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Title:
Got Varves?: Reconstructing Holocene Climate Change in Seneca Lake, NY
Authors:
Rogers, C. E.; Curtin, T. M.
Affiliation:
AA(Geoscience Department, Hobart & William Smith Colleges, Geneva, NY 14456 ; ), AB(Geoscience Department, Hobart & William Smith Colleges, Geneva, NY 14456 ; )
Publication:
American Geophysical Union, Fall Meeting 2004, abstract #PP33B-0932
Publication Date:
12/2004
Origin:
AGU
AGU Keywords:
9350 North America, 3344 Paleoclimatology, 1815 Erosion and sedimentation, 1845 Limnology
Bibliographic Code:
2004AGUFMPP33B0932R

Abstract

The sedimentary deposits of Seneca Lake, one of eleven Finger Lakes in New York State, contain a valuable record of post-glacial climate and environmental change. Paleoenvironmental interpretations depend on knowing what transport and depositional processes controlled the formation of the laminae. In this study, we examine Holocene rhythmites in two profundal cores collected from the northern half of Seneca Lake to determine possible mechanisms of formation of the alternating olive gray-black layers. Magnetic susceptibility was measured at a 2 cm interval prior to splitting the ∼5 m cores. Split cores were described, photographed, and sampled for loss-on-ignition and grain size analysis at a 10 cm interval. When possible, core one was sampled on a lamination-by-lamination basis whereas core five was sampled at a one cm interval. Sequential LOI was used to estimate the total organic and carbonate content of sediments. Grain size analyses were performed using a Coulter LS 230 laser diffractometer after removal of calcium carbonate. Temporal control of paleoenvironmental changes in cores will be established by two accelerator mass spectrometer radiocarbon dates. The cores contain proglacial pink clay overlain by Holocene mm- to cm-scale alternating olive gray and black laminations of fine-grained sand and mud. Magnetic susceptibility changes are distinct, ranging from 1-22 x 10-6 SI units in core one and 4-57 x 10-6 SI units in core five. Sediment is dominantly composed of siliciclastic material (50-99 weight %) with varying amounts of calcite (0.1-75 weight %) plus lesser amounts of organic matter (0.5-4.1 weight %). A distinct variation in sediment color, organic matter, and carbonate content, and median grain size (φ 50) occurs in the laminated sediment. The olive gray layers are characterized by well-sorted very fine silt and clay with a φ 50 <6 μ m, high carbonate content (>25%), and low organic matter content (<1%). The black laminae are generally higher in organic matter (>1%), lower in carbonate content (<25%), and have a φ 50 >6 μ m. Samples are well-sorted and positively skewed. Cores were correlated based on their carbonate content, magnetic susceptibility, and median grain size. We interpret these laminae to reflect changes in sediment sources. The olive gray layers may reflect deposition during the late spring through the summer. Fine-grained calcite precipitates mainly in the summer during maximum phytoplankton productivity in temperate, mid-latitude lakes. Carbonate content and thickness variations of the gray laminae may provide clues to the length of the productivity season, and thus perhaps the duration of the stratification season. A decrease in carbonate content may reflect cooler climate conditions. The coarser-grained black, siliciclastic-dominated layers may reflect deposition by underflows and/or turbidity flows during spring melt or episodic storms. It is unlikely the rhythmites in Seneca Lake are annual because the number of laminae preserved is insufficient to span the entire Holocene if resolved annually. Bioturbation or current re-working may blur the annual signal. Fabric analysis of thin sections will be used to further constrain the environmental conditions during deposition.
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