Testing the Utility of Natural Variations in Isotopes of Si as a Proxy for Silica Production in the Sea

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ABSTRACTOCE - 0350576In the mid 1990's it was discovered that diatoms fractionate isotopes of silicon during silicon biomineralization opening the possibility of using natural variations in isotopes of silicon in dissolved Si and in diatom frustules as a tool for assessing silica production in both the past and present day ocean. Diatoms with their silica ballast are key vectors of organic matter out of the euphotic zone. Their obligate need for Si means that their contribution to primary productivity and the biological carbon pump can be controlled by the availability of silicic acid in ocean surface waters. Natural variations in isotopes of Si in diatoms and silicic acid offer a possible means of assessing relative silicic aid use on longer time scales and larger spatial scales than any other technique currently available. Though the results thus far support the use of natural variations in isotopes of Si as proxies for relative silicic acid use, two critical gaps in our knowledge remain.To improve the research performed to date, a scientist from the University of California- Santa Barbara will continue to evaluate and calibrate the d30Si proxy by examining the variability in the magnitude of the fractionation factor (e) as well as whether silica dissolution in the water column and silica dissolution and exchange processes during sediment diagenesis affect on the d30Si signature of diatom frustules. Currently, it is unclear whether e varies regionally in the sea as only two estimates of e based on field data are available. This imposes great uncertainty in the general applicability of the fractionation factor determined for cultured diatoms to the global ocean. Also, fractionation during silica dissolution or during exchange and precipitation reactions may alter d30Si values of diatom frustules biasing interpretations made solely on the basis of an assumed biological fractionation. The goal of the proposed research will be to obtain new field estimates of e and to address fractionation during silica dissolution and sediment diagenesis while also perfecting of an entirely new method of preparing samples for silicon isotope analysis through the acid decomposition of Cs2SiF6. .