DYNAMIC HABITATS AT THE MCKITTRICK TAR SEEPS REVEALED BY STABLE ISOTOPE ANALYSIS AND RADIOCARBON DATING

The McKittrick tar seeps in Kern County, California have produced a remarkable assemblage of late Pleistocene fossils. While McKittrick has often been compared to the Rancho La Brea (RLB) Tar Pits (Los Angeles County), the assemblage represents a unique ecosystem as a fully inland habitat. In addition, McKittrick has a substantially different distribution of species and relative abundances, offering insights into another California Pleistocene environment. However, the age of the McKittrick fossils is poorly constrained, limiting interpretations of its paleoenvironment. Previous stable isotope analysis suggests an arid ecosystem during the Pleistocene, but has lacked the temporal context to resolve ecosystem shifts through time.

We collected new ¹⁴C ages and stable isotope compositions (δ¹³C, δ¹⁵N) from a variety of taxa, including carnivores, herbivores, reptiles, and insects. We combined these data with previously published results to investigate temporal trends and ecological dynamics at McKittrick. The ¹⁴C ages for McKittrick fossils indicate that the UCMP collections represent at least three phases of fossil accumulation; two Pleistocene phases: one from ~37 to 28 ka, a second ~20 to 12 ka, and a third Holocene phase. The oldest phase is dominated by open habitat grazers (horses, bison), mixed feeders (llamas), and carnivores (dire wolves, coyotes), whereas the middle phase shows a shift towards mixed habitat taxa (elk, short faced bear, sloth). Overall carbon isotope compositions suggest a mixed C3-C4 ecosystem with a shift from higher to lower δ¹³C values from the oldest to middle phases, which might signify a transition from open to mixed habitat before another shift to higher δ¹³C values during the Holocene.

Finally, we compared these results to previously published data for the RLB fauna. We calculated six community-wide metrics (Layman metrics) to assess trophic diversity and community structure within isotopic space for each locality. Our results show that McKittrick exhibits greater trophic diversity and a broader spacing of ecological roles compared to RLB. McKittrick's food web demonstrates higher trophic redundancy and a more even distribution of trophic niches. While the localities share many of the same taxa, these data suggest different ecosystem structures, likely reflecting higher habitat heterogeneity at McKittrick relative to RLB.