Climate Change Recorded by Sedimentary Organic Matter in Soda Lake, Southern California

Jennifer Eigenbrode, Cara Davis, Lisa M. Pratt

Department of Geological Sciences, Indiana University, Bloomington, IN 47405

J Ramon Arrowsmith

Department of Geology, Arizona State University, Tempe, AZ 85287

Dallas D. Rhodes

Department of Geology, Whittier College, Whittier, CA 90608

Lisa A. Rossbacher

Department of Geology, Dickinson College, Carlisle, PA 17013

Aridity, low aquatic productivity, and benthic anoxima make Soda Lake an ideal location to investigate climate signals preserved in organic matter of terrestrial sediments. This permanent saline lake occupies a hydrologically closed basin bounded by the San Andreas Fault and Temblor Range on the east, and by the Caliente Range on the west. Grasses surround Soda Lake on the Carrizo Plain, while juniper-oak woodland dominates at higher elevations. Recent lake expansions and contractions are indicated by broad mud-cracked pavements on the basin floor. The absence of previous shorelines on the adjacent valley walls suggests Soda Lake has never had a pronounced depth. Surface drainage is minimal and groundwater discharge appears to sustain the water level in Soda Lake.

In order to evaluate the climate record of this basin, we hand collected an exploratory sediment core using a 12.5 cm diameter acrylic core barrel. Organic carbon content and stable carbon isotope composition (delta C-13) of organic matter were determined for 17 samples spanning 65 cm of muddy lake sediments. Desalted organic carbon contents drop rapidly in the top 10 cm from just over 2 wt.% at the sediment-water interface to a background value of approximately 0.2 wt.%, which remains relatively constant through the lower 60 cm. Delta C-13 values of total organic matter average -18 to -20 per mil through most of the core. At a 20 cm depth, a pronounced increase in organic carbon content to 1 wt.% is associated with a negative shift in delta C-13 values to -25 per mil in a sediment layer containing woody fragments dated by C-14 at 320 years before present. This age coincides with the widely recognized interval of wetter climate and coastal flooding in the region. Thus, Soda Lake sediments appear to preserve a high-resolution record of climate change in southern California.