DATING THE CERUTTI MASTODON: U-TH ISOTOPE ANALYSES OF BONE TO DERIVE A 130-KA AGE SUPPORTING AN EARLY HOMININ SITE IN THE AMERICAS

The age of arrival of hominins in the Americas has long been debated and revised repeatedly in response to discoveries of new and better-dated sites. The Cerutti Mastodon (CM) site in San Diego County, CA (Holen et al., 2017, doi:10.1038/nature22065), pushes the timing back dramatically. The CM site includes archaeologic and taphonomic evidence in a clearly defined and undisturbed geologic context: in particular, large cobbles interpreted as hammerstones and anvils are present amongst concentrations of percussively broken mastodon limb bones within strata of otherwise fine-grained sediments. Fracture patterns, bulbs of percussion, and cone flakes indicate that bones were broken while fresh, likely for tool material or bone marrow. Radiocarbon dating of bone was unsuccessful due to the absence of collagen, and multiple attempts to use luminescence dating of enclosing silts only determined that ages were greater than 60 to 70 ka.

Subsequently, bone specimens were dated using U-series disequilibrium. Living bone tissue has low U concentrations (0.005–0.05 µg/g); however, much higher values are present in CM fossil bones (mean=162±67 µg/g) reflecting substantial post-mortem U incorporation. Evaluating the U-uptake history is critical to establishing an accurate age. Multi-sample profiles consisting of 13 to 30 individual analyses across cortical layers of 2 limb and 1 rib bones were analyzed by thermal ionization mass spectrometry. Conventional closed-system ²³⁰Th/U ages range from 100.5±0.7 to 126.9±1.1 ka. Both U concentrations and ²³⁰Th/U ages yielded simple, υ-shaped profiles, which is consistent with models of continuous diffusion and adsorption of U into porous compact bone. Application of iDAD modeling (Diffusion-Adsorption-Decay; Sambridge et al., 2012; doi:10.1016/j.quageo.2012.02.010) using U-Th isotope compositions and positions across each profile yielded maximum-likelihood ages of 137.0±5.6 ka, 126.4±5.5 ka, and 130.2±2.7 ka (±2σ). Initial ²³⁴U/²³⁸U values are consistent with local water sources, and there is no evidence of U leaching that would result in erroneously old ²³⁰Th/U ages. The mean of the 3 iDAD ages weighted by their uncertainties yields a mean age of 130.7±9.4 ka. This date implies that hominins reached the Americas more than 100,000 years earlier than previously thought.