THE REGIONAL HYDROLOGIC RESPONSE OF DESERT WETLANDS IN THE AMERICAN SOUTHWEST TO QUATERNARY CLIMATE CHANGE

Abstract

Desert wetlands are keystone ecosystems in arid environments, and represent a myriad of spring hydrologic environments that are preserved in the geologic record as groundwater discharge (GWD) deposits. In the Las Vegas Valley, Nevada, we investigated the stratigraphy, chronology, and paleoenvironments of the Las Vegas Formation, a middle Pleistocene to early Holocene sequence of GWD. The resulting high-resolution stratigraphy and chronology revealed that desert wetlands in the valley responded dynamically to abrupt climate change by expanding and contracting repeatedly during the last glacial period, in synchrony with warming and cooling events documented in the Greenland ice core record. All spring environments were affected, most strikingly evidenced when valley-wide marshes desiccated abruptly in response to warming and groundwater lowering during Dansgaard-Oeschger (D-O) events. Multiple temporally constrained, discrete spring discharge intervals are punctuated by aridification events (essentially “megadroughts”) that led to wetland collapse, followed by reinitiation of vigorous spring activity. This cycle occurred repeatedly through the middle-late Pleistocene and into the early Holocene, including the Younger Dryas climate event. We have also investigated GWD deposits at multiple sites elsewhere in the Mojave, Great Basin, and Sonoran Deserts, and have found evidence of past wet and dry cycles that are strikingly similar in timing and character to the GWD sequence of the Las Vegas Formation. The recognition that widespread desert wetland ecosystems closely track one another shows that they responded to changes in synoptic-scale climate patterns operating over a broad geographic region during the middle to late Quaternary. Understanding how, when, and why these ecosystems responded to abrupt climate stressors provides unique insight into the threatened future of arid land ecosystems throughout the American southwest.