The Pleistocene footprints are younger than we thought: correcting the radiocarbon dates of Ruppia seeds, Tularosa Basin, New Mexico

Abstract

Past studies have demonstrated that Ruppia cirrhosa (Ruppia), which typically grows in brackish water, is far too unreliable to serve as the chronological basis for radiocarbon dating because of the hard water effect (HWE). Despite this unreliability, Ruppia seeds have been used to date footprints along the margins of paleo-Lake Otero in southern New Mexico to around 23,000–21,000 cal yr BP. In this study, we employ a modern analog approach using δ13C values and radiocarbon dates from modern Ruppia plants growing in Salt Creek to calculate a maximum limiting age range for the footprints. Those plant samples with higher δ13C values produced greater age discrepancies. This simple relationship can be used to correct for the HWE and demonstrates that the human footprints purported to have been made during the local last glacial maximum could be at least ~7500 yr younger.