A TALE OF TWO LAKES: HOLOCENE CLIMATE VARIATIONS FROM PYRAMID LAKE AND BAKER LAKE, NEVADA

Cores from Pyramid and Baker Lakes, Nevada, were taken to compare sediment records over the last 500 years from a human-disturbed terminal lake and a pristine alpine lake, respectively. Lead-210 and Cs-137 age dating has been done on the cores and the records can be extrapolated back about 500 years in Pyramid Lake and 1300 years in Baker Lake. Analysis of the Pyramid Lake cores indicate that variations in trace metal concentrations and carbon, oxygen and nitrogen isotopic signatures correlate with lake stage height, some decreasing and others increasing. Lake stage fell by over 25 m from 1908 to 1967 due to water diversion from the Truckee River. Calcium concentrations and carbonate content both decrease in our core taken near the Truckee River delta when lake stage falls. This finding is similar to previous work that determined that lake sediment carbonate decreases in the shallow portions and increases in the deeper areas during declining lake stage because of resuspension of near-shore fine sediment. Trace metal concentrations are variable and a simple explanation for terrestrial sediment inputs is not possible. Organic contaminants in the core are relatively low, but increases of up to four times the background concentrations for PAHs are found in samples from around AD 1970 and also in the past 10 years. These periods correspond to lake stage increases indicating that increased runoff from the Reno/Sparks urban areas may entrain more organic contaminants in the Truckee River. The dating of the cores from Baker Lake indicates low depositional rates of about 0.25 mm/year. Trace metal and major element concentrations increase between AD 1300 and AD 1900, the period referred to as the Little Ice Age (LIA). The colder and wetter conditions present during the LIA may have increased atmospheric inputs to Baker Lake, a situation noted in chemical records from ice cores taken from the ice-cap of central Greenland. Differences in the sediment records from these two lakes indicate that terrestrial riverine inputs and resuspension processes dominant in Pyramid Lake, whereas atmospheric inputs dominate in Baker Lake. Comparison of these two lakes climate record underscores the importance of understanding the hydrology of lake systems before climate signatures can be reconstructed.