Iridium abundance maxima in the Upper Cenomanian extinction interval

Two iridium abundance peaks, both 0.11 ppb (whole‐rock basis) over local background of 0.017 ppb, have been found in Middle Cretaceous marine rocks near Pueblo, Colorado. They occur just below the 92‐million‐year‐old Cenomanian‐Turonian (C‐T) stage boundary.

No other peaks were found in 45 meters of strata (∼2.5 million years of deposition) above and below the boundary interval. The broad lower peak straddles the first in a series of extinctions of benthic and nektonic macro biota which comprise the C‐T extinction event. The sharp upper peak occurs stratigraphically about 1.2 meters above the lower peak. The excess Ir might be from meteoroid impacts although no microspherules or shocked‐mineral grains have yet been found and several elements not normally associated with meteorites (Sc, Ti, and Mn) are enriched at the Ir peaks. Alternatively, several terrestrial Ir enrichment processes are suggested.


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