USING MULTIPLE ROCK VARNISH SAMPLES FROM THE LAHONTAN MOUNTAINS, NEVADA TO ASSEMBLE A RECORD OF CLIMATE CHANGE

The Lahontan Range, which lies east of the Carson Sink, is home to a suite of Paleolake Lahontan shorelines; these landforms reflect wave activity that dates from the last pluvial maximum at approximately 14 ka. Boulders on these shorelines are heavily varnished, offering an opportunity to study varnish that has formed during an established time period.  Previous studies on desert varnish have identified chemical patterns within microlaminations that appear to reflect changes in climate. Abundant manganese in microlaminations is hypothesized to result from enhanced bioaccumulation of manganese oxides during more humid periods; decreased manganese is accompanied by increased silicon, which is hypothesized to reflect higher quantities of windborne detritus during dry periods. Varnish samples for this study were collected from four vertical transects to explore chemical patterns. The goal of this study was to construct as complete a record of  chemical variations in microlaminations within each sample as possible. Both microbial communities and desert varnish are preferentially found in microscopic pits on rock surfaces. Scanning Electron Microscopy (SEM) and Energy Dispersion Spectometry (EDS) of microlaminations within a single pit yields a distinct trend in Mn and Si abundances, but it is unlikely that any single pit holds a complete 14,000-year record of varnish development.  Spalling from freeze/thaw cycles, thermal expansion, and salt weathering combined with wind abrasion is likely to produce truncated or discontinuous growth patterns.  Analysis is also complicated by the presence of tufa, which in some cases has delayed exposure to the subaerial environment. We present EDS results from multiple pits in each sample from the four shoreline transects in an effort to compose a complete varnish growth record that may be correlated to known climate change in this region. In conjunction with semi-quantitative EDS analyses, element concentrations were determined by microprobe analysis, while Inductively Coupled Plasma Mass Spectrometry (ICPMS) was used for bulk trace element analysis.