24-14 Investigating the Varved Sediments in Lower Mystic Lake, Medford, Massachusetts: a multiproxy analysis to reconstruct the lake’s environmental history of the last ~200 years.

Session: Lake Sedimentary Records of Past Climate and Environment (Posters)

Presenting Author:

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Abstract:

A sedimentological analysis of continuously varved records can reveal changes in physical and environmental conditions on a yearly scale. Lower Mystic Lake is an open, meromictic lake in Medford, Massachusetts, that preserves varved sediment due to a saltwater lens that causes anoxic conditions within its bottom waters. We measured the bathymetry of the lake using a fishfinder and QGIS software. We retrieved four gravity cores (70-98 cm in length), two from the northern basin with a maximum depth of 15 m, and two from the southern basin in 22 m water depth. We took CT scans of one northern and one southern basin core to gain high-resolution imagery of laminations. After core opening, we measured physical parameters including magnetic susceptibility, grain size and dry-bulk density. Next, we analyzed the biogenic component through microscopic inspection of smear slides and CHNS elemental analysis. For our age model, we extracted leaf fragments for radiocarbon dating, along with correlation of historical events in the lake's watershed to distinct core intervals. Each core is composed of dark, organic-rich mud with visible laminations. The lower 30-70 cm consist of green-brown organic-rich mud, while the upper section is dark, water-saturated mud. In the lower section, carbon-to-nitrogen ratios are between C/N = 6.3-9.9, indicative of organic matter with an aquatic origin. This is supported by abundant organic matter in the form of aquatic algae (diatoms) in smear slides. The lower section has smaller grain sizes, with a mode around 34.6 µm, and varying magnetic susceptibilities between -1.3 and 5.2 10^-5 SI units. In the upper section, we measured higher C/N = 7.6-11.2, a larger grain size, and a higher range in magnetic susceptibilities possibly indicating larger terrestrial input. Overall higher magnetic susceptibility in the northern basin might indicate input of more clastic material compared to the southern basin or different redox conditions. While the base of the core dates to ~1800 CE, the major change in the sediment record is likely linked to increased pollution from industrial activities in the late 1800s (Besonen, 2006). Our findings contribute to studies on the sedimentation history of Lower Mystic Lake, as well as paleoenvironmental reconstructions for the greater Boston metropolitan area.

Besonen, M. R. (2006), Ph.D. dissertation, Univ. of Mass., Amherst.

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