Page 190 - Introduction to Paleobiology and The Fossil Record
P. 190
MASS EXTINCTIONS AND BIODIVERSITY LOSS 177
North America 0 500 km
Coastline at the
end of the
Cretaceous
Gulf of Mexico
Atlantic Ocean
Chicxulub Cuba
Crater Yucatán Haiti
peninsula (a) (b)
Figure 7.11 Evidence for a KT impact in the
Caribbean. (a) Shocked quartz from a KT
boundary clay. (b) A glassy spherule from the
Pacific Ocean
KT boundary section at Mimbral, northeast
Mexico, evidence of fall-out of volcanic melts
from the Chicxulub Crater (about 1.5 mm in
evidence
of large waves South America diameter). (Courtesy of Philippe Claeys.)
non-marine
There has been considerable debate about
Figure 7.10 The KT impact site identifi ed. the exact dating of the impact layers. Some
Location of the Chicxulub Crater on the evidence suggests that the Chicxulub impact
Yucatán peninsula, Central America, and sites of happened up to 300,000 years before the KT
tempestite deposits around the coastline of the boundary and extinction level. This is hotly
proto-Caribbean (open circles). Continental KT debated and the idea has been rejected by
deposits are indicated by triangles.
many paleontologists. But, if the impact hap-
pened at a different time from the main pulse
of extinction, then the simple KT killing model
and disturbed sedimentary blocks indicate would have to be revised.
either turbidite (underwater mass fl ow) or Thus, the geochemical and petrological
tsunami (massive tidal wave) activity, presum- data such as the iridium anomaly, shocked
ably set off by the vast impact. Further, the quartz and glassy spherules, as well as the
KT boundary clays ringing the site also Chicxulub Crater give strong evidence for an
yield abundant shocked quartz (Fig. 7.11a), impact on Earth 65 million years ago. Pale-
grains of quartz bearing crisscrossing lines ontological data support the view of instan-
produced by the pressure of an impact. In taneous extinction, but some still indicate
addition, the KT boundary clays within longer-term extinction over 1–2 myr. Key
1000 km of the impact site also contain glassy research questions are whether the long-term
spherules (Fig. 7.11b) that have a unique geo- dying-off is a genuine pattern, or whether it
chemistry. Volcanoes can produce glassy is partly an artifact of incomplete fossil col-
spherules – melt products of the igneous lecting, and, if the impact occurred, how it
magma – deep in the heart of the volcano. The actually caused the patterns of extinction.
KT spherules, though, have the same geo- Available killing models are either biologi-
chemistry as limestones and evaporites, sedi- cally unlikely, or too catastrophic: recall that
mentary rocks that lay on the seafl oor of the a killing scenario must take account of the
proto-Caribbean, so the volcanic hypothesis fact that 75% of families survived the KT
cannot explain them. Sedimentary rocks can event, many of them seemingly unaffected.
be melted only by an unusual process such Whether the two models can be combined so
as a direct hit by an asteroid. Farther afi eld, that the long-term declines are explained by
the boundary layer is thinner, there are no gradual changes in sea level and climate and
turbidite/tsunami deposits, spherules are the final disappearances at the KT boundary
smaller or absent, and shocked quartz is less were the result of impact-induced stresses is
abundant. hard to tell.
