THE DYNAMIC RESPONSE OF DESERT WETLANDS TO ABRUPT CLIMATE CHANGE AND THE POTENTIAL EFFECTS ON LATE PLEISTOCENE VERTEBRATE FAUNAS

Abstract

Desert wetlands are keystone ecosystems in arid environments that are preserved in the geologic record as groundwater discharge (GWD) deposits. Because animals are drawn to these spring-fed water sources, GWD deposits commonly contain vertebrate fossils. In the Las Vegas Valley, Nevada, we investigated the stratigraphy, chronology, and paleontology of the Las Vegas Formation, a middle Pleistocene to early Holocene sequence of GWD deposits that entombs the Tule Springs local fauna, the largest and most diverse late Pleistocene vertebrate fossil assemblage from the Mojave and southern Great Basin deserts. 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. Vertebrate fossils occur in multiple temporally constrained, discrete spring discharge intervals punctuated by aridification events that led to wetland collapse, followed by reinitiation of vigorous spring activity. This cycle occurred repeatedly throughout the late Pleistocene and into the early Holocene, including the Younger Dryas climate event. The stratigraphic and chronologic framework of Las Vegas Formation provides a unique opportunity to discern if and when vertebrate faunas responded to these climate events, and we have established similar contexts throughout the Mojave, Great Basin, and Sonoran Deserts – all sites with documented vertebrate fossil remains. We find that sites throughout these regions reveal 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 are responding to changes in synoptic-scale climate patterns operating over a broad geographic region during the middle to late Quaternary. Consequently, understanding the temporal and depositional context of inset late Pleistocene assemblages offers utility in determining how and when vertebrates responded to these climate stressors regionally in the American southwest, up to and including the terminal Pleistocene extinction event.