Carbonate Precipitating Cyanobacterial Microbial Mats in the Saline, Alkaline Lakes of the Cariboo Plateau, B.C. as Analogues of Stromatolite Forming Communities

Extensively developed, carbonate precipitating microbial mats found in saline, alkaline lakes on the Cariboo Plateau, B.C. present a unique opportunity to investigate the potential for biosignatures associated with evaporative systems that might have occurred on early earth or Mars. These evaporitic lakes have pH values > 10, salinities of up to 33 psu and alkalinities of > 15, 000 mg CaCO₃/L. The ability to identify microbial biosignatures in modern analogues is crucial to the interpretation of ancient carbonate (stromatolite) structures found in the geologic record and microbial ecosystems in extreme and/or exobiological environments.

Both autotrophic and heterotrophic metabolic processes have been linked to biologically induced carbonate precipitation. The controls on precipitation in the Cariboo Plateau microbial mats are unknown. Microelectrode studies show that photosynthetic oxygen production occurs in the upper 5 mm of the mat resulting in supersaturation of oxygen in surface waters. Depletion of this oxygen generally occurs just below 5 mm and sulfide production begins at 10 - 15 mm from the mats surface. Methane concentrations vary depending on lake geochemistry as lakes with high sulphate (>5 mM) show low methane production and lakes with low sulphate concentrations have high levels of methane production with highly depleted ¹³C values.

Variations in microbial phospholipid fatty acid (PLFA) distribution demonstrate that Cariboo Plateau microbial mat community structure varies seasonally and spatially. Isotopic analyses (δ¹³C) were used to investigate the potential for microbial biosignatures in these carbonate rich systems. Bulk cell analysis of microbial mats show non-CO₂ limited inorganic to organic offsets of 23-25 ‰. Some Cariboo Plateau lakes and microbial mats demonstrate ¹³C-enriched DIC and carbonate resulting from photosynthetic drawdown of ¹²C. Biologically induced ¹³C-enrichment of the local residual DIC pool has the potential to be preserved in ancient carbonate structures.