MORPHOLOGY AND PARTICLE SIZE ACROSS THE THREE BASINS OF WALDEN POND - A RECONSTRUCTION OF LANDSCAPE CHANGES AND MASS WASTING EVENTS OF THE PAST 700 YEARS

Walden Pond is the deepest lake in Massachusetts and holds a Late Pleistocene to Holocene sedimentary record of environmental change. The lake has three basins, a deep, intermediate, and shallow basin with 30 m, 20 m, and 16 m depth, respectively. Grain sizes were measured on short gravity cores up to 67 cm length taken from the center of all three basins. The sediments are composed of dark brown to black organic-rich silts that appear relatively homogenous. To prepare the sediments for laser diffraction grain size analysis, organic matter and biogenic silica were removed through chemical digestion, leaving behind the clastic component. Radiocarbon dating, pollen stratigraphy, and concentration of industrial contaminants as well as magnetic susceptibility data allow for a detailed age model and correlation between the basins. Compared with the other basins, the shallow basin overall exhibits coarser grain sizes. The main grain size in the three basins is silt with the uppermost 10-15 cm composed of mostly fine to medium silt (10-40 microns) and the lower part composed of coarse silt (30-60 microns). A few, up to 2 cm thick, horizons with coarser particles (50-80 microns) occur throughout the record predominantly in the deep and shallow basins. The finer clastic component deposited since the 1800’s is likely due to the erosion of soils from heavy recreational land-use around the lake, while the thin horizons of distinctly coarser material in the record suggest occasional mass wasting events. It is likely that coarser material is deposited close to the shoreline by runoff and further transported into the center of the basin through mass wasting. One potential trigger could be ground shaking from strong earthquakes that are known historically or suspected in the prehistoric record. Bathymetric and slope maps were created using ArcGIS and QGIS to visualize locations of potentially unstable slopes between 10-20 degrees, which is the ideal steepness found in other lakes for sediments to both accumulate and eventually fail. The slopes in Walden Pond are up to 40 degrees steep with the intermediate basin having the smallest area of slopes susceptible to mass wasting. The more homogeneous grain size record of the intermediate basin might also be caused by its position in the center of the lake and partial shielding from nearshore processes.