A HYDROCLIMATIC HISTORY OF THE SOUTHWESTERN UNITED STATES: RECONSTRUCTIONS FROM ROCKY MOUNTAIN DEGLACIAL LAKE RECORDS

Several studies suggest that the Southwestern United States underwent significant warming coupled with severe drought from the Late Pleistocene to the Holocene. However, the scale and forcing of these hydrological changes remains poorly understood. To address this gap, we analyzed a series of deglacial lake records spanning approximately 20,000 years from across the southeastern Rocky Mountains. We reconstructed temperature using the MBT’_5ME index and precipitation from the hydrogen isotope composition of sedimentary leaf wax compounds. Across all records, temperature changes showed consistent changes: rapid warming during deglaciation, mild cooling during the Younger Dryas (~1°C), and a Holocene thermal maximum followed by a gradual cooling of 0.8°C towards the present. Similarly, isotopic signals displayed region-wide coherence, indicating systematic shifts in atmospheric circulation throughout the Late Pleistocene and Holocene epochs. The glacial period was characterized by wetter conditions with more negative isotopic values, transitioning swiftly to drier conditions with more positive isotope values at the onset of the Holocene. The driest phase occurred between 11,000 to 8,000 years ago, coinciding with peak boreal summer insolation. A gradual recovery during the Mid- to Late-Holocene was interrupted by a significant millennial-scale shift to wetter conditions between 8,000 to 6,000 years ago, evident in isotopic records across the western US. The Younger Dryas period exhibited relatively minor cooling with no clear isotopic anomaly, inconsistent with expectations of a major climatic event. Despite the similarity between isotopic trends, the magnitude and abruptness of changes varied considerably, particularly towards southern and eastern regions, highlighting the spatial variability in large-scale atmospheric and hydrological processes that occur across the Rockies.