Náttúrufræðingurinn

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Náttúrufræðingurinn - 1991, Side 21

Náttúrufræðingurinn - 1991, Side 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

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