Late Quaternary seismic stratigraphic framework and paleolimnology of Walker Lake, Nevada

Abstract:

Lake deposits can be used to assess past hydrological and atmospheric conditions and aid in understanding regional climate of the western Basin and Range. Walker Lake, Nevada has a maximum water depth of ~35 m and is situated in a half-graben basin that formed during late Cenozoic transtension, in the Walker Lane tectonic belt. Small-scale climatic variations are preserved in the sediments of this hydrologically closed lake basin. Ten Kullenberg sediment cores (~2 to ~10 m in length), and ~300 km of CHIRP seismic reflection data were collected in Walker Lake to assess late-Quaternary stratigraphic framework and paleoclimate history. Core 4A is one of the longest cores acquired (9.19 m) and contains the oldest recovered sediments. Analyses of total inorganic carbon, total organic carbon, carbon and nitrogen abundances, and carbon stable isotopes from core 4A yield a valuable multi-proxy paleoclimate record. This record documents changes in effective moisture in the eastern Sierra Nevada and western Basin and Range. Sediments range from laminated to massive mud with three tephra deposits 0.5 - 4 cm thick. Total percentage of calcium carbonate, ranging from 3 to 35%, shows oscillations we interpret to be millennial forcings. The average C/N ratio of core 4A is 7.71 (2.5 - 11.3 range), and the stable carbon isotope measurements range from -21.0 to -25.3‰, and average -23.8‰. At ~3.7 m depth a basin-wide angular unconformity is observed in the seismic data and is also reflected in the geochemical data. An estimated age of this surface, 2000-2500 ka, is consistent with previous interpretations of partial diversion of the Walker River into the Carson sink. A δ¹³C_organic excursion, -25 to -20.8‰, occurs at the depth of the unconformity. We interpret this to be a result of enhanced deposition of aquatic organic matter during the Walker Lake drawdown. From 3.7 to 7.5 m depth, the carbon and nitrogen abundances and isotopes are surprisingly consistent down core and may be the result of protracted stability. Below 7.5 m depth the carbon and nitrogen abundances decrease, as do %CaCO₃ and δ¹³C_organic values. These signatures may reflect past connectivity between Walker Lake and other Lahontan sub-basins. AMS radiocarbon analyses are underway, which will help constrain the paleolimnologic history of the basin.