Reconstruction of the late Pleistocene grassland of the Columbia basin, Washington, USA, based on phytolith records in loess Mikhail Blinnikov a; *, Alan Busacca b , Cathy Whitlock c a Department of Geography, St. Cloud State University, St. Cloud, MN 56301-4498, USA b Department of Crop and Soil Science, Washington State University, Pullman, WA 99164-6420, USA c Department of Geography, University of Oregon, Eugene, OR 97403-1251, USA Accepted 16 August 2001 Abstract Silica phytoliths preserved in three loess sections in southeastern Washington State revealed a 100 000-year history of the Columbia Basin grassland. Changes in the proportion of different morphotypes indicate large shifts in vegetation composition during the last 100 ka. A low-elevation section (677 m asl) near the center of the basin provided a record of alternating xeric Festuca^Poa and mesic Festuca^Koeleria grassland. The middle-slope section (1095 m asl) supported Picea^Abies or Pinus ponderosa forest or non-analog parkland at different times. Some trees were present at or near the site even during the Last Glacial Maximum. The highest site (1220 m asl) supported Stipa-, Festuca- and Poa-dominated grassland with some Artemisia shrub during most of the late Pleistocene, but supports a coniferous forest today. Variations in vegetation can be explained as a response to changes in large-scale climatic controls. Grasslands and shrub steppe were apparently more widespread and forests more restricted than today during the marine isotope stages 2 and 4, probably as a result of cooler and drier conditions. The three new records are well correlated with previously published paleo-reconstructions based on phytolith, cicada burrow and stable isotope data from a nearby KP-1 loess section, Carp Lake pollen record, and global ice volume variations. ß 2002 Elsevier Science B.V. All rights reserved. Keywords: Columbia Plateau; grasslands; modern analogs; paleoecology; phytoliths; upper Pleistocene 1. Introduction The nature and causes of terrestrial environ- mental change on di¡erent spatial and temporal scales during the late Pleistocene remain poorly known at the regional level. In the Paci¢c North- west, terrestrial data are progressively scarcer as we move back in history beyond the last 15 ka. Existing environmental reconstructions of the Paci¢c Northwest (Barnosky et al., 1987; Heusser and King, 1988; Whitlock, 1992; Thompson et al., 1993) are based mostly on pollen records ob- tained from lakes in forested areas west of the Cascade Range, while the dry interior of the re- gion has received little attention due to lack of such sites. 0031-0182 / 02 / $ ^ see front matter ß 2002 Elsevier Science B.V. All rights reserved. PII:S0031-0182(01)00353-4 * Corresponding author. Fax: +1-320-529-1660. E-mail address: mblinnikov@stcloudstate.edu (M. Blinnikov). Palaeogeography, Palaeoclimatology, Palaeoecology 177 (2002) 77^101 www.elsevier.com/locate/palaeo