Isotopic biosignatures in carbonate-rich, cyanobacteria-dominated microbial mats of the Cariboo Plateau, BC

Photosynthetic activity in carbonate-rich benthic microbial mats located in saline, alkaline lakes on the Cariboo Plateau, B.C. resulted in pCO(2) below equilibrium and C-13(DIC) values up to +6.0 parts per thousand above predicted carbon dioxide (CO2) equilibrium values, representing a biosignature of photosynthesis. Mat-associated C-13(carb) values ranged from similar to 4 to 8 parts per thousand within any individual lake, with observations of both enrichments (up to 3.8 parts per thousand) and depletions (up to 11.6 parts per thousand) relative to the concurrent dissolved inorganic carbon (DIC). Seasonal and annual variations in C-13 values reflected the balance between photosynthetic C-13-enrichment and heterotrophic inputs of C-13-depleted DIC. Mat microelectrode profiles identified oxic zones where C-13(carb) was within 0.2 parts per thousand of surface DIC overlying anoxic zones associated with sulphate reduction where C-13(carb) was depleted by up to 5 parts per thousand relative to surface DIC reflecting inputs of C-13-depleted DIC. C-13 values of sulphate reducing bacteria biomarker phospholipid fatty acids (PLFA) were depleted relative to the bulk organic matter by similar to 4 parts per thousand, consistent with heterotrophic synthesis, while the majority of PLFA had larger offsets consistent with autotrophy. Mean C-13(org) values ranged from -18.7 +/- 0.1 to -25.3 +/- 1.0 parts per thousand with mean C-13(inorg-org) values ranging from 21.1 to 24.2 parts per thousand, consistent with non-CO2-limited photosynthesis, suggesting that Precambrian C-13(org) values of similar to-26 parts per thousand do not necessitate higher atmospheric CO2 concentrations. Rather, it is likely that the high DIC and carbonate content of these systems provide a non-limiting carbon source allowing for expression of large photosynthetic offsets, in contrast to the smaller offsets observed in saline, organic-rich and hot spring microbial mats.