Paper No. 46-3
Presentation Time: 1:30 PM-5:30 PM
QUANTIFYING VOLUME OF ANTHROPOCENE SEDIMENT STORAGE IN VALLEY BOTTOMS OF SELECTED NEW ENGLAND WATERSHEDS
Large-scale human modification of the northeastern U.S. landscape began in the 17th century with forest clearing and milldam construction. In the mid-Atlantic Piedmont region of the U.S., Walter and Merritts (2008) found that millpond deposits persist for centuries after dam breaching, resulting in fill terraces composed of Anthropocene sediment (in this region defined as material deposited in the past 300-400 years). Stratigraphic observations in the mid-Atlantic indicate that these laminated to massive fine-grained Anthropocene layers typically overly a prominent Holocene hydric soil which, in turn, overlies a Pleistocene basal gravel. We seek to test whether this set of processes applies to formerly glaciated New England, by quantifying Anthropocene sediment storage in valley bottoms over entire watersheds. This study focuses on two New England watersheds: the Sheepscot River in Maine and the South River in Massachusetts. We use stratigraphic analysis and radiocarbon dating to identify Anthropocene deposits, map planar terrace extents in each watershed using lidar digital elevation models, and estimate thickness of Anthropocene sediment found behind breached or removed milldams. The Sheepscot River watershed has 45 dam sites; 9 dams have been field checked and 3 show evidence of Anthropocene sediment storage. The South River watershed has 37 dam sites; 16 dams have been field checked and 14 dams show evidence for Anthropocene sediment storage. Preliminary stratigraphic analyses of cut banks show a massive fine sand and silt Anthropocene layer which sometimes is underlain by gravel and/or clay; no buried Holocene hydric soil has been found. Further evidence for Anthropocene milldam sedimentation comes from radiocarbon dating. The Sheepscot River watershed has median calibrated radiocarbon dates ranging from cal AD 1765-1838 from 4 basal cut bank locations at depths of 0.76-1.87 m. The South River watershed has 3 dates of median probabilities ranging from cal AD 1765-1781 at depths of 1.06-1.23 m. Due to limitations of radiocarbon dating for the past 400 years, these dates suggest either sedimentation just before or soon after milldam emplacement. Combining these dates with our stratigraphic analysis, our preliminary interpretation is that all observed historic sediment is related to mill damming.