MULTIPROXY ANALYSIS OF UNUSUAL SEDIMENTARY UNITS OF POSSIBLE SEISMIC ORIGIN IN THE LACUSTRINE RECORDS OF SLUICE AND WALDEN PONDS, EASTERN MASSACHUSETTS, USA

The 1755 AD Cape Ann earthquake, the most damaging historic earthquake in New England, caused ground motion of modified Mercalli intensities (MMI) = VI – VII equivalent to peak ground acceleration (pga) = 0.08 – 0.12 g in eastern Massachusetts, most likely affecting lake sediments in the area. Here we present multiproxy data of two well-dated sedimentary records from Sluice Pond, Lynn, MA, and Walden Pond, Concord, MA, that record an unusual depositional event of possible seismic origin. Sediment cores retrieved from the deepest basin of each lake in 20 m and 30 m water depths, respectively, were interpreted using a combination of physical, biological and geochemical proxies, including among others bulk density, grain size distributions, palynofacies, stable isotope data and elemental compositions. Previously published age models for both lakes are based on radioisotopes and the well-established 1630 AD European settlement horizon placing the mid-18th century in a few decimeter core depths. Both sedimentary records are composed of dark brown, organic-rich silts, which mostly appear homogeneous but show some faint laminations within the upper 10 - 20 cm of the sedimentary record. Sluice Pond sediments in a core depth of 30 - 32 cm display a distinct increase in siliciclastic particles of very fine to fine sand size grading into clayey silts. Within the same interval palynofacies analysis reveals an influx of characteristic nearshore material. Deposition of this unit can be attributed to ground shaking in 1755 AD that mobilized coarser nearshore sediments either through mass wasting or oscillation of a seiche within the lake basin. An increase in clay to silt size particles, an abundance of nearshore palynofacies as well as significantly higher C/N ratios in 15-19 cm core depth of the Walden Pond sedimentary record suggest a large influx of terrigenous material into the deep lake basin. This interval is stratigraphically higher than the 1755 AD horizon and might record a mass wasting event of non-seismic origin. Further analysis of older sediments as well as comparison to shallower sub-basins are necessary to fully understand the paleoseismic record of Walden Pond.