NEW RADIOCARBON DATES EXTEND THE RECORD OF ENTRAPMENT FOR THE MCKITTRICK TAR SEEPS

The asphaltum deposits near the town of McKittrick in the southwestern part of the San Joaquin Valley of California are the most extensive of several naturally-occurring seeps in central California. The McKittrick collections are composed of vertebrate, insect, and plant remains that were entrapped in the asphaltum. Discovered and primarily excavated in the early 20th century, the fauna has historically been considered late Pleistocene in age based on the recovered extinct megafauna. However, the small number of published dates from insects have been early Holocene in age, and three published bone collagen dates are between 20 and 12 ka BP, with the oldest dates within the Last Glacial Maximum (LGM).

To better establish the timing(s) and duration of entrapment at McKittrick, we undertook a broad sampling strategy for new ¹⁴C dates. Collagen preservation is generally low for vertebrates from McKittrick, so a large sample (n=257) was pre-screened for collagen preservation using Fourier Transform Infrared Spectroscopy. The most likely bone samples were processed to remove tar and extract ultra-filtered collagen. Insect chitin was also subjected to asphalt removal, and both chitin and collagen were dated at the UCI Keck Accelerator Mass Spectrometer facility. New dates were obtained for 38 vertebrate specimens representing 11 taxa and 5 insects representing two taxa. Analysis of these dates shows three main periods of entrapment, two Pleistocene and one Holocene. The vast majority of the new dates fall well within the late Pleistocene from ~37 to 28 ka. A second period spans ~20 to 12 ka, and the Holocene dates span ~6 to 3 ka.

These are the first reliable dates demonstrating the McKittrick seeps formed prior to the LGM and have remained periodically active. Furthermore, the large number of dates during this oldest interval suggest much of the megafauna may date to this period. Significant gaps in the dates, corresponding to the early LGM and early Holocene may reflect periods during which the seeps were less active or in which local conditions were less conducive to animal entrapment.