HOLOCENE WETLAND DEVELOPMENT AND PALEOECOLOGICAL HISTORY DURING THE HOLOCENE ALONG A VALLEY BOTTOM IN SOUTHEASTERN PENNSYLVANIA

Here, we present paleoecological research in the Appalachian Piedmont region in southeastern Pennsylvania at Great Marsh (150 m asl), a late Pleistocene to Holocene freshwater wetland that became established in a formerly periglacial (tundra) environment after permafrost thaw. A key goal of this study is to examine the stability of the spring-fed, valley-bottom ecosystem in the context of spatiotemporal changes in climate and vegetation. Subsurface sediment cores (~1 m depth) were extracted along a transect using a Russian peat borer. Cores were sampled at 1-cm intervals for loss-on-ignition (LOI) analysis, radiocarbon dating, and plant macrofossil identification. LOI was conducted by drying subsamples at 105°C to remove moisture, followed by combustion at 440°C to quantify organic matter content. Calibrated radiocarbon dates on fragments of unburned wood at depths of ~100 to 50 cm indicate rapid sedimentation from 11,311–11,747 cal BP (2 sigma) to 10,717–11,097 cal BP (2 sigma), with organic matter content generally increasing from ~11% (at 100 cm) to 75% (at 50 cm) during that period. The organic matter content generally decreases above 45-50 cm to 11% today at the ground surface. Plant macrofossils, primarily seeds, were extracted from each 1-cm subsample through wet sieving and identified under a microscope using reference libraries (digital and modern analogs). Plant macrofossil identification reveals the dominance of obligate wetland species throughout the core, representing the entire Holocene Epoch. Although the rate of organic matter accumulation decreased after ~8200 yrs BP (calibrated radiocarbon date of 8193-8369 cal BP for bulk sediment at sample depth of 35 cm), this work indicates long-term resilience and stability of the valley-bottom wetland ecosystem at Great Marsh throughout the entire Holocene.