Investigation of the Origins of Modern Firmgrounds in Walker Lake, Nevada: Implications for Lacustrine Climate Records and Hardgrounds in the Rock Record
Wednesday, 17 December 2014
Walker Lake, Nevada, contains locally abundant Pleistocene and Holocene carbonate structures, including tufas, stromatolites, and lithified carbonate hardgrounds. Hardgrounds are classically interpreted as forming via geobiological processes in shallow sediments. To better understand the origins of ancient hardgrounds, we investigated modern firmgrounds which we hypothesized to be incipient hardgrounds. The studied firmgrounds were brown, 1-2 cm thick with a mm-scale white or brown crust, cohesive enough to retain sharp erosional features, and were located on the western shoreline where alluvial fans enter the lake. We analyzed the elemental composition of the lakewater and porewaters in order to model mineral saturation states under lake conditions. In addition, we analyzed 16S rRNA gene diversity of the firmgrounds, porewater of adjacent sediments, and lakewater to investigate potential biological involvement. This, combined with SEM imaging, XRD analyses, and EDS analyses indicate that the firmgrounds are likely dominated by clays and granitic minerals rather than carbonate like the ancient hardgrounds. We suggest that Walker Lake firmgrounds are formed by weathering of unstable granitic minerals with no clear biological influence, likely resulting from physio-chemical processes related to groundwater-lake water mixing. Such firmgrounds may undergo subsequent replacement by calcium carbonate to form hardgrounds, resulting in carbonates with δ13C and δ18O compositions that do not reflect the lake water in which they formed. Given the apparent influence of groundwater on their formation, our work suggests that caution should be used when interpreting lacustrine hardground isotopic records as indicators of lakewater chemistry and climate.