Radiocarbon calibration curves,summed probability distributions,and early Paleoindian population trends in North America

Archaeologists increasingly examine summed probability distributions of radiocarbon dates to search for temporal trends in ancient human populations, including early North American population trends across the onset of the Younger Dryas climatic period (10,900 BC). We use both IntCal04 and IntCal09 to simulate and calibrate sequences of radiocarbon dates that represent evenly spaced calendar ages from 12,000 to 9000 BC and use IntCal09 to simulate two different demographic processes across this interval. The shapes of the calibrated summed probability distribution from our first set of simulations mirror many trends that have been interpreted in demographic terms and this shape shows clear links to the shape of the radiocarbon calibration curve. This is true for both IntCal04 and IntCal09, although these different curves produce different probability distributions. The shapes of the calibrated summed probability distribution from our second set of simulations differ somewhat but show virtually identical trends at points where the actual frequencies of calendar dates are very different. We conclude that changing frequencies of radiocarbon dates over time probably do contain demographic information, but that extracting this information is more difficult than archaeologists have acknowledged.