MS) will be used to generate the downcore trace element and stable isotope records in this study.