Late Pleistocene To Middle Holocene Paleolimnological Variability Preserved In The Sediments Of Walden Pond, MA.

Abstract

After the Last Glacial Maximum, the climate of the Northern Hemisphere exhibited a general warming pattern until a period of time recognized as the Holocene Climatic Optimum. The trend of gradual warming was disrupted by multiple climatic events. Of these events, the most notable is the Younger Dryas. This is the cooling event that defines the end of the Pleistocene Epoch. The purpose of this study is to reconstruct paleolimnological environmental conditions in Eastern Massachusetts during the period of time bracketed by the Younger Dryas (YD) and the Holocene Climatic Optimum (HCO) and to test the hypothesis that these changes are sensitive to regional climate change. We utilized the sediment archival record preserved in Walden Pond (Concord, MA) for this study, specifically the proxy records of stable carbon, nitrogen, and sulfur isotopes. Initial sub-bottom SONAR data taken in 2016 indicated a thick sediment package in the deep western basin of the pond, and in 2017 a Livingstone sediment core recovered approximately 9 meters of sediment. Eleven AMS radiocarbon dates were used to calculate an age model of the core, and the model calculates a basal age of 13,775 ± 280 cal BP. Sediment samples taken at 2 cm intervals down the depth of the core reveal a relatively low %OC at the base of the core. This value is about 5.00% and starts drastically increasing to high of 18.00% OC at age 11,671 BP. This trend in increasing %OC matches with a display of an increasing sedimentation rate which occurs as we pull out of the Younger Dryas. With an increasing sedimentation rate and an increasing %OC, it becomes evident that there is an increase in productivity within water column. When comparing the sedimentation rate and the %OC to the δ13Cvalues become less depleted ranging from -28.00‰ to -23.00‰ between the base of the core up to 4000 yrs. BP with some fluxes in between. Increased productivity and poor mixing are both contributors to increased δ13C. Walden Pond is a contained environment with no input of water from rivers or streams. The main sediment source is surrounding matter, windblown sediment and precipitation. The Younger Dryas displays an evident signature in the data with abrupt changes in sedimentation rates and low percentages of organic carbon. Coming out of the cold, arid conditions of the YD and into relatively warmer temperatures, increased productivity and influx of organic matter can be expected. δ15N can also indicate rates of productivity, although the current data shows a stable curve with values remaining between 2.00‰ and 4.00‰.