Establishing a Framework for Interpreting Sediment Grain size as a Proxy for Hydroclimate Changes at Searles Lake, CA

Water availability in southern California has changed greatly over millennia. Today the region experiences extreme droughts that are predicted to continue and have the potential to impact ecosystems and millions of people. Lake sediment cores from the region are a means of assessing past hydroclimate conditions to help interpretations of climate mechanisms and for making future predictions. Sediment cores from the Searles Lake Basin, located in southern California contain a 700 m-long record of alternating mud and evaporite layers which represent wet and dry conditions. Here, we begin establishing a lake size proxy by comparing grain size changes of the clastic materials preserved in mud and evaporite layers. We examine the grain size distributions of ~250 samples between the depths of 35 - 135 m. This interval extends from the bottom of the lower salt unit, through the bottom mud unit and into the mixed layer unit established by Smith et al. 1983 which were deposited between ~30 ka to ~350 ka. Most grain sizes range from sand to clay. We focus on comparing the grain sizes of clastic materials contained in evaporite layers (interpreted as dryer conditions - shallow to dry lake conditions) with mud layers in the lower salt (interpreted to correspond with deeper lake conditions) as a means of providing an initial framework for interpreting grain size changes in the record. We find salt layers contain higher percentages of larger grain sizes than mud layers, consistent with salt layers reflecting shorter distances from the shoreline to the basin floor. This framework is the first step in understanding grain size variations and will enable the application of grain size to detect lake level variations in sections of the core record dominated by muds (e.g., bottom mud) where lake level variations may have occurred but did not trigger evaporite deposition. Detection of these smaller variations may be significant for understanding millennial scale climate changes. Future work will apply grain size measurements in conjunction with a range of other proxies in a new core from the Searles Basin spanning the last 150-200 ka to examine southern California hydroclimate across the last two glacial-interglacial cycles.