THE ROLES OF SEDIMENT SUPPLY, SEDIMENT AVAILABILITY, AND WIND IN DRIVING THE LOESS ENGINE OF THE PALOUSE, PACIFIC NORTHWEST

The Palouse Loess has one of the most complete Quaternary paleoclimate records in the Pacific Northwest. Dozens of loess-paleosol sequences have been preserved with the most recent sequences closely tied to late Quaternary ice age climate variability. The ultimate controls of loess generation and accumulation (as opposed to paleosol development), here called the loess engine, is related to the complex interplay of sediment supply, sediment availability, and dust-transporting winds. Sediment supply, at least for the late Pleistocene, is tied to glacial outburst flooding (e.g., Missoula Floods) that deposited sand- and silt-rich sediment in prominent depocenters. Sediment availability is controlled by climate and vegetation density, with the loess source areas characterized by low density shrub-steppe during the cold and arid climate of the last glacial maximum (LGM). Wetter conditions correspond with the expansion of grasslands. The wind regime also likely changed, with the glacial anticyclone suppressing prevailing dust-transporting southwesterly winds during the LGM. As documented in latest Pleistocene loess-paleosol sequences, the calcic paleosols that predominate the loess represent cold and arid climates when loess accumulation was minimal due to the lack of strong, dust-transporting winds. As the glacial climate destabilized and the prevailing winds returned, multiple episodes of glacial outburst flooding also replenished the source areas. New luminescence dating of loess suggests that as the loess engine restarted, loess accumulated rapidly, coeval with and immediately following glacial outburst flooding episodes. Once wetter conditions of the Holocene prevailed, the loess engine slowed. Therefore, at least in the late Pleistocene, most of the loess-paleosol record is tied to glacial climates. Stratigraphic repetition of calcic paleosols and their striking similarity of features, including the ubiquitous cicada burrow fabric, spans the time prior to the last magnetic reversal (ca. 781 ka) and suggests that the thick loess-paleosol sequence is controlled by the interplay of arid glacial climates promoting soil formation followed by glacial outburst floods that resupplied the sand- and silt-rich sediment that was reworked by returning strong winds to produce sand dunes and loess.