A HIGH-RESOLUTION MULTI-PROXY RECORD OF LATE PLEISTOCENE AND HOLOCENE CLIMATE CHANGE FROM GREAT MARSH, SOUTHEASTERN PA

Late Pleistocene and Holocene climate change records derived from freshwater wetland deposits are rare in the unglaciated Mid-Atlantic region of the U.S. An exception is Great Marsh, a sedge-dominated valley bottom wetland in the Brandywine Creek watershed of southeastern Pennsylvania. Great Marsh was not dammed for water-powered milling, was not buried by historic sediment, and has remained relatively pristine since at least the late Pleistocene. The scientific value of Great Marsh was recognized over 50 years ago (Martin, 1958; Bricker and Moss, 1958).

Here we present the first multi-proxy climate reconstruction of this valley bottom sequence recovered from a 2-m vibracore. The bottom of the core (2.00-1.2 m) is a blue-grey silty clay deposited in an oligotrophic lake. Pollen species include Spruce, Jack Pine, and Fir are characteristic of a taiga/periglacial environment. Diatoms are absent. Quartz-gravel lenses in a brown silt matrix (1.29-1.08 m) occur above the blue-grey clay, which we interpret to be from mass wasting and/or freeze-thaw processes due to warming climate. This sequence ends abruptly into a brown organic-rich clayey silt (1.08-0.89 m), interpreted to be a more temperate late-Pleistocene climate. An overlying grey silty clay (0.89-0.68 m) represents a transient return to a cooler climate, perhaps the Younger Dryas, where pollen taxa include both taiga and temperate species. Dominant diatom taxa indicate a periglacial lake environment. This sequence is overlain by organic-rich silty clay (0.68-0.59 m) that gradually transitions to a freshwater peat (0.59-0.00 m), where micro- and macrofossils indicate a high-quality (low N and P) marsh and temperate climate. Carya seeds collected at the bottom contact (0.59 m) yield a ¹⁴C date of 10,760-11,140 cal. yrs. BP (2-sigma).

Great Marsh reveals an apparently continuous record of climate change since the Late Pleistocene, and provides a paleo-analog for today’s thawing Arctic regions. Contrary to common perception, our work throughout the Mid-Atlantic Piedmont shows that such Holocene valley bottom wetlands are not rare, simply obscured under stacks of anthropogenic sediment. This recognition provides a rich, new opportunity to reconstruct high-resolution late-Pleistocene to Holocene climate records throughout the unglaciated Mid-Atlantic.