PALEOENVIRONMENTAL SIGNIFICANCE OF SEDIMENT CHARACTERISTICS AND MAGNETIC FABRICS PRESERVED BY POST-GLACIAL DEPOSITS OF SENECA LAKE, NY

Seneca Lake is the largest by volume of the Finger Lakes located in New York. Two hypotheses have been proposed to explain the formation of erosional surfaces observed at ~20 m water depth around the perimeter of the lake: 1) a drop in lake level and subsequent erosion along the shoreline that occurred at the end of the Middle Holocene or 2) an increase in active water circulation (stronger currents) at the transition from the Middle to Late Holocene. Two cores were recovered to reconstruct environmental and climate changes during the Late Glacial and Holocene (16,500 years BP to present). We used loss-on-ignition, grain size, and magnetic parameters (bulk magnetic susceptibility and anisotropy of magnetic susceptibility) to determine whether faster lake currents associated with increased internal seiche activity occurred at the end of the middle Holocene and into the Late Holocene.

During the warmer Early to mid-Middle Holocene (10-6 ka cal yr BP), the mean grain size (7.3φ), % sand (up to 4.8%), carbonate concentration (10-42%), and bulk magnetic susceptibility (6 x 10^-4SI units) are all at their highest and show decreasing trends upcore. The organic matter concentration is low (2%) and increases upcore. Well-defined magnetic fabrics occur during this warm interval: a high degree of anisotropy (P'), which records the degree of alignment of magnetic grains (1.02-1.06) and oblateness (T), which reflects the shape of the magnetic ellipsoid (0.40-0.95). The relatively cooler late Middle Holocene and Late Holocene (6 ka cal yr BP and present) is characterized by a smaller mean grain size (8.1φ), less sand (<1%), lower carbonate (4%), slightly lower bulk magnetic susceptibility (4 x 10^-4 SI units), and higher organic matter concentrations (3-6.9%). The cold period is distinguished by a small degree of anisotropy (P': 1.006-1.035) and anomalous shape factors (T: 0.2- -0.624). Results suggest a strong relationship among magnetic fabrics, sediment characteristics and climate. The abrupt change in both sediment and magnetic properties may result from a decrease in the current strength during the Holocene. Increased effects of bioturbation or diagenetic dissolution of magnetite grains on development of anomalous magnetic fabrics cannot be ruled out for the Late Holocene. Petrographic analysis supports bioturbation as the cause of anomalous fabrics.