At the sites studied, independent radiocarbon (1) and optically
stimulated luminescence dates that tend to cluster near 13 ka were
used to establish the age of the YDB. For example, the end-Clovis
stratum (theYDB)is well dated at Murray Springs,AZ, (eight dates
averaging 10,890 14Cyr or calendar 12.92 ka) and the nearby Lehner
site (12 dates averaging 10,940 14C yr or 12.93 calendar ka). Haynes
(2) correlated the base of the black mat (the YDB) with the onset
of YD cooling, dated to 12.9 ka in the GISP2 ice core, Greenland
(see GISP2 chronology in SI Fig. 6) (18). Therefore, we have
adopted a calendar age of 12.9 0.1 ka for the YD event.
We propose that the YD event resulted from multiple ET
airbursts along with surface impacts. We further suggest that the
catastrophic effects of this ET event and associated biomass burning
led to abrupt YD cooling, contributed to the late Pleistocene
megafaunal extinction, promoted human cultural changes, and led
to immediate decline in some post-Clovis human populations (19).