Faunal burrows as indicarors of paleo-vegetation in eastern Washington, USA

The Palouse loess deposits in the Pacific Northwestern United States contai n buried paleosols with distinctive biological fabrics. The Washtucna Soil, formed 40,000-15,000 yr ago during the last full glacial, contains up to 9 0% by volume distinctive cylindrical pedotubules that are 1-2 cm in diamete r formed by burrowing fauna. The overlying post-glacial loess and modern su rface soils that support perennial grasslands lack these cylindrical burrow s. We investigated the paleoecological significance of the burrows. Observa tions of several types of burrowing fauna under native vegetation indicated that nymphs of cicadas (Homoptera: Cicadidae) are responsible for cylindri cal, back-filled burrows that match those in the Washtucna Soil. We measure d the abundance of active cicada burrows in soils in four native vegetation zones (Soil sites), establishing that cicada burrows comprise 19% of the r ooting zone volume in sagebrush steppe, but just 1-3% of the rooting zone v olume in bunchgrass steppe, meadow steppe and coniferous forest. This sugge sts strongly that abundant cicada burrows in paleosols are a proxy for the geographic extent of plant communities that contained sagebrush. To assay t he activity of cicada nymphs through time, we measured burrowed volume by d epth from the soil surface down through the Washtucna Soil (i.e. from the p resent to ca. 40,000 yr BP) at five stratigraphic research sites (Paleosol sites). Cicada-burrowed volume is generally less than 5% in the rooting zon e of the surface soils and throughout Holocene loess, but is up to 94% in t he rooting zone of the Washtucna Soil at Paleosol sites that currently supp ort bunchgrass steppe outside the present-day sagebrush steppe zone. Cicada host preferences in vegetation zones today suggest that sagebrush was the dominant shrub as the Washtucna Soil formed. From this reconstruction, comb ined with information from studies of pollen and plant opal, we infer that periglacial steppe plant communities dominated by sagebrush spread, from 40 ,000 to 15,000 yr up, into areas of the Columbia Plateau that support bunch grass steppe, meadow steppe, and even some coniferous forest today. Trace f ossils of soil fauna in paleosols are indicators of fluctuations in biologi cal activity in response to climatic changes of the Quaternary Period in Pa louse loess deposits. (C) 2001 Elsevier Science B.V. All rights reserved.