Asteroid and comet impacts seem to be obvious culprits in mass extinctions on Earth. The heavily cratered Moon reminds us how severe earlier bombardments have been, and it’s an easy segue to note that 23 extinction events are now thought to have occurred since the beginnings of life on our planet. In the past 540 million years (the period during which abundant animal life has existed), we can identify five mass extinctions, with huge losses in particular to marine plants and animals.
The Chicxulub crater in the Yucatan is a striking piece of evidence for this scenario, but massive volcanic activity may well have played a role, and perhaps a major one. And what of the other extinctions? A new theory published in Nature seems to put a damper on the easy correlation of extinctions with impacts. Indeed, Shanan Peters (University of Wisconsin-Madison) argues that the largest factor may have been changes in ocean environments related to sea level. Says Rich Lane (National Science Foundation): “Impacts, for the most part, aren’t associated with most extinctions. There have also been studies of volcanism, and some eruptions correspond to extinction, but many do not.”
What Peters argues is that the expansion and contraction of the world’s oceans, caused by the shifting of tectonic plates and changes in climate, caused massive marine extinctions as sea levels declined. A case in point is the shallow sea that covered much of North America during the era of the dinosaurs. As it drained, animals like mosasaurs and giant sharks went extinct while life on the marine shelves changed irrevocably. Peters calls sea level changes a ‘forcing mechanism,’ one that correlates with many — not all — mass extinction events. “These results,” says the scientist, “argue for a substantial fraction of change in extinction rates being controlled by just one environmental parameter.”
All of which adds a cautionary note as we discuss the still vital need to catalog and perhaps one day intercept Earth-crossing objects. The danger they pose is real and one that should be a driver for new space technologies in support of a defensive mission we should hope we never have to fly. But in presenting that case, we should also be aware that extinction events have a varied and still unfolding set of causes, one in which impacts from the skies may play a smaller role than we had previously suspected. Good science demands that we get the facts right as we work to place impact events in a sound historical context.
The paper is Peters, “Environmental determinants of extinction selectivity in the fossil record,” published online in Nature (15 June 2008). Abstract available.