LATEST PLEISTOCENE TO HOLOCENE PALEOCLIMATE RECORDED IN EOLIAN DEPOSITS OF THE PACIFIC NORTHWEST

The Palouse loess and its paleosols document climate fluctuations in the Pacific Northwest during the latest Pleistocene and Holocene. Unlike other areas such as the Great Plains, Palouse loess paleosols tend to reflect colder and drier conditions that supported shrub-dominated vegetation. Conversely, loess between paleosols is associated with wetter conditions and bunchgrass-dominated vegetation. During the last glacial maximum, the glacial anticyclone weakened prevailing southwesterly winds in the Pacific Northwest resulting in thin loess accumulations accompanied by the development of the Washtucna Soil. As the anticyclone dissipated, prevailing southwesterly winds returned and dust began to aggrade rapidly. Along the upwind margin of the Palouse at Juniper Canyon, Oregon, the Mt St Helens set S and the Glacier Peak tephras bracket 4.3 m of loess that accumulated between 15,400 and 13,100 yr B.P. High mass accumulation rates (MAR) calculated over this latest Pleistocene interval are a factor of 10 higher than average MARs during the Holocene. Dust production and accumulation slowed after ca. 13,000 yr B.P. and corresponds to the formation of the Sand Hills Coulee Soil. This soil contains similar components to those found within other Palouse paleosols including: 1) an opal phytolith assemblage consistent with the dominance of shrub-steppe vegetation, 2) cicada burrows that indicate shrub vegetation, and 3) calcium carbonate indicative of arid conditions. The age and features of the Sand Hills Coulee Soil suggest that it is a signature of the Younger Dryas (ca. 13,000 to 11,500 yr B.P.) in the Palouse loess. Dust accumulation increased following the Younger Dryas and continuing into the Holocene. Opal phytolith assemblages within loess post-Younger Dryas suggest bunchgrass steppe was dominant but occasionally interrupted by drier shrub-steppe intervals. Dune sands near Juniper Canyon and other parts of the Columbia Plateau contain Mt Mazama tephra (ca. 7,600 yr B.P.) that suggests dune migration in the early- to mid-Holocene. These intervals of sand dune activity correspond to increased wind velocities and/or decreased soil moisture and vegetation density in the Palouse.