ORIGIN AND TRANSPORT OF CLASTIC LAKE BOTTOM SEDIMENTS IN WALDEN POND, CONCORD, MASSACHUSETTS, USING PARTICLE SIZE AND ELEMENTAL COMPOSITION

Walden Pond is a steep-sided kettle lake in northeastern Massachusetts. The lake has no inflow or outflow and is divided into three sub-basins with maximum depths of 30 m, 20 m and 16 m in the western, central and eastern basin, respectively. Three gravity cores were taken from the deepest area of each basin with up to 70 cm length in 2015. The lake bottom sediment is composed of dark brown organic-rich mud with only 5-15% of clastic particles. Proxy data includes: magnetic susceptibility, elemental compositions from polarized energy dispersive x-ray fluorescence spectroscopy, and particle size from laser diffraction analysis. In the deepest basin core and below 15 cm core depth, the mean size of clastic particles is coarse silt; above 15 cm the sediment is finer with a mean in the medium silt range. Based on an age model by Koster et al., 2015, this change occurs in the late 19^th century, a time when the area around the lake was increasingly used for recreation. A concurrent increase in Fe concentration points towards the mobilization of iron-rich soils during this time. Magnetic susceptibility readings allow for correlation of the upper sedimentary record between basins. Sedimentation rates are only one half in the central and one third in the eastern basin, likely an effect of higher organic matter preservation under anoxic conditions as well as sediment focusing in the deepest and largest western basin. While all three basins show a similar trend of finer sedimentation towards the top, the eastern basin receives overall coarser sediments, possibly because of its vicinity to shore. Throughout the record coarser horizons occur and mark the sudden influx of nearshore sediment. One layer in 50 cm core depth in the western basin is 4 cm thick and exhibits normal grading indicative of settling from suspension. It can tentatively be correlated through all three basins. Mobilization of nearshore sediments in Walden Pond can occur during heavy runoff or destabilization of basin slopes potentially triggered by lake level variations, excess sediment loading or strong seismic groundshaking.

Koster D., Pienitz R., Wolfe B.B., Barry S., Foster D.R., Dixit S.S. 2005, Paleolimnological assessment of human-induced impacts on Walden Pond (Massachusetts, USA) using diatoms and stable isotopes. Aquat. Ecosyst. Health Manage. 8: 117–131.