Late Cretaceous Osmium Isotope Stratigraphy

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ABSTRACTOsmium isotope analyses will be performed on a suite of marine sediment samples in order to reconstruct seawater 187Os/188Os variations during the Maastrichtian and late Campanian. This sample suite will provide the global coverage required to determine empirically whether the resulting composite Os isotope record is representative of whole-ocean changes in 187Os/188Os. The specific study sites will include both outcrop samples collected from classic Scaglia Rossa sections in the Italian Apennines, and samples from archived ODP/DSDP material recovered from the Western Pacific, South Atlantic and Southern Ocean. Os isotope analyses will be performed by single collector magnetic sector ICP-MS at the University of Hawaii. This high through-put method is essential for this study in order to achieve the high temporal resolution required to resolve the influence of specific geologic events. Preliminary results reported in the text of the proposal demonstrate the viability of this approach, and indicate that there is a step-wise decline of approximately 50% in sediment 187Os/188Os ratios during the last 3 million years of Maastrichtian time, prior to the K-T boundary event. The specific phenomena this study focuses on are (1) emplacement of the Deccan Traps in the latest Maastrichtian and (2) a putative eustatic sea-level fall in the early Maastrichtian. Numerous stable isotope studies of benthic and planktic forams reveal evidence of a brief episode of global warming 200-300 thousand years prior to the K-T boundary. Several investigators suggest that this warming is caused by Deccan volcanism. The marine 187Os/188Os ratio is expected to decline as a result of unradiogenic, mantle-derived Os (low 187Os/188Os) associated with Deccan volcanism. By directly comparing Os isotope and oxygen isotope data from the same cores it will be possible to test the hypothesis that Deccan volcanism played a causative role in global warming.Published sequence stratigraphic data from the New Jersey continental margin documents a significant regression near the base of magnetochron C31R. This sea-level low stand is coincident with a shift to heavier oxygen isotope ratios in several published planktic and benthic foram records, an association that is suggestive of continental glaciation. Previous Os isotope work across the Eocene-Oligocene transition, and in recent glacial moraines, indicate that glacial cycles enhance the delivery of radiogenic continental Os (high 187Os/188Os) to the ocean. This study will seek evidence of high seawater 187Os/188Os coincident with the putative early Maastrichtian glaciation. If such evidence is found this will strengthen the case for Cretaceous glaciation, a controversial claim with broad implications for our understanding of Earth's climate evolution. It will also lay a foundation for seeking Os isotope evidence for older Cretaceous glacial events.The broader impacts of this research effort include heavy emphasis of graduate education and an opportunity for undergraduate participation in research at the University of Hawaii, a culturally and ethnically diverse university.