B21C-0449
Geochemical and Textural Comparison of Modern and Ancient Freshwater Microbial Mud: A Direct Calibration in the Florida Everglades

Tuesday, 15 December 2015
Poster Hall (Moscone South)
Chelsea Pederson1, James Klaus2, Donald F McNeill2 and Peter K Swart1, (1)University of Miami, Department of Marine Geosciences - RSMAS, Miami, FL, United States, (2)University of Miami, Miami, FL, United States
Abstract:
The goal of this study is to directly calibrate a freshwater microbial carbonate deposit currently forming in the Florida Everglades, to ancient deposits formed during Pleistocene marine transgressions of the South Florida platform. Through textural and geochemical analyses, this project tracks the deposition and early diagenesis of microbial carbonates and their associated organic content as an analogue study applicable to ancient microbialites.

In the modern, fine-grained low-Mg calcite crystals are precipitated and buried on a low-lying coastal plain (Paurotis Pond). Geochemical parameters were measured to provide baseline characterization of the deposition and early diagenesis of the freshwater muds. Stable isotopic values of the precipitated muds range from d18O of -1.5 to -2.5‰ and d13C 0.5 to -0.5‰, showing slight increases with depth. This change is likely due to waters from the underlying Pleistocene bedrock, diffusing up and mixing with the surface water. Total organic carbon (TOC) is depleted with depth from 10.4 to 4.2% as a result of the degradation of highly labile organic matter (heavier in d13Corg), leaving the resulting pool of organics increasingly negative (-23‰ to -26‰ in the modern).

Pleistocene freshwater limestones were observed in core borings ~20 km north of Paurotis Pond. These units closely resemble the modern muds, and allow us to assess the preservation potential of original microbial, geochemical, and textural signatures. Diagenesis has resulted in lighter d13C values relative to the adjacent altered marine units, as well as their modern equivalent. Organics in the Pleistocene are also characterized by more negative C and O isotopic compositions from interpreted original values, and TOC was reduced to <0.1%.

Results suggest that both carbonate and organic fractions are pervasively altered during the burial process, and may not be effective paleoclimate indicators as some research suggests. However, these components may still provide some evidence of a microbial origin, in both textural components as well as the remaining d13Corg values are typically more negative than non-microbial organics.

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