THE EFFECT OF LAND PLANT EVOLUTION ON PALEOZOIC ATMOSPHERIC O2

Modeling of the biogeochemical carbon cycle supports the idea that the rise and evolution of large vascular land plants, especially trees, during the mid-to-late Paleozoic had a profound effect on atmospheric O2. Increased organic burial due to the development of lignin, a new substance that is resistant to biodegradation, resulted in the increased burial of organic matter globally and the increased production of O2. This led to a Permo-Carboniferous maximum in atmospheric O2 at levels possibly as high as 30%. Experiments with modern organisms, along with the fossil record, indicate that this elevated O2 likely contributed to an increase in the size of organisms, such as insects and amphibians, which breathe via various diffusive processes.

During an approximately 20 million year period across the Permian-Triassic boundary, a decline in land plant production brought about decreases in global organic burial, global O2 production and CO2 consumption, resulting in a drop in O2 and correspondingly large rise in CO2. The O2 drop and biologically induced CO2 rise must have been contributing factors to the Permo-Triassic extinction. The cause of the decrease in land plant production is not clear but could have been due, at least partly, to an input of hydrogen sulfide to the atmosphere from an highly anoxic ocean. If true, this would have led to positive feedback via (1) prolonged plant kills, decreased organic burial and O2 production, drop in atmospheric and oceanic O2, increased oceanic sulfate reduction and H2S release, more plant kills and/or (2) prolonged plant kills, decreased organic burial and CO2 consumption, CO2 rise and global warming, decreased oceanic circulation and decreased solubility of O2 in seawater, drop in oceanic O2, increased sulfate reduction and H2S release, more plant kills.