Where is the Bone Spike?

Bonnie Blackwell

One of the leading theories for the extinction of "non-avian" dinosaurs 65 million years ago is that the impact of a large object from outer-space had a devastating, and ultimately deadly, affect on the environment. The leading candidate as the site of this impact is the Chicxulub impact crater on the tip of the Yucatan Peninsula.

The theory arose when a thin layer of clay found worldwide at the K/T boundary (the boundary between the last period of the Mesozoic, the Cretaceous, and the first period of the Cenozoic, the Tertiary) was found to contain high levels of Iridium. Iridium is rare on earth but common in extra-terrestrial bodies, such as asteroids.

What is not found at the boundary is a thick layer of fossils; indeed, few fossils are found within a foot or so below the boundary. A question often asked in regards to the impact theory is "If so many animals died, as a result of the impact, that many species became extinct, wouldn't we find a thick layer of fossil bone, a bone spike in the fossil record, from the time they died?"

Bonnie Blackwell, of the Department of Geology at Queens College, City University of New York, gives one possible explanation for the lack of such a bone spike. -- ed.

If the impact occurred and caused the acid rain suggested by the models it is not hard to see why a bone spike would be the last thing that you would expect to find. Consider for a moment the boundary clay. What ever produced that had to erode many rocks to produce that clay everywhere we see the boundary. That sort of erosion implies very large quantities of very acidic water falling out of the sky. Now consider the average bone. It will dissolve within an hour if put into 6M HCl [Hydrochloric acid]. Put it under water in water that has a pH < 6 [acidic] and it will dissolve within 1 week to 1 month depending on the size of the bone and its bone type (relative percentage of compact to cancellous bone is important here). The acid rain falling under the K/T impact model scenario is more than sufficient to destroy any bone on the surface within a short period. Moreover, the water would have also penetrated the upper soil zones. There, combined with bacteria (which can survive very acidic conditions!), [it] would have produced even more acidic water (just as occurs today in many soil zones) because of the dead vegetation and animals. That would have been capable of dissolving the calcium phosphate from any bones in the active vadose zone. Even bones in the upper phreatic zone might have been affected because the upper groundwater table likely would have become acidified also. This sort of geochemical scenario can also explain why few bones (if any??) have been found just below the K/T boundary (correct me there if I am wrong Tom/George/....) The only bones not likely to have been severely affected by such events would be those that had already been remineralized to silica, which is a process that normally takes a long time, and would not likely have happened to bones less than 100 ka [100,000 years old] to maybe 10 Ma [10,000,000 years old] (at 65 Ma [65,000,000 years ago, the time of the K/T boundary]).