Reconstruction of the late Pleistocene grassland of the Columbia basin, Washington, USA, based on phytolith records in loess

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.