Isotopic investigations of carbon cycling and microbially influenced carbonate precipitation in freshwater microbialites and carbonate-rich microbial mats

Modern microbialites and microbial mats are the focus of ongoing research as they provide an opportunity to understand microbial-mineral interactions during carbonate precipitation and the generation of biosignatures that can inform our interpretation ofthe geological record. This study determined the natural abundance isotopic compositions (13C, 14 C) ofthe primary carbon pools and microbial communities associated with modern freshwater microbialites located in Pavilion Lake and in carbonate rich microbial mats on the nearby Cariboo Plateau in British Columbia, Canada.;Isotopic enrichment of calcium carbonate within microbial communities associated with the microbialites was identified as a biosignature of microbial photosynthetic influence driving precipitation. Elevated oxygen concentrations and pH within the microenvironment of small, sporadic nodular microbial surface communities was concurrent with in situ precipitation of carbonate with delta13C values higher than predicted abiotic values and delta13C of bulk organic matter and phospholipid fatty acids (PLFA) that were consistent with a photosynthetically dominated community. Elevated carbonate delta13C values were also noted in the thin surface microbial mat recovered from shallow (11 m) microbialites. These samples showed increased biomass during summer sampling periods as compared to deeper samples, consistent with expected high rates of photosynthetic activity due to higher light levels and temperature at these depths. These results contrast other recent studies of modern microbialite systems that identified biosignatures of heterotrophic influences on precipitation of carbonates. PLFA profiles demonstrated that the surface microbial mat community consisting of both photosynthetic and heterotrophic microbes was stable over seasonal and spatial changes in light and temperature. However, changes in microbial biomass with depth and season indicated that microbial activity and growth plays an important role in the development of isotopic biosignatures.;Biosignatures of high levels of photosynthetic activity were also observed in carbonate, rich microbial mats that exhibited undersaturated p CO2 concentrations during the summer and DIC delta 13C values enriched above values predicted for isotopic equilibrium with atmospheric CO2. Seasonal and annual shifts in the balance of heterotrophy and autotrophy in the lakes and microenvironment of the mat accounted for observed variations in DIC and associated carbonate delta 13C values. In contrast to other organic rich microbial mats, bulk organic delta13C values were not enriched and the systems did not show evidence of CO2 limitation. Rather, these results indicated that low bulk organic delta13C values and large isotopic discriminations can exist under conditions of high DIC concentrations and carbonate content that provide a non limiting carbon source to replenish photosynthetic drawdown.;Natural abundance 14C analysis of carbon pools associated with the Pavilion Lake microbialites demonstrated that structures were actively growing and that groundwater carbon inputs to the lake and microbialites were minimal. Rather, ambient dissolved inorganic carbon (DIC) was the primary carbon source for both microbial communities and recent carbonate.