Biofilm-Mediated Carbonate Precipitation in Mono Lake Tufas: Evidence of Seasonal Variations in Primary Minerals and Microfabrics

We have been studying the role of microbial biofilms in the precipitation of Mono Lake tufas and their potential for capturing and preserving microbial biosignatures. In 2007, we explored submerged tufa surfaces distal to active vents south of Lee Vining Creek (0-15 m depth). In spring and summer, deeper surfaces are dominated by coarse-grained gaylussite. Microscopy and molecular assays show that gaylussite precipitation occurs in close association with cyanobacterial biofilms dominated by Leptolyngbya and Synechococcus. Gaylussite crystals nucleate preferentially on and within biofilms, the crystals entombing dense aggregations of microbes and their exopolymer matrix. Biofilms are thin to non-existent at vents, probably due to the higher rates of carbonate (calcite and aragonite) precipitation, driven by inorganic (disequilibrium-mixing) processes. In shallow nearshore areas (<2 m depth), gaylussite is only common on shaded surfaces where lower UV levels favor growth of cyanobacterial biofilms having a community composition shown to be gaylussite associated . Gaylussite is metastable and recrystallizes to form subrounded masses of micrite and microgranular aragonite. This process is accompanied by the pervasive loss of microbial biosignatures. In contrast, upper sunlit surfaces in shallow water support thin, sometimes algal-dominated biofilms of variable species composition. These surfaces are dominated by popcorn-like intergrowths of aragonite and contain rare cellular microfossils mostly preserved as filament molds.

During winter, as temperatures fall below ~5° C, tufa surfaces near shorelines accumulate precipitates of metastable ikaite. These mostly occur as small (centimeter to decimeter high) friable mounds having dark anoxic cores with framboidal pyrite present. In early spring, ikaite transforms to microcrystalline calcite which preserves the ikaite crystal form (i.e., pseudomorph known as thinolite). In older tufas, winter fabrics are recognized as lenses of porous calcitic tufa having a distinctive “microthinolitic” fabric and variable pyrite framboids.