CARBON ISOTOPE SIGNATURES IN MICROBIAL MAT CARBONATES – HOW DO DIFFERENT CARBONATE COMPONENTS RECORD INFORMATION ABOUT LOCAL VERSUS GLOBAL CARBON CYCLING? (Invited Presentation)

Carbon isotope geochemistry has the potential to be a powerful tool to study deep-time carbon cycling. However, this application requires a framework for understanding how secular variation in seawater chemistry, local processes, and post-depositional alteration impact carbon isotope signatures in marine carbonates. Geological evidence suggest that a majority of Precambrian marine carbonates were formed in microbial mat environments. In order to improve our ability to extract paleoenvironmental information from carbon isotope records deposited in these environments, we need to document the sedimentological and geomicrobiological processes that influence carbon isotope signatures in modern microbial mat ecosystems. We present carbon isotope data from carbonate components of microbial mats in Little Ambergris Cay, Turks and Caicos Islands, and compare them to carbon isotope values of ooids from the surrounding platform, which are in isotopic equilibrium with seawater dissolved inorganic carbon. The two most abundant types of carbonate grains in the mats are ooids transported to the mat from the surrounding platform and irregularly shaped authigenic grains. The ooids in the mat have carbon isotope values that are ~0.5‰ lower than ooids on the surrounding platform, likely due to early cementation of cyanobacterial microborings and/or the dissolution of ooid laminae in the mat. The authigenic grains have lower and more variable carbon isotope value than the platform ooids: those in a mat in a tidal channel with a high proportion of transported ooids have carbon isotope values ~3‰ lower than ooids on the surrounding platform, whereas mats in mangrove zones where platform-derived ooids are only deposited by large storms have carbon isotope values ~6‰ lower than platform ooids; together these observations suggest that transported ooids play a key role in carbon buffering. We also examined the carbon isotope signatures of carbonate-rich layers deposited during hurricanes, which were more similar to the carbonate-rich mat from the tidal channel. These results highlight the importance of sedimentological and petrographic context in interpreting carbon isotope signatures deposited in microbial mat environments.