Response to Comment on “Evidence of humans in North America during the Last Glacial Maximum”

Madsen et al . question the reliability of calibrated radiocarbon ages associated with human footprints discovered recently in White Sands National Park, New Mexico, USA. On the basis of the geologic, hydrologic, stratigraphic, and chronologic evidence, we maintain that the ages are robust and conclude that the footprints date to between ~23,000 and 21,000 years ago. Madsen et al . ( 1 ) question the veracity of calibrated radiocarbon ages used to constrain the antiquity of human trackways discovered recently at White Sands National Park (WHSA) Locality 2, New Mexico, USA ( 2 ). The ages were derived from seeds of the aquatic plant Ruppia cirrhosa , which they suggest may suffer from hard-water (or reservoir) effects, making them too old, potentially by thousands of years. We were well aware of this possibility, investigated it, and presented several lines of evidence that argued against such a problem. Here we respond to each of their four primary points.