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Science 26 October 1984:
Vol. 226. no. 4673, pp. 437 - 439
DOI: 10.1126/science.226.4673.437
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Articles

Iridium Anomaly in the Upper Devonian of the Canning Basin, Western Australia

PHILLIP E. PLAYFORD 1, DIGBY J. MCLAREN 2, CHARLES J. ORTH 3, JAMES S. GILMORE 3, and WAYNE D. GOODFELLOW 4

1 Geological Survey of Western Australia, Perth 6000
2 University of Ottawa, Ottawa, Ontario, Canada K1N 6N5
3 Los Alamos National Laboratory, Los Alamos, New Mexico 87545
4 Geological Survey of Canada, Ottawa, Ontario, Canada K1A 0E8

A moderate iridium anomaly, about 20 times the local background, has been found in Upper Devonian rocks in the Canning Basin. It occurs at or near the Frasnian-Famennian boundary, which is known to be associated with a major massextinction event of global extent. The anomaly occurs in an extremely condensed limestone sequence laid down under quiet deepwater conditions. Its occurrence suggests a causal link with some form of meteoroid impact. Moreover, carbon isotope data indicate that a large reduction in biomass could have occurred at this level. However, the anomaly coincides with a stromatolite bed containing the fossil cyanobacterium Frutexites; iridium, platinum, iron, manganese, cobalt, arsenic, antimony, and cerium are preferentially concentrated in filaments of this organism, with concentrations ranging from two to five times that of the matrix. It is possible that Frutexites extracted these elements directly from seawater, without the need for their derivation from an extraterrestrial source.

Submitted on July 5, 1984
Accepted on August 8, 1984


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