Page 188 - Introduction to Paleobiology and The Fossil Record

MASS EXTINCTIONS AND BIODIVERSITY LOSS 175

244

248

252

256
Depth (m) 260 coal

carbonaceous
shale
264 clay shale
mudstone
siltstone
268
sandstone

272

276
10 1 10 2 10 3 10 –1 10 0 10 1 10 2
Ir abundance (ppt) Angiosperm
pollen/fern spores

Figure 7.9 The iridium (Ir) spike and fern spike, as recorded in continental sediments in York Canyon,
New Mexico. The Ir spike, measured in parts per trillion (ppt), an enhancement of 10,000 times
normal background levels, is generally interpreted as evidence for a massive extraterrestrial impact.

The fern spike indicates sudden loss of the angiosperm flora, and replacement by ferns. (Based on Orth
et al. 1981.)

backwards (Box 7.3), that a killing impact indeed in 1991 the crater was identifi ed at
would have to extend its effects worldwide, Chicxulub in Mexico.
which meant a dust cloud that encircled the A catastrophic extinction is indicated by
globe. Based on studies of experimental sudden plankton and other marine extinc-
impacts, and on known major volcanic erup- tions, and by abrupt shifts in pollen ratios, in
tions, he calculated that the crater would have certain sections. The shifts in pollen ratios
to be 100–150 km across to produce such a show a sudden loss of angiosperm taxa and
large dust cloud, and this implied a meteorite their replacement by ferns, and then a pro-
10 km in diameter. The 1980 Science paper gressive return to normal fl oras. This fern
attracted instant press coverage on a huge spike (Fig. 7.9), found at many terrestrial KT
scale, and scientists from all disciplines boundary sections is interpreted as indicating
were alerted to the dramatic new idea the aftermath of a catastrophic ash fall: ferns
immediately. recover first and colonize the new surface,

The Alvarez et al. (1980) paper was hugely followed eventually by the angiosperms after
controversial, partly because the idea was so soils begin to develop. This interpretation has
outrageous, partly because its chief author been made by analogy with observed fl oral
was a physicist and not a geologist or paleon- changes after major volcanic eruptions.
tologist, and partly because the evidence The main alternative to the extraterrestrial
seemed flimsy in the extreme. But Alvarez and catastrophist model for the KT mass extinc-

colleagues were vindicated. Since 1980, evi- tion was the gradualist model, in which
dence has piled up that they were right, and extinctions were said to have occurred over