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ABSTRACT: The Miocene Phosphogenic Episode on the Florida Platform: Implications for the Positive ^dgr13C Excursion

J. S. Compton, S. W. Snyder

We propose that deposition and later alteration of the enormous phosphorite deposits of the southeastern United States best explain the Miocene ^dgr13C excursion. The positive ^dgr13C shift in the early to early middle Miocene resulted from increased organic carbon burial rates when transgressive seas flooded the Florida platform and forced the Gulf Stream to migrate landward. Gulf Stream flow around topographic highs produced upwelling of P-rich water capable of sustaining high surface productivity. Organic carbon burial lowered PCO2 and eventually resulted in global cooling (reverse greenhouse effect). Cooling intensified oceanic circulation (coastal and equatorial upwelling), increased ice volume, and lowered eustatic sea level. The ove all decrease in sea level exposed shallow-water shelf sediments to subaerial and supergene weathering. Most of the buried early Miocene organic matter was oxidized and caused ^dgr13C to shift back to its preexcursion level by the middle Miocene. Meteoric alteration removed organic matter and oxidized pyrite, but much of the diagenetic phosphorite, dolomite, and silica remained. The greenhouse effect from increased PCO2 eventually led to a marked slowdown in cooling and ice buildup as reflected in the slope of the global ^dgr18O curve. Available age dates for the organic-rich phosphatic member of the Monterey Formation suggest it was deposited after the maximum ^dgr13C positive shift. Oxidation of large amounts of organic carbon on the Florida p atform may have lowered atmospheric O2 and enhanced organic matter preservation in deep-water basins of the circum Pacific during the middle Miocene.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990