Preliminary Bat Guano Isotope Evidence for Past Vegetation and Climate Change in Southwest Virginia

Abstract

The Appalachian Mountains, one of the most biologically diverse regions in the temperate world, have been heavily altered by human activity for millennia yet the relative roles of human and other disturbances and climate change in creating modern landscapes are not well understood. Holocene paleoenvironmental records could provide a window into past Appalachian landscapes, but are restricted by a dearth of appropriate sites, such as natural lakes. Recent research suggests that bat guano deposits can serve as valuable archives of past environmental change. Carbon isotope (δ13C) values of guano from insectivorous bats can reflect the relative abundance of forest (C3) versus grassland (C4) vegetation at a regional scale, while guano nitrogen isotope (δ15N) values have been linked to landscape-scale N-cycling and precipitation. To investigate the paleoenvironmental history of an Appalachian site in southwest Virginia, we collected a 170 cm guano core from a limestone cave on Salt Pond Mountain. Bats are no longer active in the cave, but recovered bones indicate the past presence of Myotis species. Preliminary results show clear trends of increasing δ15N values and decreasing δ13C values from deep in the core toward the surface. δ13C and δ15N values are also significantly negatively correlated (R2=0.6, p=5.6e-15). The increase in δ15N values may signal a moistening climate toward the present, a finding consistent with regional Holocene pollen records. Higher δ13C values deep in the core likely indicate a greater prevalence of C4 grassland vegetation on the landscape, which could be linked to drier climate, indigenous burning, or both. Increased variation in δ13C and δ15N values below about 65 cm may indicate more variable climate or increased disturbance during the time represented. Forthcoming radiocarbon dates will anchor these trends in time and inform correction for the Suess effect, while other proxies could help disentangle the drivers of landscape change.