Extinction [The iridium-bearing layer]
Discussions by Yann Oliver
Up: Extinction [Proofs for an extraterrestrial impact]
The average iridium flux on the earth (on the oceanic basin) is roughly five to ten nanograms per square centimetre and per million years. It has its main origin in the continuous fall of micrometeorites.
The layer of iridium at the K-T limit contains order of ten nanograms per gram (with important local variations). The average in the earth crust is of 0.3 nanograms per gram. In meteorites, the iridium concentration can be as much as 500 nanograms per gram.
If this iridium were of non-extraterrestrial origin, it would have required around ten million years to accumulate (and this would also mean that the normal sedimentation of the other materials drastically slowed down). However, palaeomagnetic studies indicate with certainty that the layer was formed within less than one half million years (during magnetic reversal 39). Judging by the sediment thickness, the iridium was deposited in less than a few thousands of years.
The iridium-bearing layer is world-wide nearly exactly synchronous to the previous assignation of the K-T boundary. It has been found in more than 150 different sites.