A STRATIGRAPHICALLY INTACT, PRE-LAST GLACIAL MAXIMUM WOODRAT MIDDEN AND ASSOCIATED VEGETATION FROM RANCHO LA BREA

Rancho La Brea (RLB; the La Brea Tar Pits, Los Angeles, California) is a Late Pleistocene lagerstätte comprising a series of >100 asphaltic fossil deposits that have yielded millions of specimens, collectively representing ecosystems moving through time. Research at this locality has largely been limited to megafaunal vertebrates, despite the abundance of well-preserved and identifiable plant macrofossils. However, the applications of this rich vegetation record to paleoecological studies of community assembly and use as a paleoclimatic proxy have so far been hindered by a lack of stratigraphic control at RLB.

Here, we report the first stratigraphically intact unit for RLB in the form of a woodrat (Neotoma) midden from the deposit “Box 1” (Project 23 mitigation project). Neotoma middens are commonly used in the arid southwestern US for paleoenvironmental reconstructions. We show that the midden- consisting of hundreds of fecal pellets accompanied by plant and insect macrofossils- is taphonomically and temporally distinct from other Project 23 deposits. Seven radiocarbon dates confirm a pre-Last Glacial Maximum age (~50,000 years BP) for the unit, and new sediment processing approaches revealed a finer-grained sediment composition and significantly higher asphalt content in the midden-associated matrix as compared with matrix from ‘typical’ megafauna-bearing grids.

For the first time at RLB, we subsampled excavated grids to permit reliable calculations of plant abundance at a single temporal interval. Isotopic analyses of the dominant plant taxa reveal that woodrats likely consumed oak and juniper, reflecting a specialized diet requiring tolerance of plant toxins such as phenols. When considering the relationship of mammalian body size to temperature (Bergman’s Rule), analyses of fecal pellet size imply past conditions were within ~4°C of present-day mean July temperature for the Los Angeles Basin. Critically, this quantitative approach allows for the integration of RLB into a network of existing pollen and sediment records for southern California. As extant woodrats and their middens are still abundant in Los Angeles county, this discovery offers an important opportunity to connect the past with the present in this biodiversity hotspot.