2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 5
Presentation Time: 8:00 AM-12:00 PM


WHEATLEY, Gwendolyn and CURTIN, Tara M., Geoscience Department, Hobart & William Smith Colleges, Geneva, NY 14456, GW2690@hws.edu

Anisotropy of magnetic susceptibility (AMS) has been widely used to assess a range of geologic phenomena ranging from depositional conditions in marine and terrestrial environments to tectonic stress and strain. In this study, we link magnetic properties (bulk magnetic susceptibility (MS) and AMS) to paleoclimatically significant sediment properties (% organic matter, % carbonate, % terrigenous material, mean grain size, % sand, % silt, and % clay) of Late Glacial to Holocene Owasco Lake, NY sediments. We hypothesize that if a magnetic parameter exhibits a similar temporal trend with one or more paleoclimatically sensitive properties, then the AMS parameter may be controlled by the paleoclimate-driven mechanism.

Two ~4.5 m long sediment cores collected from the profundal zone of Owasco Lake were subjected to a suite of analyses: bulk magnetic susceptibility (MS) and AMS, loss-on-ignition, and grain size by laser diffraction. There is a strong correlation between two AMS parameters, P' (reflecting the degree of alignment of magnetic grains) and T (shape of the magnetic ellipsoid), and sediment composition. When the % carbonate and bulk MS are high and the % organic matter is low, P' and T values are high. In contrast, when the % carbonate and bulk MS are low and the % organic matter is high, P' and T values are low. We infer that when summer temperatures were warmer during the Early to Middle Holocene based on high concentrations of carbonate in the sediment, the detrital magnetite grains are strongly aligned and display oblate magnetic fabrics, typical of gravitational settling in a subaqueous setting influenced by lake currents. When summer temperatures were cool during the Late Holocene, as inferred from lower carbonate concentrations, the magnetite grains are not as strongly aligned and anomalous magnetic fabrics occur. One sediment core preserves evidence for three slumps just prior to the change in AMS properties near the Middle Holocene-Late Holocene transition. This lithologic evidence provides some support for a hypothesized change in lake current strength if strong lake currents are responsible for the slump events. P' values increase concurrent with slump initiation, suggesting that the two might be related and lending additional support for the intensified current hypothesis.