GROUNDWATER EVIDENCE OF PAST CLIMATE CHANGE IN THE AMARGOSA DESERT

Isotopic and Major ion chemistry from wells in the Amargosa Desert region, was analyzed intending to better understand the general flow system and recharge patterns around Fortymile wash near Yucca Mountain. Corrected ¹⁴C dating of groundwater beneath Fortymile Wash presents ages between 8 thousand years in the upper region and 14 thousand years in the lower region near the Amargosa Valley. This range in ages corresponds to the end of the Pleistocene and early Holocene, marking the end of the Wisconsin glaciation and beginning of the current interglacial period. In contrast, groundwaters just adjacent to Fortymile Wash appear to be significantly older. This suggests that the recharge processes beneath the wash differ from those influencing the adjacent areas and is likely due to focused infiltration of surface runoff along the course of Fortymile Wash. The range of the ¹⁸O data beneath the wash correspond to a relatively cold clime precipitation and have a similar spatial signature to the ¹⁴C data. ¹⁸O values in the upper part of Fortymile Wash correspond to the warmer climate of the cold clime range, but still cold relative to the present; whereas values found in the lower portion of the wash correspond to colder climates. This signature is evidence of changes to the ground water system as the climate became warmer and dryer during the past 14 thousand years. Beginning at the source of the wash in the north and following south along the path, waters become sequentially older therefore suggesting that the average reach of runoff events and recharge have diminished over time as the climate became warmer and dryer. The old signature hypothesis appears to be supported by a Principal Component Analysis (PCA) of the major ion data and Cluster Analysis of these results group wells with a similar genesis. The resulting principal axes and well groups are presented as biplots and overlain on a digital elevation map of the region. These analyses demonstrate a flow path that follows the surface topography more closely than would be predicted with contemporary groundwater levels in the region. Plots of ²H versus ¹⁸O compared with the Global Meteoric Water Line (GMWL), demonstrate a humid climate type of precipitation with little surface evaporation before infiltration.