Lacustrine turbidites as indicators of Holocene storminess and climate: Lake Tahoe,California and Nevada |
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Authors: | David A Osleger Alan C Heyvaert Joseph S Stoner Kenneth L Verosub |
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Institution: | (1) Department of Geology, University of California, Davis, CA, USA;(2) Desert Research Institute, Reno, NV, USA;(3) College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR, USA |
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Abstract: | Sediment cores from Lake Tahoe permit the discrimination of turbidites initiated by seismic-induced debris flows from those
generated by severe storms and associated hyperpycnal currents over the last 7000 years using integrated textural, magnetic,
and geochemical signatures. Relative to fine-grained ‘background’ sediments, the majority of Tahoe turbidites exhibit coincident
trends of increased mean grain size, increased magnetic susceptibility, decreased TOC, higher δ13Corg and variable C/N. We interpret these characteristics to record the rapid influx of terrigenous sediments within runoff from
the watershed triggered by high-intensity storms. Correlation of multiple, individual turbidites between cores suggests a
synchronicity of occurrence, supporting the model of extreme hydrologic events as the trigger for most turbidity currents
into Lake Tahoe. In contrast, turbidites generated by seismic collapse of steep lake margins would have textural, magnetic
and geochemical signatures that would reflect a homogenized mix of autochthonous biogenic debris and multiple older turbidites.
Only one of the turbidites in the cores appears to be seismically generated. A second component of this study tested the hypothesis
that turbidite clustering reflects phases of increased storminess, paleoprecipitation and lake level. We correlated broad
patterns of turbidite frequency in the Tahoe cores with climate proxies from (1) elsewhere in the Tahoe watershed, (2) the
western Great Basin (primarily Pyramid Lake) and (3) the San Francisco bay estuary. The reasonable degree of temporal overlap
suggests that apparent trends in severe storm frequency recorded by clusters of turbidites provides a measure of long-term
regional paleoprecipitation and lake level. A key finding is an extended phase of dryness and a near absence of major storms
between ~3000 and ~900 cal yr B.P. in the Tahoe watershed. |
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Keywords: | Lake Tahoe Holocene Turbidites Paleoclimatology Storminess Drought |
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