LATE QUATERNARY PHYSICAL STRATIGRAPHY OF WALDEN POND

Walden Pond, Massachusetts, is an iconic symbol of humanity's place in Nature and one of the most famous lakes in the world, thanks to the writings of Henry David Thoreau. In recent decades short sediment cores have been utilized to examine environmental conditions over the past ~1.5 millennia. Our group sought to explore the sedimentary record of deeper time, and have worked in collaboration with the MA Department of Conservation and Recreation. In 2015 we conducted a sub-bottom SONAR survey of the lake and observed lacustrine sediment overlaying glacial outwash sediments. Lacustrine sediments vary in thickness with thickest accumulations in the three basins. Depth, area, and lacustrine sediment thickness of the basins increased from east to west with maximum lacustrine sediment thicknesses ranging from 6.7 m in the eastern basin to 10.3 m in the western basin. Assuming that each basin has had continuous sedimentation since the formation of the lake, these observations show that sedimentation rates are related to depth and basin area, likely as a combined result of increased organic matter preservation and sediment focusing. Based on the sediment thickness of the basins, we revisited the lake in June 2017 to obtain a Livingstone core from the western basin. We recovered nine ~1 m long continuous core drives, which represent 8.41 m of uncompressed sediment. The basal unit that ultimately refused penetration was a bedded clastic unit with elevated magnetic susceptibility and density. Above this unit the sediment was a massive dark brown gyttja with low magnetic susceptibility and variable density. Eleven AMS ¹⁴C dates were obtained and a compaction-corrected Bayesian age model was calculated, confirmed by regional pollen zones. The basal age of the core is calculated as 13,700 cal BP, which is thousands of years younger than regional deglaciation. Our basal sediments are of Younger Dryas age, and the core system was not able to penetrate these clastic sediments. Geophysical data reveal that there is approximately 1.9 m of additional late Pleistocene lacustrine sediment that were not recovered from the western basin. Nonetheless, this new record is an extremely valuable archive of late Holocene paleolimnologic conditions in eastern Massachusetts, providing a high sedimentation/high resolution and continuous record over the past 13.7 millennia.