Marine peloids; a product of bacterially induced precipitation of calcite

High-magnesian calcite peloids are a common constituent of cemented marine carbonate accumulations. They are abundant and well developed within both isolated microcavities and surface crusts. The nuclei of marine peloids are composed of fossil bacterial clumps encased within anhedral, submicron- to micron-sized, high-magnesian calcite crystals. The marine peloids are similar to silt-sized particles found in travertine deposits that also are composed of fossil bacterial clumps and their surrounding calcite coronas. These similarities include size (20-60 mu m), a cloudy brown nucleus, and rims of clear euhedral crystals. In addition, previous studies have established the presence of fatty acids of bacterial origin within peloidal deposits, and enriched stable isotopic signatures have been interpreted as recording organic fractionation concomitant with calcite precipitation. These characteristics all indicate that the nuclei of many peloids originate as calcite precipitates within and around clumps of bacteria and that this precipitation was induced by the vital activity of bacterial colonies.