ASSESSING MICROBIAL INFLUENCE ON DEPOSITION OF FRONDOSE LACUSTRINE CARBONATE TUFAS FROM WINNEMUCCA DRY LAKE, NV, USA

Lacustrine carbonate deposits (“tufas”) are commonly observed in the Lahontan lake basins of the western United States. In Winnemucca Dry Lake, NV, tufas are widespread along the western margin of the dry lake bed, and a variety of morphologies and textures are observed. The most common and laterally extensive variety of tufa is a frondose form that exhibits draping and branching textures and is deposited on hard substrates. This form of tufa occurs both at spring-associated sites on the lake bottom and at high elevations along bedrock cliffs. The tufa meso- and microfabrics are similar to those observed in modern microbialites. The widespread distribution of these tufas suggests that groundwater flux is not the most significant controlling factor on deposition, and we hypothesize that frondose tufas were deposited upon microbial substrates. Samples of frondose tufa were collected from a spring-associated site and a high elevation bedrock outcrop site. Hand samples exhibit a porous, branching mesofabric, with some samples containing areas exhibiting stromatolitic laminations. Thin sections show that frondose tufas are dominated by micrite and shrubby calcite fabrics, with varying degrees of secondary calcite; some samples contain preserved microbial filaments. Organic geochemistry, including C_org content, δ¹³C_org, pigment analysis, and DNA sequencing indicate that tufa deposition at both spring-associated and bedrock sites may be influenced by photosynthetic microbes (Cyanobacteria, Chloroflexi). Radiocarbon age dating of carbonate and organic carbon lends insight into the timing of these processes and the relationship to lake basin history. These results have implications for paleoenvironmental interpretations of Lahontan tufa deposits, and may have broader implications for studies of microbialites throughout the rock record.