EOLIAN DEPOSITIONAL AND ENVIRONMENTAL HISTORY FOR THE MONAHANS DUNE SYSTEM, WEST TEXAS FOR THE PAST CA. 0.5 MA BY THERMAL-TRANSFER OPTICALLY STIMULATED LUMINESCENCE (TT-OSL) DATING

Monahans eolian system in west Texas was studied using 15 Geoprobe cores, > 5 m-long, which provides new stratigraphic records spanning the past 500 ka. Stratigraphic modeling of this basin shows eolian sediment infilled a fluvial landscape of the ancestral Pecos River. Quartz grains from the oldest part of the record was dated using thermal-transfer optically stimulated luminescence (TT-OSL) protocols, yielding ages 60-515 ka. The younger TT-OSL ages on quartz grains of ~240-60 ka correspond well with single aliquot regeneration (SAR) ages on the same grains. The TT-OSL chronometric signal is 1-2% of the corresponding SAR signal, yielding 1000s of photons; sufficient for dating. Notably, the TT-OSL emissions show sensitivity to solar resetting, with > 90% signal reduction after 48 hr sunlamp exposure. Modern analogs show a statistically insignificant TT-OSL signal corresponding to a near-zero age. Pedostratigraphic facies analysis and associated OSL ages are the bases for identifying twelve Eolian Depositional Phases (EDP). These EDP are distinct events of eolian sedimentation, often succeeded by pedogenesis and/or erosion. This stratigraphic sequence shows two major divisions of six lower EDPs spanning from > 515 to 96 ka demarked by the presence of truncated, carbonate-rich paleosols (stage II to IV) or with a well-developed argillic horizon. These lower EDP infill an antecedent fluvial landscape, with terrace forms and sinkholes to depths > 40 m. The upper-most six EDP spanning the past 70 ka, with modest pedogenesis showing cambic horizons. Eolian activity occurred mostly during interglacials or interstadials with 2 to 10 meters’ thick eolian sand with TT-OSL ages of ca. 515-465, 380, 255, 222, 188, 96, 70-40, and the past 12 ka. Pedogenesis is mostly confined to cooler and wetter glacial/stadial periods. A playa lake formed adjacent to the Monahans dune field between 335 and 200 ka, coincident with intervals of pedogenesis in the dune field stratigraphy. This record reveals prolong periods of pedogenesis, > 20 ka, and landscape stability with intervening intervals of eolian infilling for the Monahans Dune Field between 515 and 70 ka. Eolian deposition dominates the past 70 ka with a landscape mosaic of weak to modest pedogenesis; pronounced eolian deposition occurred from 70 to 40 ka and in the last 12 ka.