HR: 1340h
AN: PP33B-0932 [Abstracts]
TI: Got Varves?: Reconstructing Holocene Climate Change in Seneca Lake, NY
AU: * Rogers, C E
EM: caitlin.rogers@hws.edu
AF: Geoscience Department, Hobart & William Smith Colleges, Geneva, NY 14456
AU: Curtin, T M
EM: curtin@hws.edu
AF: Geoscience Department, Hobart & William Smith Colleges, Geneva, NY 14456
AB:
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 $\sim$5 m cores. Split cores were described,
photographed, and sampled for loss-on-ignition and grain size analysis at a 10 cm interval. When possible, core one was
sampled on a lamination-by-lamination basis whereas core five was sampled at a one cm interval. Sequential LOI was used to
estimate the total organic and carbonate content of sediments. Grain size analyses were performed using a Coulter LS 230
laser diffractometer after removal of calcium carbonate. Temporal control of paleoenvironmental changes in cores will be
established by two accelerator mass spectrometer radiocarbon dates.
The cores contain proglacial pink clay overlain by Holocene mm- to cm-scale alternating olive gray and black laminations of
fine-grained sand and mud. Magnetic susceptibility changes are distinct, ranging from 1-22 x 10$^{-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 ($\phi$ $_{50}$) occurs in the
laminated sediment.
The olive gray layers are characterized by well-sorted very fine silt and clay with a $\phi$ $_{50}$ $<$6 $\mu$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 $\phi$ $_{50}$ $>$6 $\mu$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.
DE: 9350 North America
DE: 3344 Paleoclimatology
DE: 1815 Erosion and sedimentation
DE: 1845 Limnology
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting