AN ANALYSIS OF GRAIN SIZE CHANGES IN THE UPPER FIVE UNITS OF THE SEARLES LAKE, CA STRATIGRAPHY

The Southwestern United States is presently known for its arid climate. However, numerous paleoclimate records reveal wet and dry fluctuations occurred on centennial to millennial time scales. The sediments preserved in the Searles Lake basin, CA consist of alternating mud and evaporite layers which have been interpreted broadly to indicate wetter and drier conditions, respectively. In 2017, the SLAPP core (76 m-long, >200 ka) was collected with the goal of constructing a detailed multi-proxy climate record that builds on a significant body of prior work. Within this core, five units were identified: the Mixed Layer, Bottom Mud, Lower Salt, Parting Mud, and Upper Salt (from bottom to top). Here, we analyzed the grain sizes of these five units to understand basin sediment transport and interpret these changes within a paleoclimate framework. We removed evaporite minerals and organic materials from ~700 samples to isolate the clastic fraction. Grain size distributions were measured with a Coulter LS 13 320. We examined unit to unit and within unit changes in grain size distributions. Identified characteristic grain size patterns can be used to compare units. The Mixed Layer and Bottom Mud were enhanced in clay to fine silt relative to the core mean. Within the Bottom Mud, a 3.8 m interval has a distinct bimodal grain size distribution with a coarser fraction not observed in the rest of the unit. The Lower Salt coarsens upward, and the Parting Mud is nearly identical to the core’s mean grain size distribution. Lastly, the Upper Salt contains a finer sediment fraction compared to the core mean. We also observed that units and subunits have distinct grain size distributions that can be tracked by comparing changes in grain size peak frequencies. This may be helpful in identifying short-term changes in sedimentation. The current analysis suggests finer grain sizes occur during wetter conditions and coarser grain sizes occur during drier conditions. Our investigation of grain size distribution is working toward a more unified set of proxy records that link evaporite mineralogy, shoreline data and other proxies, which together will provide a detailed history of the past climate conditions in the region.