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temperature to fall to the freezing point. Darkness came at noon, and
remained for months. Photosynthesis halted and the food chain that
depended upon it ceased to function.
The blast wave acted as a chemical catalyst, causing atoms of
oxygen and nitrogen to combine to form various noxious com-
pounds, many found in today's smog. Sulfur oxides joined them, for
in a coincidence unfortunate for life at the end of the Cretaceous,
the Yucatan rocks at ground zero included sulfate deposits. As hap-
pened in the modern eruptions of Pinatubo and El Chichon, sulfur
dioxide formed tiny droplets that further obscured the sun and low-
ered visibility even more. Kevin Pope, Kevin Baines, and Adriana
Ocampo have calculated that the impact into the sulfur-rich de-
posits of the Yucatan would have produced over 200 billion tons of
both sulfur dioxide and of water, leading to a decade-long impact
winter. 6 4
As precipitation washed out the nitrogen and sulfur compounds,
it generated acid rain that may have destroyed the remaining sus-
ceptible plants. Gregory Retallack of the University of Oregon 6 5 has
found evidence in the boundary clay in Montana of severe acid
leaching, possibly enough to have dispersed the iridium and dis-
solved the shocked minerals and spherules. Thus, Retallack says,
some impacts might be "self-cleaning," eliminating traces of their
own existence. Because some soils naturally buffer acids and others
do not, acid rain might also explain some of the K-T extinction
selectivity. For example, the floodplains of ancient Montana would
have remained above a pH of 4, which according to Retallack would
spare small mammals, amphibians, and fish, but harm plants, non-
marine mollusks, and dinosaurs. Acid-vulnerable plants such as the
broadleaf evergreens would have suffered, whereas the acid-tolerating
plants would have done better, more or less consistent with the evi-
dence.
The rain may have acidified the surface layers of the oceans suf-
ficiently to kill the surface-dwelling plankton and phytoplankton,
which would have caused a breakdown of the oceanic food chain
that was based upon them. The reactions that formed nitrogen
oxides also absorbed ozone, reducing the earth's protective ozone
layer and allowing ultraviolet radiation to penetrate to the surface,
causing further loss of life.
Some of the vast amount of water vapor that was blasted into
the atmosphere froze; the rest formed a vapor cloud that lasted for
years. There it was joined by the most insidious long-term effect of
the impact—a worldwide cover of carbon dioxide, generated by
impact into the thick limestones (calcium carbonate) that also were
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