Climatic and human controls on Holocene floodplain vegetation changes in eastern Pennsylvania based on the isotopic composition of soil organic matter

A paleoenvironmental time-series spanning the Holocene was constructed using 29 radiocarbon ages and 149 standardized delta C-13(som) values from alluvial terrace profiles along the middle Delaware River valley. There is good agreement between increasing delta C-13(som) and Panicoideae phytolith concentrations, suggesting that variations in C-4 biomass are a major contributor to changes in the soil delta C-13. A measurement error deconvolution curve over time reveals two isotope stages (II-I), with nine sub-stages exhibiting variations in average delta C-13(som) (average %C-4). Stage II, similar to 10.7-4.3 Ica, shows above-average delta C-13(som) (increase %C-4) values with evidence of an early Holocene warming and dry interval (sub-stage IIb, 9.8-83 ka) that coincides with rapid warming and cool-dry abrupt climate-change events. Sub-stage IId, 7.0-4.3 ka, is an above average delta C-13(som) (increase %C-4) interval associated with the mid-Holocene warm-dry hypsithermal. The Stage II-I shift at 4.3 ka documents a transition toward below average delta C-13(som) (decrease %C-4) values and coincides with decreasing insolation and hydroclimatic change. Sub-stages Ib and Id (above average %C.4) coincide with the first documented occurrence of maize in the northeastern USA and a substantial increase in human population during the Late Woodland. These associations suggest that people influenced delta C-13(som) during the late Holocene. (C) 2013 University of Washington. Published by Elsevier Inc. All rights reserved.