USING GRAIN SIZE ANALYSIS TO RECONSTRUCT THE MOISTURE BALANCE OF A CLOSED LAKE SYSTEM: ~14,000 YEARS OF WATER LEVEL CHANGES AT WALDEN POND

In closed lake environments, analysis of grain size variation over time can reveal shifts in water level changes, anthropogenic land use patterns, and sudden depositional events. Walden Pond, located in Concord, Massachusetts, is a closed lacustrine environment where an 8-meter-long Livingstone sediment core was extracted from the deepest basin in 30-meter water depth. Samples were collected from the core at 16 cm intervals to analyze the grain size variations. Due to their high content of biogenic components, sediments were treated chemically, removing organic matter and abundant diatom frustules that can mask the particle sizes of minerals. The effectiveness of our methodology was compared via microscopic analysis of smear slides across two common treatment techniques. The isolated mineral grains were measured using a laser diffraction particle size analyzer. A BACON age model of 11 radiocarbon ages dating back to 14,000 cal yr BP was used to assign median ages to each depth. Resulting grain size distribution center around a mode of ~40 micron (coarse silt). Coarser intervals are characterized by an increased volume of very fine to fine sand (70-200 micron) and are interpreted as periods of lower lake levels when coarser shore sediment is exposed to erosion. Intervals of larger grain sizes occur at depths of 700 - 583 cm (9600 - 7850 cal yr BP), 553 - 496 cm (7350 - 6650 cal yr BP), and 159 - 119 cm (1550 - 1150 cal yr BP). In comparison to lake level reconstructions from nearby Sluice Pond in Lynn, Massachusetts, coarser-grained intervals correlate with periods of lower lake level at Sluice Pond. We also observe significantly finer sediments at depths of 36-0 cm, spanning from the time of European settlement (~315 cal yr BP for Concord, MA) to present: this interval likely reflects anthropogenic changes in land use with increased erosion of finer soil sediments. Our findings contribute to a comprehensive understanding of Walden Pond’s sedimentation history and support interpretations of water level changes in surrounding lacustrine environments, offering valuable insights into eastern Massachusetts' paleoclimate. Looking ahead, we aim to correlate other geochemical and sedimentological proxies measured along this core to further refine the reconstruction of past environmental conditions at Walden Pond.