ACTIVE TECTONICS AND CLIMATE: A COMPLEX INTERACTION IN LAKE TAHOE SEDIMENT CORES

The climate signal in Lake Tahoe, measured by variations in δ¹⁵N, δ¹³C, TOC, total Nitrogen, C/N ratio, and %Opal is masked and modified by the effects of active faulting in and near the basin. Episodic debris flow and turbidite deposits, identified in a suite of sediment cores from the lake, are interpreted as paleoseismic in origin. The isotopic and chemical signals as well as radiocarbon dating suggest reworking of preexisting lacustrine material in the episodic deposits. The Holocene-Pleistocene transition and other Holocene climate variations are recorded in “normal” sedimentation between the episodic deposits. The H/P transition is marked by an immediately more positive δ¹⁵N and an increase in %Opal. Both signals have large fluctuations over the following ~1000 years, with δ¹⁵N remaining relatively flat and %Opal decreasing overall. δ¹³C is more negative overall, but also shows large fluctuations. TOC and total Nitrogen, increase over ~1000 years following the transition and continue to increase throughout the Holocene. The C/N ratio also increases over the ~1000 years that follow the H/P transition, and then levels off for the remainder of the Holocene. The largest signal throughout the Holocene in “normal” sediments is δ¹⁵N, which becomes much less positive following an episodic deposit that is dated between 7890-7190 calBP. The δ¹⁵N signal appears to respond to lake level variations and periods of time when the lake is not spilling or spilling intermittently due to both climate and active tectonics. Previous workers show the outlet, the Truckee River, was not spilling or only spilling intermittently to Pyramid Lake, the terminal lake in the system, from ~8000 to ~3000 calBP. At least two paleoseismic events in the past 7890 years likely occurred on the Dollar Point segment of the West Tahoe fault, potentially affecting the Truckee River outlet that is located in the footwall of the fault. The coincidental timing between the drought conditions and faulting may indicate that the outlet was tectonically cut off while drought conditions worsened, which leads to the rather large shift in δ¹⁵N at this time. Sediments from the latest Pleistocene and Holocene in Lake Tahoe provide perspective on the interaction between active tectonics and climate and on the interplay between the two in the geologic record.