An Investigation of Holocene Microbial Microstructures from Sediments in Lower Pahranagat Lake, Nevada

Microbial carbonates have had significant influence on the global geochemical environment for much of Earth's history and can play a substantial role in petroleum plays. Hickson et al. (2016, J. Paleolim.) established the occurrence of surficial microbial carbonate mat communities and fossilized domal stromatolites from Lower Pahranagat Lake (LPAH), a small spring-fed alkaline lake in southern Nevada, characterized by high carbonate deposition (up to 80%), high salinity, and low silicate input. These factors inhibit the success of benthic metazoans in LPAH, making preservation of microbial communities possible, although the degree of preservation is uncertain. In this study, we use scanning electron microscopy (SEM) and petrography to examine carbonate-rich sediments for spatially/texturally distinct microbial microstructures from LPAH. We find evidence of a distinct assemblage of microbialite microstructures. Samples of interest were selected from a lake core collected in 2016 that spans the last 5800 years. We applied a new "face-freezing" technique that preserves fragile microbial fabrics in wet sediments during the sub-sampling process. Thin sections were prepared from these samples, including short intervals featuring mm-scale wavy to crinkly laminations and clotted textures, which we suspect were either microbially influenced or induced. Our initial SEM analyses of these sequences reveal the presence of well-defined filamentous cyanobacteria and potential calcified extracellular polymeric substances (EPS). These structures are commonly associated with microbially-facilitated carbonate mineralization. Based on macroscopic features in modern and recent sediments and lithified domal stromatolites found along the lake margin, Hickson et al. (2016) also recognized LPAH as a robust modern analogue for late Oligocene to Miocene lacustrine microbial carbonates found at the Horse Spring Formation (HSF) in the Lake Mead region. Our petrographic and SEM analyses of LPAH microbialite microstructures allows for a more detailed comparison with HSF microbialites and the opportunity to gain a better understanding of the biogeochemical interactions, lithification, and paleoclimatic controls that influenced the microbial carbonate communities of the ancient HSF.