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Student Research

Going With the Flow: Evidence for Changes in Circulation in Seneca Lake, NY During the Holocene

Meghan Crocker

Seneca Lake (42°40'N, 76°55'W) is one of 11 Finger Lakes located in western New York State. Holocene laminated sediment, 1.52 m thick, was recovered from 47.1 m water depth in the northern part of the lake and yields a ~14 ka record of environmental and climate variability. We used a combination of loss-on-ignition (LOI) measurements, mineralogical analysis, grain size analysis by laser diffraction, and magnetic parameters (magnetic susceptibility and anisotropy of magnetic susceptibility (AMS)) to reconstruct changes in paleocirculation patterns throughout the Holocene. Magnetic susceptibility was measured at a 1 cm interval on the unopened core. Once the core was split, it was photographed and described. Samples for LOI, mineralogy and grain size analyses were collected every 2 cm. A 2.1 x 2.1 x 2 cm plastic cube with sediment was obtained continuously downcore for anisotropy of magnetic susceptibility analysis. The MS, AMS, mineralogy, LOI, and grain size data record two significant changes in environmental conditions in the lake, one at the beginning of the Hypsithermal (~9 ka) and the other during the late Neoglacial (<~4 ka). MS values range from 0 to 5.2 x 10-5 SI. Susceptibility values are highest during the beginning of the mid-Holocene and lower throughout the rest of the Holocene with minor fluctuations. P' values, which record the strength of the magnetic fabric, range from 1.02 to 1.11. There are two peaks in P' values during the Holocene, one at ~9 ka and the other at <~4 ka as mentioned above. The coarsest mean grain size are coincident with the peaks in P' values, with the largest mean grain size (35um) found at the beginning of the Hypsithermal. A concurrent increase in P' and mean grain size likely reflects an increase in depositional energy from a low velocity, quiet water setting. The relatively high P' values most likely do not result from a change in magnetic mineralogy. The highest P' values occur at the beginning of the Hypsithermal when carbonate concentrations are highest, between 30-35%. The combination of relatively finer grain sizes and low P'values of sediment deposited during the late Hypsithermal and part of the Neoglacial suggests there was more extensive reworking by currents or organisms, eliminating any preferred depositional alignment of grains as a function of lake currents or low current influence during this time. Overall, variations in median grain size, MS, and P' indicate varying current strengths are responsible for deposition of sediment and reflect changes in lake circulation in response to changes in air temperatures and the position of the jet stream.