A MULTIPROXY ANALYSIS OF SEDIMENT CORES FROM WALDEN POND, CONCORD, MA: INVESTIGATING NATURAL PROCESSES AND HUMAN IMPACTS

Most commonly known as the title location of Henry David Thoreau’s Walden, Walden Pond, a three-basin kettle lake in Concord, MA, has been a popular destination as a recreational, historical, and geologic landmark for centuries. This project focuses on discovering possible sedimentological markers of human activity and natural processes, allowing for the reconstruction of the history of Walden’s surroundings. Events of interest include the settlement of European migrants in 1635, the Cape Ann earthquake of 1755, the construction of the railroad in 1844, the establishment of a recreational park in 1866 and its destruction by a fire in 1906. A multiproxy analysis of three cores up to 70 cm long from Walden Pond’s three basins with depths of 16, 20, and 30 m was conducted. Each core was examined for sedimentary characteristics, such as magnetic susceptibility, grain size distributions from laser diffraction analysis, elemental concentrations using XRF technology, and composition as identified on smear slides. Age constraints are from a previously published age model as well as the identification of the Ambrosia rise in 28 cm core depth for the deepest basin core, which correlates with land clearing and the first European settlements in this area. At about 20 cm, there is a steep increase in magnetic susceptibility up to 13 x 10^-5 SI units for the deepest basin core, possibly reflecting increased industrialization of the area. The grain size distribution for the same core reveal that from 54-15 cm, the mean grain size of the clastic component of the lake sediment is in the coarse silt range with a mean of 40 μm and a median of 25 μm. At 2, 9, 26, 30, 42, and 46 cm there are unusually coarse horizons indicating a sudden influx of coarser material. From 15 cm to the top of the core, the grain sizes decrease with a mean of 32 μm and a median of 16 μm. Our age model corroborates that the shift towards smaller grain sizes started in the mid-19^th century. This can possibly be attributed to soil erosion from the logging of the surrounding forest, starting with the construction of the railroad in 1844 and heavy recreational use since the 1920s. The data from sediments of the last four centuries will be compared to pre-European sediments across all three basins in order to examine the environmental history of Walden Pond and the effects of anthropogenic impacts.