A PALEOSEISMIC RECORD FOR THE LAST 800 YEARS FROM TWO LAKES IN EASTERN MASSACHUSETTS

Sedimentary records retrieved from Walden and Sluice Ponds show evidence of mass wasting likely triggered by strong historic and prehistoric groundshaking in eastern Massachusetts. The two lakes of glacial origin form steep-sided basins with 30 m and 20 m maximum water depth, respectively, and are well-suited to accumulate continuous records of past environmental change and instantaneous sedimentation events. We carried out a multiproxy analysis of six short gravity cores retrieved from each of the three basins of Walden Pond as well as two Kullenberg cores from the deepest part of Sluice Pond including measurement of physical characteristics, geochemical proxies, palynofacies, and elemental compositions. Independent age models for all three basins of Walden Pond as well as Sluice Pond are based on industrial contaminants, pollen stratigraphy and radioisotopes and offer robust age control. Deposited before the arrival of Europeans settlers, sediments in both lakes are composed of organic-rich mud reflecting a forested landscape. The rise of non-arboreal pollen including Ambrosia record the widespread logging practices of European settlers in New England starting in the mid-17^th century. Subsequent erosion of soils is marked by grain size decrease, lower organic matter content, and elemental compositions characteristic of local soils. Separate from these long-term trends we identified sediment horizons that mark sudden-onset events. These are typically characterized by larger grain size, influx of nearshore palynomorphs, higher bulk densities, and/ or lower organic matter content. We interpret these horizons as underwater mass wasting events. If synchronous across different basins a regional trigger such as strong groundshaking is possible. For Sluice Pond, we are able to identify two event horizons, one associated with the M=5.9 1755 Cape Ann earthquake, the largest historic earthquake in New England, and another one dated to between 1430 -1620. For Walden Pond, sediments show synchronous mass wasting in at least two subbasins potentially related to the historic 1755 and the 1638 New Hampshire earthquakes, as well as two older events dated to between 1390-1460 and 1100-1330. Given the spatial distribution of deformation the potential earthquake source could be within the Littleton, MA seismic zone.