EXTENDING THE ESTANCIA BASIN PALEOCLIMATE RECORD BACK WITH U-SERIES DATING OF GYPSUM SAND LAYERS

One of the most significant paleoclimate records in the American Southwest was derived from the study of Estancia Basin lacustrine sediments 75 km southeast of Albuquerque, New Mexico. This record, like many lake sediment records, provided a high-resolution paleoclimate story extending back to the limits of ¹⁴C dating. Many lacustrine-derived paleoclimate records are restricted by this limit. We present U-series data on gypsum sand layers that extend the Estancia Basin record further to the base of the lacustrine sequence, where beyond that sequence are non-layered gypsum- and clay-rich sediments interpreted to be playa deposits. This novel approach of constructing a lacustrine chronology from U-series dating of gypsum sand layers should be applicable to semiarid regions globally.

Pleistocene Lake Estancia was a large perennial lake in the Estancia Basin in central New Mexico during the last ice age. The lake was largest and deepest during the Last Glacial Maximum, and was completely desiccated by the early Holocene. The ¹⁴C-based chronology established the Estancia Basin paleoclimate record back 31 ¹⁴C ky (36.4 cal. ky). Potential U-series datable authigenic minerals in the basin deposits include calcite, magnesite, and gypsum. Beyond 20 ka, only gypsum occurs in relatively pure form as gypsum sand layers. These layers represent gypsum that formed along the lake margins and was blown into the basin during drier climatic episodes. These gypsum sand layers therefore represent dry periods of climate. The U-series chronology, tainted by relatively high concentrations of thorium (U/Th = ~10), becomes robust at 30 ka, and matches the ¹⁴C chronology at that time period. Presence of gypsum sand layers near the base of the lacustrine sequence indicate that Pleistocene Lake Estancia existed from 65 cal. ka to about 12 cal. ka. Thus our U-series chronology pushes the lake record back another 25 to 30 ky. The period that Pleistocene Lake Estancia existed matches the period of stalagmite growth (56 to 11.4 cal. ka) in nearby Fort Stanton Cave. Lake sediment deposition and speleothem growth took place during the last ice age, and both indicate that effective moisture was greater and both provide high-resolution records of climate change. For example, of the seven gypsum sand layers dated, five correspond to Dansgaard–Oeschger events.