Isotope record of groundwater recharge mechanisms and climate change in southwestern North America

Understanding the response of groundwater systems to changes in climate is crucial at a time when human -caused climate change appears to be increasing in magnitude and rate. Groundwater preserves records of past effects resulting from climate change at the time-scale of late Pleistocene-Holocene climate evolution. Detailed regional datasets provide opportunities for evaluating past changes as a means of anticipating future climate -groundwater relations. Isotope parameters delta 18O, delta 2H and uncorrected 14C, considered together in large groundwater datasets from southwestern North America, provide evidence of changes in groundwater recharge mechanisms and the climate changes that led to them. The evidence consists in positive shifts in delta 18O and delta 2H in paleowater recharge related to a 13-15 ka shift recorded in in delta 18O of speleothem deposits, and as concurrent changes in recharge seasonality in the core area of the North American monsoon. A negative shift in delta 18O and delta 2H, most likely contemporaneous with a regional 51-55 ka change speleothem deposits, is recorded in certain basins with deep, confined groundwater. A14C threshold of 10 percent modern carbon serves empirically to distinguish paleowaters before and after the 13-15 ka event. A younger, negative isotope shift has occurred in Baja California. The 13-15 ka shift is regional but is not recorded in all basins studied and appears to be absent at the northwestern and southeastern limits of the study area. Relations among the isotope parameters may be complicated by factors such as isotope altitude effects, delayed melting of Pleistocene ice, changing degrees of evaporation in river water and introduction of anthropogenic 14C. Recharge mechanisms fall into two patterns: (1) dominant winter recharge with varying degrees of evaporation prior to infiltration, and (2) recharge in both summer and winter, but only during the wettest months. Pattern (2) replaced pattern (1) at 13-15 ka in the core area of the North American monsoon. Later, the present-day pattern of recharge from summer-fall rain associated with tropical depressions replaced predominant winter recharge in southern and eastern Baja California. A post -1950 negative shift in delta 18O and delta 2H, observed in southern Nevada and northern New Mexico, may be of anthropogenic origin and related to development of large-scale irrigation in California.