AAPG ACE 2018

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Ecology and Biogeochemistry of Microbial Mats on Little Ambergris Cay

Abstract

Microbially-influenced sedimentary textures are common in the ancient rock record, especially shallow marine and lacustrine carbonates. In some cases they form commercially significant reservoirs, such as those that occur in the intrasalt of South Oman and the presalt of offshore Brazil. Evaluation of textures in these reservoirs requires an understanding of the biological processes at play in both the formation and degradation of modern microbial mats. Thick, extensive, and morphologically diverse mats dominate the interior basin of Little Ambergris Cay—a small, uninhabited island on the southern margin of the Caicos platform in the Turks and Caicos Islands. These mats provide a modern ecosystem with which we can better understand primary productivity and biogeochemical cycling in cyanobacterial mats on a carbonate platform, and examine processes that influence the geobiological signatures preserved in Archaean and Proterozoic rocks. Over two field campaigns in July 2016 and August 2017, we cataloged the extent and diversity of microbial mat morphologies across Little Ambergris Cay. Using 16s rRNA amplicon sequencing, we surveyed the taxonomic composition of the different mat types, and found—despite substantial differences in morphotypes—their microbial communities are largely uniform. To elucidate more nuanced aspects of the physiologies operating within the mats, we conducted direct metagenomic sequencing, which adds functional metabolic information to the taxonomic data. To investigate biosynthetic processes and differential microbial activity, we conducted incubation experiments with labelled activity tracers. To investigate degradation processes, we examined changes in microbial diversity, microfaunal morphology, organic biomarkers, and sulfur isotope ratios with depth in the mat. One of the most remarkable features of these ecosystems is the apparent absence of early lithification of the mats—a feature that geochemical data suggest reflects their high photosynthesis and respiration fluxes. Taken together, these data strengthen our framework for understanding the morphological and geochemical signatures preserved in ancient microbially-influenced reservoir facies.