INVESTIGATION OF POTENTIAL MICROBIALITE FORMATIONS FROM YUCATAN PENINSULA, MEXICO CENOTES

Microbially-constructed sedimentary formations are known to exist continuously in the geologic record dating back to as early as the Archean. While ancient fossils provide a rare snapshot of Earth’s earliest known biological activity, the processes governing their morphology and construction continue to be unraveled and are complicated by the wide variety of climatic and hydrologic environments in which Holocene analogues are found. Geochemical and mineralogical analyses were conducted on a recently-discovered cluster of potential microbialites from cenote (sinkhole) walls in the Yucatán Peninsula’s dynamic karst aquifer. The dendrolitic, phototropic structures are found at varying depths and circumferential orientations in the meteoric groundwater overlaying the halocline interface. Amorphous calcium carbonate matrices comprising the bulk of collected samples were found to consist entirely of magnesium-rich calcite, and sulfur anomalies in select locations reflect the influence of water chemistry in grain precipitation/absorption. Scanning Electron Microscopy revealed trace indicators of likely biogenic participation, including circular voids suggestive of photosynthetic oxygen degassing, remnants of a mucilaginous biofilm-like material, and several species of diatoms incorporated into the matrix. However, fossilized remains of cyanobacteria and/or other known stromatolite-forming microbes were not detected. Petrographic analysis uncovered unusual pore-concentric laminations and possible phosphate inclusions in addition to an abundance of microfossils, all consistent with microbialitic origins.

Further analysis is needed to confirm the biotic nature of the structures’ assemblage and morphology and to deduce the method of sedimentary accretion (i.e. trapping and binding of suspended particles versus direct calcite precipitation). If the formations are microbialites as hypothesized, however, their discovery expands the range of environments where modern microbialites exist to include semi-open groundwater systems. Other freshwater occurrences, while numerous and well-documented, have historically been limited to surface water settings such as lakes and lagoons.