A NEW LUMINESCENCE CHRONOLOGY AND PALEOECOLOGIC HISTORY OF HOLOCENE LOESS, PACIFIC NORTHWEST, USA

New luminescence ages and opal phytolith assemblages from the Palouse loess shed light onto the Holocene paleoclimate record for the Pacific Northwest. The well-documented record of climate change and eolian dynamics preserved in Pleistocene-aged Palouse loess contrasts with the less well known Holocene-aged loess. Eight infra-red stimulated luminescence (IRSL) ages spanning 3.6 m of loess document large changes in rates of loess accumulation since the latest Pleistocene. Lower accumulation rates in the latest Pleistocene and early Holocene are related to the Sand Hills Coulee Soil, a paleosol that records relative landscape stability. Five ages indicate that 2 m of Holocene loess accumulated between ca. 6 to 7 ka. Loess accumulation rates dramatically decrease after 6 ka and appear relatively constant through the rest of the Holocene. Regional records of nearby sand dune activity that peaked shortly after the Mazama climactic eruption, ca. 7,600 yr BP, corroborate the windy and arid conditions recorded in the loess. Regional lacustrine records also demonstrate a dry climate during this time. These complementary records support the hypothesis that eolian sand and loess accumulations are related genetically. Additionally, opal phytoliths extracted from the loess suggest, unexpectedly, that shrub steppe persisted at this locality from the last glacial maximum through ca. 6 ka. Phytolith assemblages from other localities in the Palouse connect the transition from shrub-steppe to grass with ice sheet recession. Shrub-steppe vegetation indicates arid conditions and serves as an effective trap for dust, promoting the preservation of loess. The transition to bunch-grass steppe after ca. 6 ka marks a change to wetter conditions. The timing of the rapid accumulation of loess and paleobotanical change in the Palouse during the Holocene corresponds with the mid-Holocene thermal maximum, recorded also in the desert southwest and Midwest US. This work helps to broaden the understanding of the widespread nature of the mid-Holocene thermal maximum across much of the western US and provides a more detailed Holocene paleoclimate history for the Pacific Northwest.