LATE PLEISTOCENE TO MIDDLE HOLOCENE PALEOLIMNOLOGICAL VARIABILITY PRESERVED IN THE SEDIMENTS OF WALDEN POND, MA

After the Last Glacial Maximum, the climate of the Northern Hemisphere exhibited a general warming trend up to the Holocene Climatic Optimum (HCO), with intermittent climate anomalies observed in many proxy records. The most notable cooling is that of the Younger Dryas (YD), which is evident in many records from the North Atlantic region at the end of the Pleistocene Epoch.The purpose of this study is to reconstruct paleolimnologic environmental conditions in Eastern Massachusetts during the period of time bracketed by the YD and the HCO and to test the hypothesis that these changes are sensitive to regional and hemispheric climate change. We utilized the sediment archival record preserved in Walden Pond (Concord, MA) for this study, specifically carbon, nitrogen, and sulfur elemental and stable isotope proxy records. Initial sub-bottom SONAR data taken in 2016 indicated a thick sediment package in the deep western basin, and in 2017 a Livingstone sediment core recovered ca. 9 meters of sediment. Eleven AMS radiocarbon dates were used to calculate an age model, which has a basal age of 13,775 ± 280 cal BP. The base of the core has low %OC (ca. 3.8%), increases rapidly to ca. 18.2% at the end of the YD, and has an increasing trend through the HCO. At the end of the YD, sedimentation rates rapidly increase and δ¹³C values become less depleted, from a low of -27.91‰ during the YD to a maximum of -23.56‰ during the HCO. Increased productivity and water column stratification are both likely contributors to increased δ¹³C. Depleted δ³⁴S, enriched δ¹⁵N, and a decreasing trend in OC/N values through the early and middle Holocene are consistent with increased water column productivity and preservation of organic matter in hypoxic/anoxic conditions. In addition to the trends described, higher frequency variability is observed in the proxy records. Ongoing analyses at higher temporal resolution in the core will help to identify smaller amplitude events that may have occurred between YD and HCO and will allow for an assessment of cyclic components preserved in the record.