OXYGEN, CARBON DIOXIDE, AND MASS EXTINCTIONS

The newest generation of models computing past oxygen and carbon dioxide levels are providing new views of the history of life on Earth. We have used latest generation findings from GEOCARB and GEOCARBSULF and compared these to times of thirteen major and minor mass extinctions (listed in Hallam and Wignall, 1997) to search for possible generalities that may provide new insight into mass extinction processes and causes. One result is that the Permian-Triassic and Triassic-Jurassic, as well as several of the lesser extinctions (Pliensbachian-Toarcian, Cenomanian-Turonian, and Paleocene- Eocene) were associated with either rising or high CO₂, dropping O₂, or both. New paleontological data, as well as various lines of isotopic data (including new delta 34S curves from the Triassic-Jurassic) from these mass extinctions, as well as new Givetian-Frasnian (Devonian) data recently retrieved from our field work in the Canning Basin of Australia indicate that these events were paleontologically protracted and composed of multiple perturbations to the carbon and in some cases the S cycle. Here we review current evidence linking rapid global warming and associated anoxia in the past with both paleontological and molecular fossil evidence to arrive at a new model for mass extinctions that greatly differs from the understanding of an impact-associated mass extinction, such as that at the end of the Cretaceous. Each of the extinction events listed above seems to have been a time when microbial populations resurged to Precambrian-like conditions, and remained so during temporally varying, but generally protracted post-extinction recovery intervals. Thus, the mass extinctions can be used as windows into a far deeper past; they are short intervals that can be used to better understand the time before animals.