The Holocene Phytolith Record from Morgan Playa in the Rolling Plains of Texas

Abstract

Phytolith analyses of playa-fill sediments from Morgan Playa yield strong evidence for Holocene vegetation and climate change. Morgan Playa is located in the Rolling Plains of Garza County, north-central Texas. Backhoe excavation of the small (about 3.2 hectares) basin exposed 250 cm of Holocene age sediments. A strongly developed Early Holocene paleosol occurs at the base (205-250 cm) of the profile. Dark gray-brown, clayey playa sediments compose the upper 205 cm. A single bulk radiocarbon sample from 170-180 em dating to 6840±140 yrs BP (Beta-48207) provides limited chronological control. Compositional changes in grass phytolith assemblages are used to define four bio-stratigraphic zones. Phytolith assemblages from the lowest zone, correlated with a paleosol, represent a mixed grass prairie. Climatic estimates derived from this zone suggest an annual temperature near present levels but significantly greater (+100 mm) annual precipitation than today. A transitional zone (145-205 cm) shows increase in warm-season short grasses (grama and buffalo grasses) and Aristida (three-awn). This change is associated with increasing aridity during the Middle Holocene. Warm-season short-grass phytoliths dominate the bulk of the profile (15-145 cm). Climatic estimates for this zone include both lower annual precipitation (-50 mm) and significantly higher (+3°C) annual temperatures than today. Reduction in short-grass phytolith percentages and an increase in phytolith percentages from cool-season grasses characterize the upper 15 cm of deposit. It is unclear whether this upper zone is the result of climate change or historic land use. The Morgan Playa record demonstrates the utility of phytolith analysis for inferring paleoclimatic change in a region where pollen analysis is unfruitful and unreliable.