Isotopic Constraints on the Formation of Microbialites in Pavilion Lake, B.C

The freshwater microbialites present in Pavilion Lake, B.C. Canada have unique morphologies that may be a signature of a biological role in their formation. It has been hypothesized that the variations in the morphologies of these carbonate structures with depth are due to variations in light levels and associated cyanobacterial photosynthetic activities. These cyanobacterial communities may be controlling microbialite formation by passive means, such as trapping and binding abiotically precipitated sediments, or by active alterations of geochemical conditions during metabolic processes such as photosynthesis. However, constraining which of these processes is controlling the formation of these structures and the potential for associated biosignatures remains challenging.

Isotopic analysis of ¹³C and ¹⁴C of surface water Dissolved Inorganic Carbon (DIC), porewater DIC, groundwater DIC and associated carbonate minerals indicates that the primary source of carbon is from the atmosphere with a 12-26% input from groundwater/subsurface sources. ¹⁴C analysis further demonstrates that these microbialites are actively growing at a rate of 6 cm/thousand years over the last thousand years.

Stable isotopic fractionation between surface water DIC and microbialite surface carbonate showed ¹³C-enrichments of up to 2 ‰ above predicted equilibrium carbonate values. These observations are indicative of biologically induced ¹³C-enrichment of the residual DIC pool due to photosynthetic activities. Such enrichment has the potential to be preserved in ancient carbonate structures. However, up to now δ¹³C values of interior carbonates from the microbialites have shown no evidence of similar isotopic enrichment suggesting that this signal may not be preserved. Microscopy to determine the physical relationship between microbes and precipitated carbonate and the crystal form of the carbonate minerals may elucidate this potential for remineralization and loss of isotopic biosignatures.