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174 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
about it than about the PT event. Before 1980,
Siberian trap volcanism
scientists had come up with over 100 theories
for what might have happened 65 million
rise in atmospheric CO 2 years ago. These theories ranged from the
reasonable (global climate change, change in
CO 2
plants, impact, plate tectonic movements, sea-
GLOBAL WARMING
level change) to the frankly ludicrous (loss of
CH 4
terrestrial
extinctions sexual appetite, increasing stupidity or hor-
monal imbalance of the dinosaurs, competi-
melting of shallow
gas hydrates reduced ocean reduced tion with caterpillars for plant food, mammals
circulation; upwelling ate all the dinosaur eggs). A number of serious
stratification
efforts had been made to document just what
productivity decline happened through the KT interval and to look
marine anoxia
at environmental and other changes. Then the
bombshell struck.
marine extinctions In June 1980, one of the most important
papers of the 20th century appeared in Science.
Figure 7.8 The possible chain of events This paper, by Luis Alvarez and colleagues,
following the eruption of the Siberian Traps, made the bold assertion that a 10 km mete-
251 Ma. Volcanism pumps carbon dioxide (CO 2 ) orite (asteroid) had hit the Earth, the impact
into the atmosphere and this causes global threw up a great cloud of dust that encircled
warming. Global warming leads to reduced the globe, blacked out the sun, and caused
circulation and reduced upwelling in the oceans, extinction worldwide by stopping photosyn-
which produces anoxia, productivity decline and thesis in land plants and in phytoplankton.
extinction in the sea. Gas hydrates may have With their plant food gone, the herbivores
released methane (CH 4 ) which produced further died out, followed by the carnivores. This
global warming in a “runaway greenhouse” simple model was based on limited observa-
scenario (shaded gray). (Courtesy of Paul tional evidence and it was, needless to say,
Wignall.)
highly controversial.
Luis Alvarez was a physicist who had won
a Nobel Prize for his work on subatomic par-
with water in the atmosphere to form a deadly ticles. He became involved with his son Wal-
cocktail of sulfuric, carbonic and nitric acids. ter’s geological work in Italy, where a relatively
The acid rain killed the land plants and they complete rock succession documented the KT
were washed away, and this released the soils boundary in detail. The geological team iden-
that were also stripped off the land. With no tified an unusual clay band right at the KT
food, land animals died. The carbon dioxide boundary, within a succession of marine lime-
from the eruptions caused global warming stones. They measured the chemical content
and this perhaps released the gas hydrates, of the clay band, and of the rocks above and
causing further global warming. Warming is below, and found an unusual enhancement of
often associated with loss of oxygen, and the metallic element iridium. This was the
seabeds became anoxic, so killing life in the famous iridium spike, where the iridium
sea. If this model is correct, it is in some ways content shot up from normal background
more startling than the KT impact because levels of 0.1–0.3 parts per billion (ppb) to 9
this represents an entirely Earth-bound process ppb (Fig. 7.9). Iridium is a platinum-group
when all normal regulatory systems, whether metal that is rare on the Earth’s crust, and
these are part of a Gaia model (see p. 25) or reaches the Earth almost exclusively from
not, broke down. And it all began with global space, in meteorites. The background low
warming . . . levels represent the results of numerous minor
meteorite impacts that go on all the time.
Alvarez proposed that the iridium spike
The Cretaceous-Tertiary event
indicated an unusually high rate of arrival of
The KT event has been subjected to intense iridium on the Earth’s crust, thus a huge mete-
scrutiny since 1980 so much more is known orite (asteroid) impact. He calculated, working
