EARLY HOLOCENE CLIMATE AND VEGETATION ACROSS TEXAS USING OXYGEN AND CARBON ISOTOPES IN LAND SNAIL SHELLS

The large humidity gradient across Texas gives rise to forests in the east and desert in the west. With a warming climate, there is potential for arid ecosystems to expand eastward, posing a significant impact on densely populated regions in central Texas. Understanding the distribution of vegetation in the past may help us understand ancient aridity gradients and human influences on ecosystems through time. Prior research has shown that the δ13C values of the shells of the land snail Rabdotus decrease with increasing canopy cover whereas oxygen isotope compositions provide an indicator of humidity. In this study, we measured the stable carbon and oxygen isotope compositions of Rabdotus shells as indicators of climate and vegetation, obtained from collections at the Texas Archeological Research Laboratory (TARL). TARL collected these shells from two early Holocene archaeological sites. The mean δ13C values of Rabdotus shells from an eastern site in Zilker Park, Austin, and a western site near San Angelo are -9.1 +/- 0.1 and -8.1 +/- 0.2 ‰, respectively. Considering the 2‰ decrease in the δ13C value of atmospheric CO2 over the past 150 years, these shell values correspond to a canopy cover greater than 70%, with the lower values in Austin possibly indicating even greater canopy cover. The mean δ18O value at both sites is 30.9 +/- 0.2 ‰, suggesting a similar humidity. The difference in canopy cover despite comparable humidity could be attributed to human influence on vegetation during the early Holocene in Central Texas.