2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 8
Presentation Time: 3:30 PM

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

SLATER, Greg F., School of Geography and Earth Sciences, McMaster University, 1280 Main St. West, Hamilton, ON L8S 4K1, Canada, BRADY, Allyson L., School of Geography and Earth Sciences, McMaster University, 1280 Main Street West, Hamilon, ON L8S4K1, LAVAL, Bernard, Civil Engineering, University of British Columbia, Vancouver, BC V6T 1Z4, Canada and LIM, Darlene S.S., Space Science and Astrobiology, NASA Ames Research Center, Mail-Stop 245-3, Moffett Field, CA 94035-1000, gslater@mcmaster.ca

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.