2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 8
Presentation Time: 3:50 PM


THOMAS, M.D. and FARMER, J.D., Geology, Arizona State University, PO Box 871404, Tempe, AZ 85287-1404, mdthomas@asu.edu

Carbonate tufa deposits in the Mono Basin have been studied and suggested to form based on mixing of Ca-rich spring waters with highly saline and bicarbonate-rich lake waters. Recent work by our lab has revealed that the microbiota associated with subaqueous tufas play an integral role in certain types of carbonate precipitation. This work details the characterization of the carbonate deposits through petrographic analysis and x-ray diffraction, as well as a suite of microbial and molecular analyses to determine the composition of the tufa microbiota.

Results indicate an intimate relationship between carbonate precipitation and microbial biofilms in this highly saline, alkaline setting. Gaylussite, Na2Ca(CO3)2∙5(H2O), has been recognized as a primary precipitate in Mono Lake. While chemical disequilibrium may be sufficient to explain some carbonate precipitation at sites of spring discharge, we have also observed gaylussite precipitation distal to spring flow, nucleating on and within microbial biofilms. In many cases these gaylussite crystals completely encapsulate portions of biofilm, and may subsequently act as substrate for further precipitation. This may result in the incorporation of biofilms into tufa deposits, with the potential for biosignature preservation.

Gaylussite crystals with enclosed microbiota were sampled and molecular protocols including PCR, cloning, and sequencing were utilized to assay the microbiota within the crystals, the majority of which are composed of cyanobacteria. While microbial controls appear to be important in carbonate precipitation in this system, long-term biosignature preservation depends upon their survival during early diagenetic transformations that replace primary gaylussite by more stable forms of carbonate (calcite and aragonite). Ongoing research seeks to trace the preservation of microbial biosignatures into older deposits where stabilization by diagenesis has occurred. Current data may be used in future interpretations of remnant Pleistocene carbonate formations found in the Mono Basin, which may contain biosignatures from similar processes during formation.