PALEO-ANALOG EVIDENCE OF PERMAFROST THAW IN SOUTHEASTERN PENNSYLVANIA

During the last glacial maximum 33,300 to 26,500 years BP, areas now encompassed within southeastern Pennsylvania and northern Maryland were not covered by the Laurentide ice sheet, but earlier work indicates that the region was subject to periglacial environmental conditions and processes. Previous analysis of macrofossils from a sediment core in a vernal pool at Kings Gap Pond, northern slope of South Mt, indicate that tundra vegetation persisted until at least 14,000 yrs BP. Using slope analysis of lidar data, we document widespread evidence of periglacial landforms that are indicative of permafrost thaw, and we attribute the youngest of these to the last glacial maximum. These fossil landforms include gelifluction lobes and sheets produced by a combination of bedrock fracturing and downslope mass movement over a frozen inactive layer of permafrost during climate induced seasonal freeze-thaw cycles. In places, sheets and lobes dominate the landscape and commonly are stepped, with multiple cases of younger lobes over-riding older ones. Along ridge lines we have identified numerous relics of nivation hollows, concave features on hill slopes that are the product of snow patch accumulation, freeze-thaw and mass wasting. We also document massive paleo-slumps along mid- to lower parts of hillslopes that we attribute to permafrost thaw, and suggest that their origin is similar to well-known examples of retrogressive thaw slump in the Arctic today, such as the Sewalik thaw slump in Alaska. Finally, the massive amounts of downslope movement of material during times of permafrost thaw impacted valley bottoms, in places leading to development of extensive debris and alluvial fans, and in some places these landforms blocked valley bottoms. We developed a website that provides examples of these features and describes their origin in the context of climate change and permafrost thaw (www.fandmgeo-breakingtheice).