A STABLE ISOTOPIC RECORD OF PALEOCLIMATE OF THE BOISE AREA, IDAHO

A 30 ka paleo-climate record of the Boise area, Idaho, USA has been delineated by analysis of the stable isotopic compositions of four groundwater systems. Groundwater ages for these systems are modern cold batholith waters, 5-15 ka thermal batholith waters, 10-20 ka frontal fault waters, and 20-30 ka Snake River plain thermal waters, respectively. Although not meteoric, the stable isotopic composition of local groundwater has been used as a surrogate for the stable isotopic composition of precipitation. Using the δ2H and δ18O composition of the groundwater, local groundwater lines (LGWL's) were defined for each system. Each LGWL has been evaluated with defined slopes of 6.94 and 8, respectively, and resulting deuterium excess values (d) were found for each groundwater system for each slope. Relative differences of the deuterium excess values for both slopes were similar. Time dependent changes in moisture source humidity and temperature, and Boise area recharge temperatures, calculated from stable isotopic data and the deuterium excess factors, agree with previous paleo-climate studies. Results indicate that from the last glacial maximum (LGM) to the present time the humidity over the ocean moisture source increased by 9%, sea surface temperature decreased 6-7 °C, and local Boise temperature decreased by 4-5 °C.