Environmental Engineering Reference
In-Depth Information
15
Evidence for volcanism triggering extinctions: a short
history of IPGP contributions with emphasis on
paleomagnetism
vincent courtillot, fr´ d´ ric fluteau and jean besse
15.1 Introduction
The question of mass extinctions came to the forefront with the discovery of
anomalous iridium concentrations in a thin clay layer at the Cretaceous
Tertiary
boundary (KTB) near Gubbio (Italy) and the proposal by Alvarez
et al
.(
1980
)thatan
impact of an asteroid was the likely cause of anomalies observed at that boundary.
This came less than two decades after the plate tectonics revolution; many scientists
who were involved in the latter became interested in the former. One of us (V.C.),
who had assisted Xavier Le Pichon in teaching the
-
first plate tectonics class in France
in 1972, became a convert. In 1982, during the
field trip to Tibet between
Chinese and Western geoscientists, Claude Allègre, Paul Tapponnier and V.C.
discussed at length what seemed to be the solution to the KTB extinction. We taught
our students the
“
Alvarez impact hypothesis
”
in the following years.
This was also the time when we started forming the paleomagnetic laboratory
at IPG (Paris). Having first analysed Mesozoic and Cenozoic red sandstones
and andesitic lavas from Tibet, we wanted to see if paleomagnetism could better
constrain the convergence history of the Indian and Asian plates. Next, we aimed at
India and focused on the thick pile of Deccan lavas that, according to older papers,
might have erupted over tens of millions of years from the Cretaceous to theMiocene.
But only one (in general) and at most two (on rare occasions) reversals were found in
thousands of meters of lava outcrops: the duration of volcanism had been quite
short. The K
first joint
Ar method indicated an age of 65 Ma, with an uncertainty of 1 to
2 Myr; a tooth of a fossil ray (found in sediments sandwiched between the lower lava
-
flows) implied the Maestrichtian. Put together, these three lines of evidence led to
the conclusion that volcanism had lasted only about 1 Myr, straddling the KTB
(Courtillot
et al.
,
1986a
,
b
;
Figure 15.1
). The Deccan Traps ranked
“
as one of the
