Out with a Bang - Four Revolutions in the Earth Sciences - page 189

which might then have delivered the final blow. Arguments over these possibilities
had been going on for decades.
At a meeting in 2007, researchers presented a new theory that purported to solve
the three mysteries in one fell swoop: a meteorite impact, or perhaps a meteorite
or group of meteorites that burst in the air, had exploded over North America and
led to an “impact winter.” They reported finding in a thin sedimentary boundary
that marks the beginning of the YD ultramicroscopic diamonds, tektite-like micro-
spherules, iridium, and other unusual minerals and materials. If replicated, these
findings would be difficult to explain by any process other than impact or airburst.
But though some said they had replicated the findings, others reported that they
could not. The failures carried the day, and the YD impact theory appeared to have
died in its crib. One group titled their 2011 article “The Younger Dryas Impact
Hypothesis: A Requiem.” They wrote that “recognized and expected impact mark-
ers were not found, leading to proposed YD impactors and impact processes that
were novel, self-contradictory, rapidly changing, and sometimes defying the laws
of physics.” In a warning that sounds familiar to us by now, they said that “the YD
impact hypothesis provides a cautionary tale forresearchers, the scientific commu-
nity, the press, and the broader public.” 19 But as we have seen, just when authorit-
ies sign the death certificate of a theory, new evidence may be bringing it back to
life.
Studies of new YD horizons, including one in a lake in central Mexico reported
in 2012, continued to yield nanodiamonds and strange spherules. 20 Such positive
evidence proved increasingly hard to dismiss. Then came a finding that fully re-
surrected the impact theory. Right at the YD boundary in a well-studied ice core
from Greenland, researchers found a large anomaly in the abundance of the rare
metal platinum. Over a twenty-one-year period straddling the boundary, platinum
concentration rose at least a hundred-fold, then returned to where it had started.
Strangely, the concentration of iridium, which usually accompanies platinum, did
not rise accordingly. The combination of high platinum and low iridium ruled out
terrestrial sources and most meteoritic ones, though it does occur in a certain class
of iridium-poor iron meteorites.
Another study focused on the glassy microspherules. The authors analyzed
nearly seven hundred spherules from eighteen YD sites, extrapolating the total de-
posited in the event to about 10 million metric tons of spherules spread over 50
million square kilometers. Their analysis ruled out all sources for the spherules
other than meteoritic. 21 Many questions remain, but the YD impact theory has re-
turned from the dead.