The Dept. of Earth and Planetary Sciences is hosting the Eldridge Moores Distinguished Visiting Scholar, Professor Lee Kump of Penn State University who will be delivering the Distinguished Lecture “How did the atmosphere become oxygen-rich?” at 4:10 pm June 1st in 55 Roessler Hall.
Please see the attached flyer for details -an abstract follows as well.
How did the atmosphere become oxygen-rich?
Two things are certain about the history of atmospheric oxygen: when Earth first formed, there was none, and today we have 21%. To piece together the rest one uses geological, isotopic and biological proxies and tolerances. Most scientists now agree that the Archean (prior to 2.5 billion years ago) the atmospheric was essentially anoxic except, perhaps, for geologically brief intervals of oxygenation. Then, with the transition from the Archean to the Proterozoic Eon, the atmosphere passed through a low threshold set by the appearance of mass-independent sulfur isotope fractionation of some 1/100,000th of the modern level. How high it rose is unclear, but the proxies suggest a Proterozoic atmosphere of 0.1 to 1% of modern oxygen levels and a second rise into the Phanerozoic to near modern levels.
This talk will focus neither on the timing of these stepwise increases nor on an assessment of the constraints on oxygen level. Rather it will explore the long-term drivers and feedbacks that together may explain why atmospheric oxygen levels followed this trajectory. The first topic is the initial rise (the Great Oxidation Event) at 2.4 Ga, an enigma because of indications that oxygenic photosynthesis was invented and exploited globally long before then. We propose that a shift in the dominant style of volcanism from submarine in the Archean and subaerial in the post-Archean, tied to the stabilization of the continental cratons, can account for this first rise in atmospheric oxygen.
The second rise, in the Neoproterozoic, is even more challenging to explain. We explore the hypothesis that it was tiedl to the “greening” of the land surface, i.e., the establishment of biotic soils with fungi that not only accelerated weathering whose metabolic wastes, together with their microbial collaborators, led to oxygen depletion in soils. To restore the balance between O2 production during organic carbon burial and O2 consumption during weathering, the O2 level had to rise.
Neither of these hypotheses has exceptionally strong support, so the take-home message is that the stepwise oxygenation of the atmosphere had to be the result of fundamental and permanent changes in the exogenic cycle; it can’t be the result of events (such as organic carbon burial events).