Dr. Dorothy Merritts
Harry W. & Mary B. Huffnagle Professor of Geosciences
Program Chair of Environmental Studies
Franklin & Marschall College
Legacy landscapes: Freeze-dried, thawed permafrost landscapes, breached dams, and wetland-floodplain stream restoration, eastern United States
Decades of research on the geomorphology of the mid-Atlantic US make it timely to synthesize long-term (104 – 106) rates of geologic erosion and sediment generation with paleo-ecological and geomorphic records of landscape change. Such a synthesis reveals that legacy landscape effects from Pleistocene cold-climate conditions pre-determined Holocene warm-climate landforms and processes throughout the mid-Atlantic region. These periglacial landscape legacies likewise pre-determined the response of modern streams to base-level change from breaching (or removal) of 18th-19th century milldams. Most modern channels are in a transient state, one characterized by incision through historic and Holocene sediment into underlying Pleistocene cold-climate deposits and landforms. North of the latitude of present-day Annapolis, Maryland (~39) relict landforms provide a record of paleo-permafrost, consistent with previous paleocological work that indicates tundra vegetation during the late Pleistocene. Using lidar, orthoimagery, and geomorphic and paleoecological studies, we and our collaborators find extensive networks of thermal contraction polygons in shale bedrock; ubiquitous gelifluction sheets and lobes up to 10s of m thick on side slopes and valley bottoms; retrogressive thaw slumps and other thermokarst features; and water tracks similar to those in the Dry Valleys of Antarctica. We dated the sand infill within a relict thermal contract wedge to 14.9 1.8 ka (1 sigma weighted mean age for two OSL ages). Gelifluction on hillslopes produced poorly sorted, matrix-supported deposits indicative of mass movement under saturated conditions. These deposits accumulated in valley bottoms, leading to local aggradation. We have identified fluvial channel networks buried by colluvium north of ~39 latitude, but this paleo-climatic signature diminishes southward beyond the limits of paleo-permafrost. Holocene landscape stability was enhanced by Pleistocene cold-climate conditions that led to widespread coarse-grained colluvial deposition on hillslopes and valley bottoms. Our radiocarbon dating at dozens of mid-Atlantic sites, combined with paleo-seed and other macro-fossil analysis, indicates that valley bottom wetlands supplied by groundwater became established on a substrate of periglacial deposits during the early Holocene in low-order (1st to 3rd) watersheds. Oldest radiocarbon dates from organic-rich wetland soils typically are ≤11.2 ka, post-dating the Younger Dryas cold period. At one valley bottom location (Great Marsh, southeastern PA), our calibrated radiocarbon dates of 19.1-19.9 kyrs BP for organic matter in silt immediately below a 12.1-13.2 kyrs BP organic-rich soil indicate a late Pleistocene-Holocene transition erosional disconformity between ~13.2 and 19.1 kyr BP. Multiple lines of evidence suggest this was a time of intense eolian activity and flushing of some sediment along valley bottoms. By early Holocene wetlands had become established at groundwater level throughout the region, and many persisted continuously until European settlement. Sedimentation rates in Holocene wetlands were low, typically <0.1 mm/yr. Widespread mill damming for water power, however, buried many valley bottom wetlands, with 18th-19th c. sedimentation rates at least 10-100 x greater. Regional studies in the eastern US indicate long-term, low erosion rates of ~8 to 15 m m.y.–1, whereas historic hillslope erosion rates are estimated to have been ~100 x greater and contemporary sediment yields up to 10 times greater (Reusser et al, 2015). Our work with LiDAR DEM differencing (acquisition in 2008 and 2014) indicates that historic reservoir sediment stored upstream of breached milldams is eroded rapidly as knickpoints propagate through this sediment. These erosion rates can be 1000 times greater than long-term geologic rates. Furthermore, Pleistocene colluvium beneath this historic sediment is more readily scoured because of high stream banks after incision and can enhance bank erosion as gravel bars of reworked colluvium develop along meander bends. In essence, valley bottoms with stored historic sediment in the eastern US are in a transient state for which “memory” strongly affects modern processes and rates of erosion.