Paleoseismic Investigations of Organic-rich Lake Sediments in New England – a Multiproxy Approach

Sedimentary records retrieved from Walden Pond, Concord, MA and Sluice Pond, Lynn MA show evidence of mass wasting likely triggered by strong historic and prehistoric groundshaking. The two lakes of glacial origin form steep-sided basins of 30 m and 20 m maximum water depth with Walden Pond being divided into three distinct subbasins. Because of the visually homogeneous nature of the organic-rich lacustrine muds we employed a combination of sedimentological, geophysical, geochemical, and palynological proxies to characterize background sediments and the fingerprint of the M=5.9 1755 Cape Ann earthquake, the largest historic earthquake in New England. We then used a similar multiproxy approach to investigate the longer record of Sluice Pond as well as the individual records of the three basins of Walden Pond extending the paleoseismic record about 800 years back in time. Robust age models using a combination of radioisotopes, industrial contaminants, and pollen stratigraphy allow for dating of event horizons as well as correlation between different lake basins. 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. For Sluice Pond, we were able to identify two event horizons, one associated with the M=5.9 1755 Cape Ann earthquake, and another one dated to between 1390 -1510 CE. For Walden Pond, sediments show synchronous mass wasting in at least two subbasins potentially related to the historic 1755 earthquake, as well as two older events dated to between 1380-1520 CE and 1050-1320 CE. Interestingly, a cluster of landslides in southern Maine within geotechnically sensitive clays dates to 1220-1450 CE and could correlate with the disturbed lake sediments. We analyzed the spatial distribution of event horizons in the two lakes in Massachusetts and landslides in Maine and identified possible source areas and magnitudes of prehistoric earthquakes with one possible source area within the Littleton, MA seismic zone. Our analysis suggests that the seismic hazard in the Boston area might be higher than previously appreciated.