The evolution of Paleoindian geochronology and typology on the Great Plains

In 1988, Fairbanks conducted a drilling expedition off the south coast of Barbados to recover submerged corals contemporaneous with the last deglaciation. Core recovery was excellent and >30 different samples were dated by conventional β-counting techniques (Fairbanks 1989). At about the same time, we developed, at Lamont, the thermal ionization mass spectrometry (TIMS) technique to obtain precise U-Th ages (Edwards 1988), and to compare them with the14C estimates measured on the same samples. A surprising result was that the discrepancy between14C and U-Th ages increased through time toca.3000–3500 yr atca.15,00014C BP (Bardet al.1990a). Because the three youngest samples yielded U-Th ages in agreement with their calibrated14C ages, we concluded initially that the TIMS U-Th determinations were not only precise, but also accurate, and that the14Cvs.U-Th data set could be used for a first-order14C calibration.

Bone would seem to be an ideal material for14C dating because this calcified tissue contains 20 weight per cent protein. Fossil bone, however, can lose most of its original organic matter and frequently contains contaminants having different14C ages. Numerous14C dates on bone have been available to archaeologists and geologists but many age determinations have been inaccurate despite over 30 years of research in the field following the first14C age determinations on bone (Arnold & Libby, 1951). This situation remained unchanged until simple pretreatments were abandoned and more bone-specific fractions were isolated. The ideal solution is to use accelerator mass spectrometer14C dating, which facilitates the use of milligram-sized amounts of highly purified compounds—an approach impossible to pursue using conventional14C decay-counting methods.

The events to do with peopling the New World archaeologically represented by ‘Clovis’ and ‘Folsom’ have been — tantalizingly — beyond the range of radiocarbon calibration. Now calibration extends further, one can ask if the aburptness of Clovis, of Folsom, and of the transition between them are realities. A calibrated chronology for those sites where the stratigraphic security is best shows these in truth are rapid human affairs.