| Summary: | Recent efforts to precisely date the florescence of the Clovis culture in North America have been hampered by both practical and theoretical problems: 1) The era of Clovis expansion (about 11,200-10,700 BP or 13,200-12,700 cal BP) coincides with the gap between the anchored central European tree-ring sequence (back to 12,400 cal BP) and the floating Bolling-Allerod sequence; 2) Clovis seems to immediately precede the onset of the Younger Dryas (YD) stadial. The "black mats" of the US Southwest appear to mark the regional occurrence of this climatic downturn. However, the timing and means of long-distance propagation of this climatic event are not yet well understood. Greenland ice cores (GISP2, GRIP, and NGRIP) remain poorly synchronized, with a discrepancy of 100 to 250 yr for the date of onset (as late as 12,700 cal BP, or as early as 12,950 cal BP); 3) The YD onset was accompanied by a rapid drop of radiocarbon ages from 11,000 to 10,600 BP in less than a century. The mechanism causing this was probably a change in overturning circulation in the North Atlantic. Do variable Clovis ages, often from what appear to be single-occupation contexts, reflect this "cliff" effect, slightly earlier minor reversals during the late Allerd, or simply the practical limitations of precision of the 14C method? 4) Dates for Fishtail or Fell I sites (with fluted, stemmed points) in southern South America are statistically indistinguishable from Clovis dates in North America. Does this imply very rapid population expansion, diffusion of tool-making techniques through long-established local populations (as argued by Waters and Stafford 2007), or abnormally large interhemispheric 14C offsets? 5) Are recent ostensibly high-precision collagen-derived dates for Paleoindian-associated fauna (e.g. horse and mammoth) reliable? Are interlaboratory blind tests of the new filtration processes necessary?
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