The Cretaceous-Tertiary Boundary
265
9.6 The Cretaceous-Tertiary Boundary
9.6.1 The Evidence for Sudden Extinction. The Cretaceous-Tertiary boundary
marks a major biostratigraphic break in the record. This break, of course, is the
reason why the boundary between the Mesozoic and the Cenozoic was placed here in
the first place. By convention, we count the time since as the age of the mammals
(Appendix A3), which replaced that of the reptiles. In shallow marine sections exposed on land, the evidence for profound change is clear everyWhere. However, one
could never be quite sure to what degree the change observed was the result of
superposition of sediments of greatly differing age. Pelagic sediments exposed on
land gave the best evidence for a rapid and fundamental change in plankton biota a
the "Kff"-Boundary, thanks to the studies carried out by a number of biostratigraphers, in the 1960's (e. g., W. A. Berggren, M. N. Bramlette and E. Martini, H.
Luterbacher, and I. Premoli-Silva). Results from deep-sea drilling fully confirmed
these early results, and demonstrated a largescale, worldwide extinction event (Fig.
9.15). Careful scrutiny of the interval associated with extinction, and global correlation, showed that the catastrophe happened in a geologically short time, say, less than
100 000 years.
9.6.2 The Causes of Extinction. From the viewpoint of paleoceanography, there is
little reason to suspect fundamental differences in circulation or heat budget between
the latest Cretaceous and the early Paleogene, although a general regression apparently was in progress at the time. This observation, combined with the suddenness of
the extinction, and the fact that it was highly selective (warm-water planktonic
species were greatly reduced, but not deep-sea benthic species, for example) makes a
buildup of gradual stress unattractive as a cause. Instead, we must look to a major
unusual global disturbance.
The number of causes that have been proposed to explain the great extinctions at
the end of the Cretaceous (including the demise of the dinosaurs) is truly remarkable.
Radiation from a supernova, poison gas from a comet entering the atmosphere, climate catastrophe from large-scale volcanism or from enormous meteorites colliding
with Earth are among the hypotheses testifying to the fertile imagination of geologists. However, the major cause of the sudden extinctions remained unknown, until L.
and W. Alvarez and co-workers (1980) and J. Smit and J. Hertogen (1980) presented
strong evidence for the impact scenario, that is, for collision with a large mass of
rock coming in from outer space (e. g., a body ca. 10 km in diameter). The evidence
rests mainly on the observation that iridium (a noble rare metal) is greatly enriched in
the Kff boundary layer, and that shocked quartz is present, attesting to sudden high
pressures. These findings made the topic of the Kff extinctions a dominant one all
through the 1980s. Even a candidate for an impact crater has been found, on the
Yucatan peninsula in Mexico, the 200-km-diameter Chicxulub structure.
Alternative scenarios are being debated as well, mainly those centered on volcanism. There is no reason, of course, why volcanism (or regression) could not have
aided in bringing about unfavorable conditions for survival in the aftermath of an
impact, or a series of impacts.
265
9.6 The Cretaceous-Tertiary Boundary
9.6.1 The Evidence for Sudden Extinction. The Cretaceous-Tertiary boundary
marks a major biostratigraphic break in the record. This break, of course, is the
reason why the boundary between the Mesozoic and the Cenozoic was placed here in
the first place. By convention, we count the time since as the age of the mammals
(Appendix A3), which replaced that of the reptiles. In shallow marine sections exposed on land, the evidence for profound change is clear everyWhere. However, one
could never be quite sure to what degree the change observed was the result of
superposition of sediments of greatly differing age. Pelagic sediments exposed on
land gave the best evidence for a rapid and fundamental change in plankton biota a
the "Kff"-Boundary, thanks to the studies carried out by a number of biostratigraphers, in the 1960's (e. g., W. A. Berggren, M. N. Bramlette and E. Martini, H.
Luterbacher, and I. Premoli-Silva). Results from deep-sea drilling fully confirmed
these early results, and demonstrated a largescale, worldwide extinction event (Fig.
9.15). Careful scrutiny of the interval associated with extinction, and global correlation, showed that the catastrophe happened in a geologically short time, say, less than
100 000 years.
9.6.2 The Causes of Extinction. From the viewpoint of paleoceanography, there is
little reason to suspect fundamental differences in circulation or heat budget between
the latest Cretaceous and the early Paleogene, although a general regression apparently was in progress at the time. This observation, combined with the suddenness of
the extinction, and the fact that it was highly selective (warm-water planktonic
species were greatly reduced, but not deep-sea benthic species, for example) makes a
buildup of gradual stress unattractive as a cause. Instead, we must look to a major
unusual global disturbance.
The number of causes that have been proposed to explain the great extinctions at
the end of the Cretaceous (including the demise of the dinosaurs) is truly remarkable.
Radiation from a supernova, poison gas from a comet entering the atmosphere, climate catastrophe from large-scale volcanism or from enormous meteorites colliding
with Earth are among the hypotheses testifying to the fertile imagination of geologists. However, the major cause of the sudden extinctions remained unknown, until L.
and W. Alvarez and co-workers (1980) and J. Smit and J. Hertogen (1980) presented
strong evidence for the impact scenario, that is, for collision with a large mass of
rock coming in from outer space (e. g., a body ca. 10 km in diameter). The evidence
rests mainly on the observation that iridium (a noble rare metal) is greatly enriched in
the Kff boundary layer, and that shocked quartz is present, attesting to sudden high
pressures. These findings made the topic of the Kff extinctions a dominant one all
through the 1980s. Even a candidate for an impact crater has been found, on the
Yucatan peninsula in Mexico, the 200-km-diameter Chicxulub structure.
Alternative scenarios are being debated as well, mainly those centered on volcanism. There is no reason, of course, why volcanism (or regression) could not have
aided in bringing about unfavorable conditions for survival in the aftermath of an
impact, or a series of impacts.