EARLY PLEISTOCENE ARIDITY TRENDS IN DEATH VALLEY, CA: TESTING ANOMALOUS OXYGEN-17 IN SOIL NITRATE AS A NEW PALEO-PRECIPITATION PROXY IN ANCIENT ARID SYSTEMS

The Neogene sedimentary record of pluvial lakes in the North American southwest is an important archive of terrestrial climate responses to Northern Hemisphere glaciation in regions characterized by an arid background climate. The spatial extents of pluvial lakes in the Pleistocene are thought to have been driven by orbitally controlled glacial-interglacial cycles, though they also reflect the interaction between hemispheric-scale climate cyclicity and more local factors (i.e., local topography, aridity gradients, etc.). In this study, we present a new depositional model for lower Pleistocene playa deposits of the Confidence Hills Formation, Death Valley paired with the first pilot test of the use of anomalous ¹⁷O (Δ¹⁷O) in soil nitrate as a paleo-precipitation proxy appropriate for ancient hyper-arid depocenters. This geochemical proxy has been developed in modern arid environments, but its application to the geologic record has not been tested.

Our results show that Early Pleistocene aridity cycles in the Confidence Hills spanned a range of ~15-225 mm (2.0-23.2‰ Δ¹⁷O) mean annual precipitation (MAP) resulting in the expansion and contraction of a playa system bounded by alluvial fans. These cyclic depositional processes resulted in three lithofacies assemblages: (1) interbedded siltstone and anhydrite deposited along the playa margin; (2) thin-bedded siltstone deposited via sheetfloods in a sandflat environment during the wettest part of the cycle; and (3) massive anhydrite deposited through extended desiccation of the playa following the wettest part of the cycle. Stratigraphic cycles are 10-15 meters thick and most likely coincide with 41 kyr glacial-interglacial cycles based on a temporal calibration using previously published paleomagnetic data and a new Ar-Ar age for an ash bed. The validity of Δ¹⁷O as a paleo-precipitation proxy in ancient arid paleo-environments is supported by a clear relationship between lithology and Δ¹⁷O values that correlate with MAP. Consistent with our depositional interpretations, siltstone beds yielded low Δ¹⁷O values (high MAP), laminated anhydrite beds yielded moderate Δ¹⁷O values (moderate MAP), and thick massive anhydrite beds yielded high Δ¹⁷O values (low MAP). The apparent relationship between lithology, interpreted depositional processes, and MAP values is promising for future applications of this tool.