Abstract

Data from three Bahamian and South Floridan cores with deeper water facies show that, contrary to current models, there is no phreatic meteoric diagenesis that can be attributed to large-scale sea-level lowstands. Models of meteoric diagenesis in carbonate sediments commonly assume major phreatic alteration during glacio-eustatic sea-level lowstands. However, the diagenesis previously attributed to large-scale (>80 m) lowstands is found in shallow-water facies that have been repetitively exposed. In order to document diagenesis from large-scale lowstands, an interval is needed that was deposited in deeper water and not immediately exposed to meteoric fluids. Three cores with such deeper-water facies indicate that the majority of phreatic meteoric diagenesis in Florida and the Bahamas happened during glacio-eustatic highstands, not during lowstands. The data also suggest that diagenetically active meteoric lenses in Florida and the Bahamas are restricted to within 60 m, and perhaps less, of the land surface. The most likely reasons deeper lenses are chemically inactive are (1) the greater percolation distance allows the water to reach saturation prior to entering the lens, and (2) the large distance exceeds the reach of soil-derived organic matter, known to drive diagenesis in meteoric lenses. Models that currently assume a constant rate for alteration in meteoric fluids need to accommodate this variation with thickness of the vadose zone. This study also questions the use of paleophreatic lenses as records of eustatic sea level, as the large-scale falls may not leave any record.

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