RECONSTRUCTING THE DEPOSITIONAL ENVIRONMENTS OF THE LATE PLEISTOCENE FROM RANCHO LA BREA

Located in Los Angeles, California, The La Brea Tar Pits (known also as Rancho La Brea, RLB) is a world-renowned lägerstatte that holds one of the planet’s most comprehensive records of Late Pleistocene life. Fossiliferous asphalt seeps (“tar pits”) contain extraordinarily well-preserved biological materials that can inform reconstructions of past and present environmental processes. Analyzing sediments found inside and among fossils can assist in determining the provenance, deposition, and taphonomy of deposits, including disentangling whether asphalt is a primary or secondary taphonomic agent. Using measurements of grain size (HORIBA particle size analyzer), shape (Scanning Electron Microscope), and mineral composition (X-Ray Diffractometer), we evaluated 42 sediment samples extracted from within megafaunal limb bones in different asphaltic fossil deposits. The limb bones, excavated in the early 1900s and stored in the La Brea Tar Pits Museum’s historical Hancock collection, come from four sampling sites representing discrete fossil deposits. Data from these samples will better inform the geologic context of fossil deposits where such evidence was previously unavailable during destructive early 20th-century excavation practices. Results from this study suggest that while sediments extracted from megafaunal limb bones are not pre-sorted and are therefore viable for use in grain size analysis, determination of primary versus secondary asphalt infiltration cannot be made due to limited sediment samples and taphonomic indicators. These results continue the novel method of grain size analysis in tar pits, expanding the possibility for additional paleontological research into both RLB sediments and “tar pits” sediments at other localities globally.