Using Soils, Tephras, and Luminescence to Reveal the Holocene Record of Loess in the Pacific Northwest

Luminescence ages, tephras, and paleosols are providing a clearer picture of the latest phase of loess accumulation in the Palouse of the Pacific Northwest. The timing of Holocene loess accumulation has been poorly understood until recently. The Sand Hills Coulee Soil (SHC), a calcic paleosol originally interpreted to have formed sometime between the late Pleistocene and the mid-Holocene, has been constrained to the latest Pleistocene with the aid of tephrochronology and luminescence chronology. The soil provides a regional marker for basal deposits of Holocene loess and represents a period of stability and aridity in the latest Pleistocene. Downwind of the Umatilla Basin, the SHC formed at the top of thick loess rapidly accumulated downwind of glacial outburst flood deposits. In Channeled Scabland settings, the soil formed within thin loess overlying flood sediments. Only one locality has revealed rapid accumulation of loess in the Holocene. At the Clyde site, approximately 2 m of loess was accumulated over about 1-2 kyr, coincident with mid-Holocene aridity. Clyde is located downwind of Eureka Flat, a deflational area that has been the major source of dust for the Palouse loess over much of its history. Mid-Holocene aridity has also been documented in sand dunes and lacustrine deposits of the region. Sites where thin, 1 m or less, Holocene loess caps the SHC correspond to the onset of cooler, wetter climates in the late Holocene, suggesting dust sources were less active compared to the latest Pleistocene when dust was liberated from expansive areas of flood sediments. The accumulation of loess in the Palouse has been punctuated, related to the availability and distribution of dust sources and climate perturbations since the last glacial maximum.