Lacustrine turbidites as indicators of Holocene storminess and climate: Lake Tahoe, California and Nevada

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 δ¹³C_org 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.

Appendix

The datum for the correlation in Fig. 9 is based on the clustering of AMS dates from pine needles and charcoal around 113–115 cm depths in the two cores (Table 1). The four raw radiocarbon dates (4480, 4570, 4660, and 4690) all intersect a particularly irregular portion of the IntCal04 calibration curve, making the assignment of a single calibrated age an arbitrary procedure. Median probabilities from these dates calculated by Calib 5.01 range from 5160 to 5420 cal yr B.P. with an average of 5300 cal yr B.P.

The fact that the four samples of large organic fragments were all found within a 2 cm interval at similar depths in both cores suggests that they were deposited relatively close in time and could conceivably be related to the same single depositional event. At most, the time span between deposition of the organic fragments may be 2–3 centuries. For the purpose of correlation, we assumed these four dates to be essentially synchronous, assigned a calibrated mean date of 5300 cal yr B.P. to the 114 cm midpoint depth of the four dates, and used this horizon as the datum.