SEM ANALYSIS OF VERTICAL AND LATERAL VARIATIONS IN DESERT VARNISH CHEMISTRY FROM THE LAHONTAN MOUNTAINS, NEVADA

Variations in the chemistry of microlaminations in desert varnish from the Lahontan Mountains appear to record sequences of climate change. Using Scanning Electron Microscopy (SEM), laminations can be identified and Energy Dispersion Spectrometry (EDS) used to determine differences in relative abundances of elements which reflect changes in climate. We hypothesize that dry periods tend to produce layers with greater quantities of aluminum and silica, reflecting more wind-blown clay, whereas humid periods produce layers higher in manganese, which is microbially accumulated in desert varnish. The study area lies within the Carson Sink east of Fallon, Nevada, and samples were selected from shorelines left behind by Paleolake Lahontan. Because this particular area lies within the Carson Sink, it represents the lowest elevations occupied by the lake and therefore the youngest shorelines. We analyzed a series of samples taken from boulders on shorelines at four vertical transects to explore vertical and lateral patterns in microlamination chemistry. All samples exhibit well-developed desert varnish, with some layers reaching thicknesses of 200 microns. Microlaminations show noticeable changes in aluminum, silica, and manganese content, with a strong inverse correlation between manganese and silica abundance. This supports the idea that sublayers reflect climate oscillations in this region. Analysis was complicated by the presence of encasing tufa, which prevented some older shorelines from developing thick varnish layers. This study is of particular importance because petroglyphs, artifacts and some of the oldest human remains in North America, such as the Spirit Cave mummy, are found in the Lahontan Mountains. Relating climate conditions to ages of various lake stands is important to understanding interactions between prehistoric humans and their environment.