ISOTOPIC EVIDENCE FOR RECHARGE OF LATE PLEISTOCENE MISSOULA FLOODWATERS INTO THE COLUMBIA RIVER BASALT AQUIFERS

Radiocarbon ages combined with stable isotopes of groundwaters provide strong evidence for the presence of Late Pleistocene Cordilleran Ice Sheet (CIS)-related recharge waters in the Columbia River Basalt Aquifers (CRBAs) in central Washington. Anomalously low δ¹⁸O values (-18.9 to -17.0‰) correspond to corrected ¹⁴C ages between 33.3 and 10.3 k yrs BP. Oxygen isotope values of Late Pleistocene precipitation in the Columbia Plateau region (-13.4 to -16.1‰), determined from studies of paleocarbonates, are within the range of average modern precipitation (-15.9‰). Therefore, it is reasonable to conclude that these isotopically depleted waters were not recharged by Pleistocene or modern precipitation, but rather by major Late Pleistocene discharges that are likely related to the CIS-associated glacial Lake Missoula outburst floods. Seven samples in the CRBAs have ¹⁴C ages between ~19-13 k yrs BP, the time when the majority of Missoula flood events are documented to have occurred based on radiocarbon age dates of flood deposits and field evidence. Four of these seven samples also correspond to Missoula flood event ages reported from 19 to 17 k yrs BP, based on low salinity anomalies determined by Lopes and Mix (2009) from freshwater diatoms in marine sediments off the coast of southern Oregon. Twelve samples from the CRBAs demonstrate ¹⁴C ages between 19 and 34 k yrs BP that likely correspond to pre Late Pleistocene CIS-related flows, which are also documented in the freshwater diatom record from the northeastern Pacific (Lopes and Mix, 2009). These may be related to pre-Late Pleistocene Missoula flood or other CIS-related flow events. Three samples have ¹⁴C ages between 10.3 and 6.7 k yrs BP that may correspond to Missoula flood events that postdate 13 k yrs BP. The presence of Late Pleistocene Missoula and/or other CIS-related floodwaters in the subsurface has important implications for better understanding the impacts of past climate change on groundwater recharge.