LATE QUATERNARY EXPANSION AND CONTRACTION OF DESERT WETLANDS IN RESPONSE TO ABRUPT CLIMATE CHANGE: TULE SPRINGS FOSSIL BEDS NATIONAL MONUMENT (TUSK), NEVADA

Tule Springs Fossil Beds National Monument (TUSK), located in the upper Las Vegas Wash of southern Nevada, contains one of the most extensive paleowetland deposits in the American Southwest. Recent investigations by the San Bernardino County Museum and U.S. Geological Survey, in cooperation with the Bureau of Land Management, have focused on determining the geologic and temporal context of thousands of late Pleistocene vertebrate fossils recovered from fine-grained paleowetland deposits at hundreds of localities within TUSK. Outcrop-based mapping and targeted chronometric dating were utilized to augment previous studies and to address several outstanding questions regarding the disposition, age, and duration of several stratigraphic units. The paleowetland deposits, informally called the Las Vegas Formation, are composed of stratigraphically ascending units A through G and intervening soils, with units B, E, D, and F containing respective subunits. The units represent discrete groundwater discharge episodes, which were punctuated by relatively brief discharge hiatuses as evidenced by soil development and/or widespread erosion events. Radiocarbon and luminescence dating of the initiation and cessation of discrete discharge episodes enabled the temporal bracketing of each discharge hiatus, providing a high-resolution record of successive wetland development and collapse for the last 35 ka and a coarser, longer-term record spanning the last ~100 ka. In sum, the TUSK hydrologic record shows a tight correlation with cold/warm oscillations in Greenland ice-core records, particularly during MIS 1-3, in which the wetlands repeatedly expanded and collapsed at multiple times in response to abrupt climate change. Additionally, evaluation of lithologic characteristics, including bedforms and diagnostic tufa morphologies, allowed for the reconstruction of the dominant discharge type (i.e., spring fed streams, spring cauldrons, marshes, wet meadows, phreatophyte flats) represented by each unit or subunit, thus providing additional hydrologic information for these intervals. Our results show that springs and desert wetlands in the southwestern U.S. are sensitive to climate change and illustrate the potential vulnerability of these fragile ecosystems to future anthropogenic warming.