Náttúrufræðingurinn - 1991, Blaðsíða 20

the length of time necessary for the depos-
ition of the boundary clay that marks the
Cretaceous/Tertiary (K/T) boundary in a
very well known sedimentary section at
Gubbio, a small town in central Italy.
Their method was to determine the iri-
dium content of the clay and compare that
with the estimated annual downfall of
cosmic dust. They chose iridium because
it is about 1000 times more abundant in
meteorites than in crustal materials. The
analytical results of their work were quite
unexpected and the group felt compelled
to visit other well documented sections
from the same geological period and col-
lected samples from Stevns Klint in
Denmark and Woodside Creek in New
Zealand. Chemical analyses of a band of
clay at the boundary in each of the three
localities showed similar if not identical
compositional patterns.
The abnormally high quantities of iri-
dium and osmium were of most interest.
They were much too higli to be explained
by influx rates of cosmic matter. The Al-
varez group proposed that exactly 65
million years ago an asteroid, 10 ± 4 km
in diameter, struck the earth and
exploded. Such an impact would result in
a crater that would be at least 150 km in
diameter. On impact, a vast cloud of dust
was thrown up into the air and shut out
the sunlight for several years. During the
time of darkness photosynthesis was supp-
ressed and food chains collapsed, causing
mass extinction on a large scale. Further-
more, the existence of soot in many K/T
boundary sections is considered evidence
of wide-ranging, even global, wildfires
raging at the time possibly caused by an
asteroid impact (cf. Wolbach et al. 1985,
Melosh et al. 1990). To begin with, the Al-
varez group drew attention to volcanism
and pointed out that atmospheric effects
of the asteroid impact had been about
1000 times greater than those at the enor-
mous eruption at Krakatoa in 1883. As
there was no suitably large volcanic erupt-
ion candidate known in the geological
record they rejected the idea of volcanic
activity having been the cause of the ext-
inctions. Numerous other workers all over
the world followed up on the work of the
Alvarez group and came to similar con-
clusions. Very soon it became quite clear
tliat the iridium anomaly was a worldwide
phenomenon and there were several other
elements that showed a similar pattern.
The same year that the Alvarez group
came forward with their hypothesis an al-
ternative scenario was suggested in which
a cometary nucleus dived into the ocean,
warming up the atmosphere somewhat
and causing widespread toxicity in the
oceans when it dissolved (Hsu 1980). A
third alternative was proposed in which
the earth met with a shower of comets
(Hut et al. 1987). While many meteoric
craters and meteoric impact structures
round the world have been proposed as
candidates, they are either too small, too
far away from the major iridium sites, or
of the wrong age (cf. Grieve 1987).
Another group of American earth
scientists (Officer & Drake 1983 and 1985)
has been in the forefront for a terrestrial
school of thought. They favour causes of
terrestrial origins having led to the mass
extinctions and insist there is no need to
look for extraterrestrial causes. Analytical
work on the airborne products of the 1983
Kilauea eruption on Hawaii showed the
presence of a small quantity of iridium
(Zoller et al. 1983), thereby lending
credence to the claim that the iridium an-
omaly at the K/T boundary could have
been caused by fall-out from a gigantic
volcanic cloud that enveloped the earth.
Officer and Drake and their co-workers
reviewed published data and demonstrat-
ed that the K/T iridium-anomaly at a num-
ber of locations from Spain and Italy to
the Pacific is not as sharp temporally as is
often assumed and the iridium could have
been deposited over periods of up to at
least 100,000 years (Officer and Drake
1983 and 1985, Officer et al. 1987). It has
also been shown that while the iridium-
concentrations are significantly higher
than the crustal average but comparable
to meteorite sources the concentrations of
arsenic and antimony at the K/T boundary
14