Geoscience Reference
In-Depth Information
concentrate the iridium from a large reservoir of seawater into a par-
ticular rock layer. Iridium might be absorbed selectively on the sur-
faces of the clay minerals, for example. Or, perhaps the clay and irid-
ium were once dispersed minutely throughout a thick, marine
limestone bed that slowly dissolved away, leaving behind only the
insoluble clay and iridium. These ideas might have applied to rocks
deposited in the sea, but not to those laid down as sediments in
freshwater, which contains even less iridium and where there is no
opportunity to tap a vast reservoir. The discovery of a strong iridium
anomaly in rocks from the Raton Basin in New Mexico and Col-
orado, rocks recognizable as having formed in freshwater, put the
idea of seawater extraction to rest.
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(Luis Alvarez, with the advan-
tage of hindsight, said that the occurrence of the iridium spike in
freshwater rocks should have been one of his predictions.] At the
exact level of the Raton iridium spike, several Cretaceous pollen
species went extinct and ferns—which are opportunistic and move
in after other species disappear—proliferated.
PREDICTION
2: Elsewhere in the geologic column, iridium
and other markers of impact will be rare.
If high iridium concentrations come from meteorites, they will not
be found in most other rocks. If the indicators of shock described in
Chapter 3—shatter cones, shocked quartz, coesite, stishovite, and
tektites—are produced only by impact, they too will be rare to
nonexistent in other geological settings.
(This is an appropriate place to note that the K-T mass extinc-
tion was one of many times during which substantial numbers of
species disappeared. Paleontologists have identified five, including
the K-T, that were especially severe. If impact is responsible for any
others of the "Big Five," they too might show an iridium spike and
impact markers. However, the presence or absence of indicators at
those horizons would have no direct bearing on the Alvarez theory,
which applies only to the K-T event. The possibility that impact
might have caused more than one mass extinction is a related but
separate theory that I will address later.)
FINDINGS
It is obviously impossible to search for iridium in every rock on the
surface of the earth. Frank Kyte and John Wasson of UCLA did the
next best thing by measuring iridium content in a long, continuous
core of sediment pulled up from the deep seafloor in the Pacific.
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It
captured the sedimentary record from about 35 million years ago all
