CLIMATIC AND HUMAN CONTROLS ON HOLOCENE VEGETATION CHANGES IN EASTERN PENNSYLVANIA BASED ON THE ISOTOPIC COMPOSITION OF SOIL ORGANIC CARBON

The δ¹³C of soil organic carbon and phytolith analysis are useful proxies for estimating the above-ground concentrations of C₃:C₄ biomass, which in turn, can be used to infer secular changes in vegetation, climate, and land-use. A soil δ¹³C time-series that spans the Holocene was constructed using 169 soil δ¹³C values, 22 radiocarbon ages, and artifacts from 6 floodplain soil profiles along the middle Delaware River valley. There is good agreement between increasing soil δ¹³C and Panicoideae phytolith concentrations (R^2=0.86, p=0.0003) within the samples, suggesting that variations in C₄ biomass is a major contributor to changes in the soil δ¹³C. A local regression (LOESS) curve through the entire dataset reveals eight isotope stages (I-VIII), with four distinct stages exhibiting more positive δ ¹³C (more C₄). Stage II, 9.5-8.5 Cal ka, is the most notable positive δ¹³C interval and is associated with a cool-dry climate and the 9.2 ka abrupt climate change event which further promoted dry conditions. Stage IV, 5.7-4.4 ka, is a positive δ¹³C interval associated with the mid-Holocene warm-dry climatic optimum and a notable drop in regional lake levels. Stage VI & VIII (more C₄) coincide with the first documented occurrence of maize in the NE USA and a substantial increase in population and maize-based agriculture during the Late Woodland cultural period, respectively. These associations suggest that humans influenced δ¹³C values during portions of the late Holocene. The trend towards more negative δ¹³C values during stage IX likely reflects the combination of forest recovery due to post-pandemic mortality and forced relocation of indigenous people. The results from this study suggest that variations in (paleo)soil δ¹³C along the Delaware River valley are partly controlled by changes in ground cover due to climate change and land-use. In particular Late Woodland farming and forest clearance had a notable effect on the soil δ¹³C signature in NE USA.