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Opal Burial in the Pacific Sector of the Southern Ocean: A Test of the 'Silicic Acid Leakage Hypothesis'

This award, provided by the Antarctic Geology and Geophysics Program of the Office of Polar Programs, supports a project to investigate the 'Silicic Acid Leakage Hypothesis' as it relates to global carbon dioxide fluctuations during glacial-interglacial cycles. This project will evaluate the burial rate of biogenic opal in the Pacific sector of the Southern Ocean, both during the Last Glacial Maximum (LGM) and during the Holocene, as a critical test of the 'Silicic Acid Leakage Hypothesis'. The 'Silicic Acid Leakage Hypothesis' has been proposed recently to explain the glacial reduction in the carbon dioxide content of the atmosphere that has been reconstructed from Antarctic ice cores. Vast amounts of dissolved Si (silicic acid) are supplied to surface waters of the Southern Ocean by wind-driven upwelling of deep waters. Today, that dissolved Si is consumed almost quantitatively by diatoms who form skeletal structures composed of biogenic opal (a mineral form of silicon). According to the 'Silicic Acid Leakage Hypothesis', environmental conditions in the Southern Ocean during glacial periods were unfavorable for diatom growth, leading to reduced (compared to interglacials) efficiency of dissolved Si utilization. Dissolved Si that was not consumed biologically in the glacial Southern ocean was then exported to the tropics in waters that sink in winter to depths of a few hundred meters along the northern fringes of the Antarctic Circumpolar Current, and return some decades later to the sunlit surface in tropical regions of wind-driven upwelling. An increase in the amount of dissolved Si that 'leaks' out of the Southern Ocean and later upwells at low latitudes could shift the global average composition of phytoplankton toward a greater abundance of diatoms and fewer CaCO3-secreting taxa (especially coccolithophorids). Consequences of such a taxonomic shift in the ocean's phytoplankton assemblage include: a) an increase in the global average organic carbon/calcium carbonate ratio of particulate biogenic material sinking into the deep sea; b) a reduction in the preservation and burial of calcium carbonate in marine sediments; c) an increase in ocean alkalinity as a consequence of the first two changes mentioned above, and; d) a lowering of atmospheric CO2 concentrations in response to increased alkalinity of ocean waters. A complete assessment of the Silicic acid leakage hypothesis will require an evaluation of: (1) Si utilization efficiencies using newly-developed stable isotopic techniques; (2) opal burial rates in low-latitude upwelling regions; and (3) opal burial rates in the Southern Ocean. This project addresses the last of these topics. Previous work has shown that there was little change in opal burial rate between the LGM and the Holocene in the Atlantic and Indian sectors of the Southern Ocean. Preliminary results (summarized in this proposal) suggest that the Pacific may have been different, however, in that opal burial rates in the Pacific sector seem to have been lower during the LGM than during the Holocene, allowing for the possibility of 'Si leakage' from this region. However, available results are too sparse to make any quantitative conclusions at this time. For that reason, we propose to make a comprehensive evaluation of opal burial rates in the Pacific sector of the Southern Ocean. Significance and Broader Impacts: Determining the mechanism(s) by which the ocean has regulated climate-related changes in the CO2 content of the atmosphere has been the focus of a substantial effort by paleoceanographers over the past two decades. The Silicic Acid Leakage Hypothesis is a viable new candidate mechanism that warrants further exploration and testing. Completion of the proposed work will contribute significantly to that effort. During the course of this work, several undergraduates will be exposed to paleoclimate research through their involvement in this project. Burckle and Anderson are both dedicated to the education and training of young scientists, and to the recruitment of women and under-represented minorities. To illustrate, two summer students (undergraduates) worked in Burckle's lab during the summer of 2002. One was a woman and the other (male) was a member of an under-represented minority. Anderson and Burckle will continue with similar recruitment efforts during the course of the proposed study. A minority student who has expressed an interest in working on this research during the summer of 2003 has already been identified.

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