Out with a Bang - Four Revolutions in the Earth Sciences - page 186

extinction, is revealed as both being pre-K-T age and having caused no species ex-
tinctions.” Instead, “Deccan volcanism and associated climate and environmental
effects may have triggered the K-T catastrophe with the Chicxulub impact an early
contributor.” 9
Thirty Years Later
In 2010, the thirtieth anniversary of the original Alvarez paper, a team of forty-
one authors from different geological specialties and nationalities took the occa-
sion to write a comprehensive review of the status of the Alvarez theory. 10 By
then, more than 350 K-T boundary sites had been discovered, and a vast amount
of new information had become available, making the rocks of the K-T bound-
ary “the most intensively investigated deposits in the geological record.” 11 Many
predictions that derive from the original Alvarez theory but which were untested
or not even conceived in 1980 had been fulfilled. Let us take one as an example:
the thickness of the K-T “ejecta” layer around the world (1215). According to the
review, drill cores from inside the Chicxulub structure reveal an impact breccia
(broken fragments of rock cemented together) over one hundred meters thick. The
greater the distance from Chicxulub, the thinner the boundary deposits become un-
til, at faraway sites like Gubbio, they are only two to five millimeters thick. These
declining thicknesses are exactly what one would predict from a giant explosion
in which the Chicxulub structure was ground zero. After considering the wealth
of information available by 2010, the authors of the anniversary article concluded
“that the Chicxulub impact triggered the mass extinction” (1214).
Three sets ofauthors wrote to Science to rebut the review article. The first rebut-
tal was signed by twenty-nine authors, some of them now in their thirtieth year of
opposing the Alvarez theory and most of them vertebrate paleontologists. 12 While
they agreed that “impact played some role” in the K-T extinctions, they rejected
the “simplistic extinction scenario” presented in the review article, instead prefer-
ring multiple causes including sea level fall, volcanism, and climate change. To
bolster their case, they made a three-pronged argument: (1) no mass extinctions
other than the K-T are associated with impact; (2) no other known impact struc-
tures are associated with mass extinctions; and (3) all five large mass extinctions
occur during times ofsea level fall, and three occur at the same time as flood basalt
eruptions, like those from India's Deccan plateau. But the Alvarez theory does not
claim that all mass extinctions are caused by impact or that all meteorite impacts
result in a mass extinction. The theory proposes that one particular impact caused
one particular mass extinction. To be consistent, the rebutting authors ought also to