Miocene Phosphogenesis and Depositional History of the Florida Platform: Implications for Changes in the Carbon Isotopic Composition of Seawater

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Previous studies of the Miocene Hawthorn Group of the Florida Platform indicate that phosphogenesis occurred primarily during the early to early middle Miocene. The timing of Phosphogenesis appears to be synchronous with a global positive excursion of the carbon isotopic composition (delta13C) of seawater. Preliminary examination of the Hawthorn in northeastern Florida shows that these sediments are rich in phosphorite, dolomite, silica and other minerals indicative of intense upwelling, but contain little organic carbon or pyrite. Burial of isotopically light carbon by deposition of organic-rich, continental margin sequences in the SE U.S. during the early Miocene likely resulted in a global increase in the delta C of seawater. Multiple reworking and supergene weathering from subaerial exposure after the middle middle Miocene oxidized and removed most of the organic carbon and diagenetic pyrite. Rapid oxidation of the buried organic carbon resulted in the delta C of seawater returning to its pre-excursion value. We will test this hypothesis by determining the mechanism and timing of phosphogenesis, the timing of sediment oxidation, and the relationship of these events on the Florida Platform to global sea-level changes and variations in the delta C and delta O of seawater during the Miocene. The research program relies on integrating the seismic sequence stratigraphy, sediment mineralogy and texture, and elemental and isotopic composition of the phosphorite and associate minerals. The Sr isotope stratigraphy of the Hawthorn sections will be compared to the delta C, delta O, and Sr isotope stratigraphy of the Miocene sediments from the southwest Pacific (DSDP Site 588).