The objective of this study is to provide insight into processes during the deposition and subsequent modification of textural and geochemical signals during the burial of a freshwater palustrine carbonates. During the Holocene marine transgression, transitional and marine sediments have progressively buried the freshwater mud forming in the paludal environment of the Florida Everglades. Sediment cores were collected in the modern freshwater palustrine environment, three transitional environments, and a full marine environment, representing snap shots of the freshwater mud during early burial, and allowing the tracking of microbial processes, textures, and geochemical signals through early stages of burial. Characterization of the cores included textural evaluation, as well as geochemical analysis of sediments, organics, and pore waters.
Initial results indicate loss of primary textures and geochemical signals during the early burial process (<4 ky). Texturally, low-Mg calcite crystals show dissolution features under SEM, Dissolution is promoted by the high surface area of the texturally complex microbial precipitates, and the continuous wetting and drying of the freshwater system. TOC is depleted from ~11-4% in the Holocene cores, and primary isotopic signatures (13C and 15N) of the organic carbon are also lost during the degradation process. Stable isotopes of the carbonate (13C and 18O) also show a range in values, probably a result of dissolution and early reprecipitation of the lime mud. Elementally (Ca2+, Mg2+, Sr2+, HCO3-, SO42-, etc.), pore waters and sediments show altered signals when compared to the modern system, which we attribute to variations in dissolution/reprecipitation, groundwater influence, and microbial processes. Characterization of Holocene mud deposits were then used as an analogue for freshwater Pleistocene deposits in order to address the preservation potential of microbial features over time. These results revealed even further alteration of the microbial carbonate mud with increased burial. We conclude that the carbonate mud is pervasively altered during even the earliest stages of burial. Inorganic and organic constituents lost their primary textural and geochemical signatures in both the Holocene and Pleistocene microbial lime mud. The preservation potential of microbial signatures within this paludal example is low, due to the alteration of both textural and geochemical signals. If this holds true for other freshwater carbonates, there are many implications for the recognition of these deposits in the geologic record as well as the validity of their use in paleoclimate reconstructions.
Topics: Biotic sedimentary processes , Topics: Microbial activity as sedimentary process , Topics: Freshwater carbonates
