BIG SODA LAKE AS A WINDOW INTO THE GROUNDWATER: IMPLICATIONS FOR DETERMINING HOLOCENE PALEOCLIMATE INDUCED FLUCTUATIONS IN GROUNDWATER INPUT

Paleolimnological research in the Great Basin has produced a rich record of late Quaternary environmental change. Few studies have been able to use changes in groundwater input to a lake as a measure of climate change. Big Soda Lake, a 63 meter deep, saline meromictic (25,000 mg/L TDS, surface water; 87,000 mg/L TDS, bottom water) volcanic lake near Fallon, Nevada is ideal for looking into the influence of groundwater on the lake sediment paleoclimate record. This is because the lake has no surface outflow and is fed by subsurface springs through regional groundwater flow. The depth of the lake provides a window into the variations in groundwater inflow that is influenced by the amount of evaporation that is occurring from the surface of the lake. Therefore variations in the patterns of oxygen and carbon isotopes of calcite precipitated in the lake should reflect changes in the amount of evaporation and groundwater flux into or possibly through the lake. Results of analyses from a 9 m sediment core recovered from the center of the lake indicate that oxygen and carbon stable isotopes of calcite co-vary as expected in a closed-basin lake down to 4.25 m below the lake bottom. Fractionation corrected oxygen isotope values show systematic mixing between present lake water (-0.3 to -4‰) and the present regional groundwater (-11 to -15‰), indicating that when more groundwater is entering the basin, isotopic signatures become more negative. At certain times in the past isotopic values have been more positive (more saline) than the current lake water composition. Analysis of diatoms preserved in the sediments indicate that when isotopic compositions are more positive (more evaporation and less groundwater) the diatom assemblages represent a more saline (higher TDS) lake, and when isotopic values are more negative (greater groundwater input and less evaporation), the diatom assemblage indicates a less saline lake (The presence of blooms of Chaetoceros muelleri indicates higher productivity and freshening of the lake during these times). Age dating of the sediment core is currently underway, but preliminary results indicate that the record extends back at least 4,000 years before present. Correlation of isotopic and diatom excursions in the core with known Great Basin dry and wet periods is also being evaluated.