COS 300-5 - Bat guano isotope evidence for Holocene 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, but the relative roles of human disturbance and climate change in creating modern landscapes are not well documented. Holocene paleoenvironmental records could provide a window into past Appalachian landscapes, but are restricted by a paucity of appropriate sites. Recent research suggests that bat guano deposits can serve as valuable archives of past environmental changes. In particular, 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, along with bat diet. 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. Radiocarbon dates on bulk guano indicate that the sampled guano pile accumulated over ~4000 years from 5320 to 1195 years cal BP. Recovered bones reveal the past presence of Myotis species. Preliminary isotope results show clear trends of increasing δ¹⁵N values and decreasing δ¹³C values from deep in the core toward the surface. Carbon and nitrogen isotope 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 (~ 3000 years cal BP) may indicate more variable climate or increased disturbance during the time represented.