Physics
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.u24c..02g&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #U24C-02
Physics
1028 Composition Of Meteorites (3662, 6240), 1115 Radioisotope Geochronology, 3344 Paleoclimatology (0473, 4900), 4901 Abrupt/Rapid Climate Change (1605), 9617 Devonian
Scientific paper
Two sections across the Frasnian-Famennian boundary were analyzed for Re and Os concentrations and 187Os/188Os ratios to evaluate evidence for a meteoritic input coincident with this boundary and its associated mass extinction. These sections are from a siltstone and shale sequence at Irish Gulf in New York, US and a calcareous shale and ferromanganese oxide sequence at La Serre in France. The Irish Gulf section, with an initial 187Os/188Os of ~0.49, does not show the characteristic meteoritic Os imprint with a 187Os/188Os value of about 0.13. Both Re and Os are retained in this section, as indicated by the construction of an isochron with an age of 388 ±41 Ma, consistent with independently determined ages for the Frasnian-Famennian boundary. Although the La Serre section, with Os concentrations as high as 33 ppb and Re concentrations ranging from 1.4 to 7.4 ppb, might be expected to show excellent evidence for a meteoritic contribution, the highly radiogenic isotopic composition (187Os/188Os ranges from 2.42-3.61) instead suggests recent massive Re loss or addition of radiogenic Os. This open system behavior prevents the reconstruction of an initial 187Os/188Os value for the boundary at La Serre. Assuming reasonable Re concentrations prior to loss, however, the Os isotopic value is inconsistent with a large meteoritic component. In addition, this study reinforces the need for Os isotopic evidence, not only enriched PGE concentrations, as substantiation for a meteoritic impact.
Gordon Gwyneth Williams
Over J.
Rockman M.
Turekian Karl K.
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