Náttúrufræðingurinn - 1991, Blaðsíða 21

are significantly higher than both the
crustal averages and meteoritic sources.
This led to a proposal of a third source,
the mantle. Officer and Drake demon-
strated that the As/Ir and Sb/Ir ratios in
the K/T boundary rocks are three orders
of magnitude higher than those in meteor-
ites but quite comparable to those found
in airborne particles emitted by the Kilau-
ea eruption.
The Deccan llood basalts in western
India have been suggested as the byp-
roducts of the gigantic volcanic outbursts
from which the iridium fall-out originated
and geochronological work bears this out,
showing the Deccan Traps to be exactly 65
± 1 Ma (Baksi 1987). Calculations have
shown that while the greatest part of the
volcanic activity will have been quiet and
effusive a small percentage of explosive
activity will have sufficed to account for
the iridium fall-out worldwide.
The extraterrestrial school of thought
has gained momentum and fresh support
with the recent discovery of tektitic glass
pellets in marine sediments at the K/T
boundary in Haiti (Haraldur Sigurdsson et
al. 1991). This discovery suggests more
strongly than before that there may ind-
eed have been a cosmic impact at the time
of the K/T boundary. Meteoritic impact
structures in the Caribbean region (cf.
Hildebrand and Boynton 1990, Smit 1991)
lend further support to the hypothesis in-
volving the cosmic impact. Much work
must be done, however, before definite
conclusions can be drawn either way.
15