Page 189 - Introduction to Paleobiology and The Fossil Record

176 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 7.3 Professor Alvarez’s equation

In proposing that the dinosaurs and many other organisms had been killed by an asteroid impact,
Luis Alvarez proposed an equation that summarized all the key features of an impact and the black-
ing-out of the sun. The equation is simple and daring, especially because it is based on limited evi-
dence. This might seem to be a bad thing – surely scientists should be careful? However, sticking
your neck out is a good thing for a scientist to do. You have to dare to be wrong; but it helps to be
right sometimes as well.
The role of a scientist is to test hypotheses (see p. 4), and that means your own hypotheses have
to be open to test by others. The more daring the hypothesis, the easier it would be to disprove. The
Alvarez et al. (1980) model for the KT mass extinction was extremely daring and could easily have
failed. The fact that it has not been disproved, and indeed that a huge amount of new evidence sup-
ports it, makes this a very successful hypothesis.
The Alvarez et al. (1980) formula is:

M = sA
.
022 f
where M is the mass of the asteroid, s is the surface density of iridium just after the time of the
impact, A is the surface area of the Earth, f is the fractional abundance of iridium in meteorites, and
0.22 is the proportion of material from Krakatoa, the huge volcano in Indonesia that erupted in
1883, that entered the stratosphere. The surface density of iridium at the KT boundary was estimated
−9
−2
as 8 × 10 g cm , based on the local values at Gubbio, Italy and Stevns Klint, Denmark, their two
−6
sampling localities. Measurements of modern meteorites gave a value for f of 0.5 × 10 .
Running all these values in the formula gave an asteroid weighing 34 billion tonnes. The diameter
of the asteroid was at least 7 km. Other calculations led to similar results, and the Alvarez team
fixed on the suggestion that the impacting asteroid had been 10 km in diameter.

Websites about the KT event may be seen at http://www.blackwellpublishing.com/
paleobiology/.

long intervals of time as a result of climatic In some interpretations, the volcanic model
changes. On land, subtropical lush habitats explains instantaneous catastrophic extinc-
with dinosaurs gave way to strongly seasonal, tion, while in others it allows a span of
temperate, conifer-dominated habitats with 3 myr or so, for a more gradualistic pattern
mammals. Further evidence for the gradualist of dying off caused by successive eruption
scenario is that many groups of marine organ- episodes.
isms declined gradually through the Late Cre- The gradualist and volcanic models held
taceous. Climatic changes on land are linked sway in the 1980s and 1990s, but increasing
to changes in sea level and in the area of warm evidence for impact has strengthened support
shallow-water seas. for the view expressed in the original Alvarez
A third school of thought is that most of et al. (1980) paper. The discovery of the
the KT phenomena may be explained by vol- Chicxulub Crater, deep in Upper Cretaceous
canic activity. The Deccan Traps in India rep- sediments on the Yucatán peninsula, Central
resent a vast outpouring of lava that occurred America (Fig. 7.10) has been convincing. Melt
over the 2–3 myr spanning the KT boundary. products under the crater date precisely to the
Supporters of the volcanic model seek to KT boundary, and the rocks around the shores
explain all the physical indicators of catastro- of the proto-Caribbean provide strong sup-
phe (iridium, shocked quartz, spherules, and port too. For example, sedimentary deposits
the like) and the biological consequences as around the ancient coastline of the proto-
the result of the eruption of the Deccan Traps. Caribbean that consist of massive tumbled