SEM ANALYSIS OF DESERT VARNISH FROM LAKE LAHONTAN SHORELINES IN THE EETZA RANGE, NEVADA

Variations in the chemistry of microlaminations in desert varnish may 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. For example, one might expect dry periods to produce layers with greater quantities of aluminum and silica, reflecting more wind-blown clay, whereas humid periods would 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 windward facing shorelines left behind by Lake 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 basalt boulders on 12 distinct shorelines. All samples exhibit well-developed desert varnish, with some layers reaching thicknesses of 200 microns. Easily identified microlaminations showed 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 tufa, which prevented some older shorelines from developing thick varnish layers. This study is of particular importance due to its close proximity to the Grimes Point Archeological Site. Samples were collected less than a mile from Grimes Point, which is host to a plethora of petroglyphs and artifacts. The Grimes Point area also houses some of the oldest human remains in North America, such as the Spirit Cave mummy. The populations that inhabited this area lived near the shores of Lake Lahontan. Therefore, constraining the climate conditions and ages of various lake stands is important to understanding interactions between prehistoric humans and their environment.