POST-GLACIAL SEDIMENTATION IN OSSIPEE LAKE, NEW HAMPSHIRE

Land cover and climate changes, attributed to natural and anthropogenic forcings, cause deviations in geomorphic processes that act to deliver sediment from watersheds to lakes. In New England, contradictory evidence exists as to the influence of deforestation associated with European settlement and major flood events on watershed erosion rates over the past two and a half centuries. Through combining sediment core analysis from Ossipee Lake, New Hampshire with geomorphic analysis of the Ossipee Lake watershed, this study quantifies Holocene through Anthropocene watershed erosion patterns and links these patterns to historic events such as major storm events or deforestation, and long-term variations related to natural climate variability and post-glacial landscape evolution. A 2.2 m core was collected during the summer of 2017, providing an undisturbed record of environmental changes within the past ~1,700 years. An 8.2 m core, collected during the summer of 2018, captured the transition from glacial to post-glacial sedimentation. These cores are spliced into a composite record to span the Holocene to modern era. Bulk composition and age-depth modeling, utilizing both short-lived radioisotopes and radiocarbon dating of macrofossils, are used to quantify changes in deposition and inferred erosion rates over time. Additional insight on sedimentary processes is provided by measurements of magnetic susceptibility and bulk geochemistry. Lake-sediment data suggests clastic sediment mass accumulation rates vary between 0.008 to 0.038 g/cm²/yr, with increased frequency of event deposits occurring between 6 and 2 ka. Geomorphic analysis is used to identify regions within the watershed that act to deliver sediment to Ossipee Lake. Calculated bed shear stress along rivers highlights areas in the watershed capable of transporting sediment to Ossipee Lake, while the supply of sediment to the channels is influenced by the surficial geology.