Morphogenesis and Taphonomy of Oncoid Microbialites, Cuatro Cienegas Basin, Coahuila, Mexico

We have studied oncoid microbialites of the Rio Mesquites, a spring-fed stream that flows south across the western side of the Bolson de Cuatro Cienegas, a 1200 km2 arid basin located on the eastern flank of the Sierra Madre Oriental in the state of Coahuila, Mexico. Our goal is to understand oncoid morphogenesis in this environment and assess the potential for capturing and preserving a cellular microfossil record. We sampled oncoids along a 1km-stretch of the Rio Mesquites, from its headspring source, to a location about 300 meters downstream where the stream channel suddenly widens. At each sample site along our transect, we measured oncoid diameters and fixed small oncoids and/or subsamples for later lab analysis. We prepared oriented thin sections of whole dried oncoids and field-fixed/stained florets to expose continuous accretion profiles from surface to core. Profiles were then sub-sampled for X-ray Powder Diffraction analysis and geochemistry. We used fluorescence, DIC and polarizing microscopy and laser confocal methods to characterize microbial fossils and microtextures in thin section and SEM imaging of cross-sections of critical point dried accretion profiles. These studies revealed the presence of 1) a lightly calcified surface community dominated by filamentous green algae, coccoid and filamentous cyanobacteria and diatoms and 2) a lower diversity understory community, dominated by an unidentified 2-um diameter branched, ensheathed cyanobacterium. The understory community was consistently associated with elevated calcification rates expressed as pervasive micritization that created favorable conditions for fossilization. Analyses of deeper oncoid profiles showed that even though biosignatures of the highly productive surface community in this system undergo rapid degradation, pervasive calcification of the lower diversity understory community enhances biosignature preservation, allowing this community to dominate throughout oncoid profiles. This is an important taphonomic bias that has relevance when interpreting the microfossil record of ancient microbialites.