Thursday, March 30, 2006

Extinctions update More evidence chicxulub was too early

A new study of melted rock ejected far from the Yucatan's Chicxulub impact crater bolsters the idea that the famed impact was too early to have caused the mass extinction that killed the dinosaurs 65 million years ago.

A careful geochemical fingerprinting of glass spherules found in multiple layers of sediments from northeast Mexico, Texas, Guatemala, Belize, and Haiti all point back to Chicxulub as their source. But the analysis places the impact at about 300,000 years before the infamous extinctions marking the boundary between the Cretaceous and Tertiary periods, a.k.a. the K-T boundary.

Using an array of electron microscopy techniques, Markus Harting of the University of Utrecht in the Netherlands has found that chemical compositions of the spherules all match what would be expected of rocks melted at the Chicxulub impact. The spherules are now found in several layers because after they originally hit the ground, they were "reworked" by erosion to create later layers of sediments, he said. It's this reworking long after the impact that has misplaced some of the spherules into sediments that, based on the fossils in the same sediments, are misleadingly close to the K-T boundary.

Upshot: Chicxulub was too early to have caused the K-T extinctions and also the irridium layer seen worldwide. The article suggests that the irridium layer may have been caused by numerous smaller particles depositing their irridium in the atmosphere without actually colliding with the earth's surface. Still missing is a cause of the extinctions. But see other posts on this elsewhere.

Also compare to this: Climate blamed for mass extictions