PALEOHYDROLOGY OF THE SOUTHWESTERN U.S. DURING THE LAST GLACIAL TRANSITION RECONSTRUCTED FROM PLUVIAL LAKE AND WETLAND SEDIMENTS OF THE SAN AGUSTIN PLAINS, NEW MEXICO

Paleoclimate records across the arid western U.S. show significant regional variation in timing of wet conditions that accompanied the last glacial-interglacial transition. To accurately reconstruct hydrologic variations across this region and make inferences about past climate forcing, sensitive and well-dated records are needed. We present new constraints on paleolake levels and paleo-groundwater discharge for the San Augustin Plains (SAP) closed basin lake system (New Mexico) since the Last Glacial Maximum, based on ¹⁴C, U-series, and OSL dating of sediments. Starting at ~25 ka, wetlands deposits record enhanced spring discharge in the upper (eastern) basin of SAP, while a saline lake existed in the lowermost (western) basin. By ~23 ka lake level rose abruptly, beginning a protracted lake transgression that culminated at ~19.5 ka with a maximum depth of ~45 m, before falling to <20 m by 17 ka. This lake cycle featured continuous carbonate tufa formation across the lower basin. Following this, the lake transgressed abruptly, reaching a greater depth of ~55 m from 16-15 ka, after which it dried to <10 m depth by ~14 ka. These two lake cycles agree well in timing with the global LGM and “Big Wet” phase of Heinrich Stadial 1, respectively, and with lake highstands recorded in many other lakes of the western U.S. This supports the hypothesis that these intervals featured south-shifted and intensified winter storm track activity during the last glacial transition. Marsh conditions prevailed in the lower and middle basins during the Younger Dryas chron, while wet meadows existed in interdune areas of the upper basin. During the Holocene, short, discrete wet intervals are recorded by wet meadow deposits in both upper and lower basins with millennial frequency at 9.9, 8.8, 8.0, 6.8, and 5.4 ka. These results provide new evidence of periodic wet periods in the early and middle Holocene, beyond the last glacial period.