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

Abstract

The Palouse loess deposits in the Pacific Northwestern United States contain buried paleosols with distinctive biological fabrics. The Washtucna Soil, formed 40,000–15,000yr ago during the last full glacial, contains up to 90% by volume distinctive cylindrical pedotubules that are 1–2cm in diameter formed by burrowing fauna. The overlying post-glacial loess and modern surface soils that support perennial grasslands lack these cylindrical burrows. We investigated the paleoecological significance of the burrows. Observations of several types of burrowing fauna under native vegetation indicated that nymphs of cicadas (Homoptera: Cicadidae) are responsible for cylindrical, back-filled burrows that match those in the Washtucna Soil. We measured the abundance of active cicada burrows in soils in four native vegetation zones (Soil sites), establishing that cicada burrows comprise 19% of the rooting zone volume in sagebrush steppe, but just 1–3% of the rooting zone volume in bunchgrass steppe, meadow steppe and coniferous forest. This suggests strongly that abundant cicada burrows in paleosols are a proxy for the geographic extent of plant communities that contained sagebrush. To assay the activity of cicada nymphs through time, we measured burrowed volume by depth from the soil surface down through the Washtucna Soil (i.e. from the present to ca. 40,000yrbp) at five stratigraphic research sites (Paleosol sites). Cicada-burrowed volume is generally less than 5% in the rooting zone of the surface soils and throughout Holocene loess, but is up to 94% in the rooting zone of the Washtucna Soil at Paleosol sites that currently support 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, combined with information from studies of pollen and plant opal, we infer that periglacial steppe plant communities dominated by sagebrush spread, from 40,000 to 15,000yrbp, into areas of the Columbia Plateau that support bunchgrass steppe, meadow steppe, and even some coniferous forest today. Trace fossils of soil fauna in paleosols are indicators of fluctuations in biological activity in response to climatic changes of the Quaternary Period in Palouse loess deposits.