Climatic and human influences on Holocene alluvial history and paleoenvironment of the middle Delaware River Valley, USA.

Abstract

The potential for future prolonged drought episodes in the Northeastern USA is alarming given that a humid climate currently provides water to +50 million people in the northeast, USA. Hydro-climatic projections are hampered by a lack of regionally-based paleoenvironmental reconstructions. The middle Delaware River Valley provides a unique opportunity to expand the Holocene alluvial history and paleoenvironment for the northeast, USA. Thirty-six soil profile descriptions, 332 grain size analyses, and 82 14C ages from trenches and auger borings show that similar alluvial landforms within the river valley have different formation histories and depict a valley that has experienced middle to late Holocene floodplain and terrace reworking. Despite erosion, secular changes in buried soil and sediment properties are closely associated with climate change and land-use. A Holocene time-series was constructed using 149 δ13Csom values from alluvial terrace profiles. There is good agreement between increasing δ13Csom and Panicoideae phytolith concentrations, suggesting that variations in C4 biomass are a contributor to changes in the soil δ13C. A measurement error deconvolution curve over time reveals two isotope stages (II and I), with nine sub-stages exhibiting variations in average δ13Csom (%C4). Stage II, ~10.7-4.3 ka, shows above average δ13Csom (increase %C4) values with evidence of an early Holocene warm/dry interval (sub-stage IIb, 9.8-8.3 ka) that coincides with rapid warming and cool/dry abrupt climate change. Sub-stage IId, 7.0-4.3 ka, is an above average δ13Csom (increase %C4) interval associated with the mid-Holocene warm/dry Hypsithermal. The Stage II-I shift at 4.3 ka documents a transition toward below average δ13Csom (decrease %C4) values, coinciding with decreasing insolation and moisture budget reorganization. Sub-stages Ib and Id (above average %C4) coincide with the first documented occurrence of maize in northeastern USA and population increase during the Late Woodland. These associations suggest that humans influenced δ13Csom during the late Holocene. The influence of land-use is further corroborated by a regionally extensive anthropogenic sedimentation event documented throughout eastern North America, pre-Colonial sediment (PCS) circa: A.D. 1,100–1,600. These data demonstrate that combined prehistoric land-use and climate change impacted eastern North American floodplains several hundred years prior to the onset of European Settlement.