Common lead dating
Harris’s paper is rather disjointed and I can see no better way to address its fallacies than by categorising my comments into those about the quality of the essay and the errors of fact and reasoning it contains.In order to put the rest of the argument in context, I include here a very brief review of the background to Harris’s claims.
There is, world-wide, an ash layer at the boundary which has been independently dated by several different radiometric techniques to approximately 65 million years (3).However, the cause of this mass extinction was keenly sought but not known until Alvarez et al suggested that it was caused by the impact of a huge asteroid (4) (5) .Subsequent work has identified the Chicxulub crater as being the site of the bolide event which most closely corresponds in time to the K-T boundary (6) .Zircon is an extremely hard and stable crystal, which incorporates uranium on crystallisation but little lead. Over time, radiogenic lead accumulates in the crystal providing a measure of age.
A great quantity of further work has confirmed the Chicxulub crater as the relevant site for the K-T extinction.Part of that further work is what Harris takes exception to.