The great oxidation event: an expert discussion on the causes, the processes, and the still unknowns. Interview by Frances Westall.

Dr. Frances Westall (FW): Let’s begin with each of you giving an overview as to the importance of biogenic processes in the rise of oxygen—oxygenic photosynthesis. Lee, would you like to begin? Dr. Lee Kump (LK): What is really interesting about the history of atmospheric oxygenation is that there is this event or interval of change from an anoxic Archean atmosphere to an oxygenated post-Archean atmosphere (Holland, 1994). When we think about the causes of the rise of oxygen and we think about the Great Oxidation Event, it is really a fundamental change in the way the Earth system functioned. It is a new state; it is the oxygenated state that is distinct from the anoxic state, and it is nonreversible. That suggests to me really only two possibilities: that it was driven by biological innovation, and that the timing of that transition is linked to that biological innovation, in other words, the origin of oxygenic photosynthesis; or that it is the result of a threshold effect for some sort of monotonic change in either oxygen supply or oxygen demand, crossing over that threshold where the potential falls below the potential supply of oxygen, and we go into that oxygenated state. These are not new ideas with me, of course, and probably the one person who defined that most clearly was Heinrich (Dick) Holland, who unfortunately recently passed away. But he set the stage for all of us in terms of our thinking about this balance between oxygen supply and demand. Presuming that the origin and spread of oxygenic photosynthesis significantly predated the apparent oxygenation of the atmosphere from the sulfur isotope record, it is about looking for either a monotonic decline in the oxygen sink or an increase in the supply of oxygen to the atmosphere (Farquhar et al., 2000). So that is what I have been focusing on. The other part of it that I have always found very interesting is that the transition seemed to be happening right around the Archean-Proterozoic boundary. At first in the earlier days when Dick Holland in particular was compiling geological evidence for the rise of oxygen, it looked like the rise of oxygen postdated the Archean-Proterozoic boundary at 2.5 billion years ago. Now it looks like that transition is very close to the Archean-Proterozoic line, so looking for this long-term driver for monotonic change and the timing seeming to get honed in on the Archean-Proterozoic transition, I have always looked for a change in the solid Earth dynamics that is linked to that, because the ArcheanProterozoic boundary was defined based on geological evidence for the stabilization of continental cratons. That is what ultimately led Mark Barley and me to look at that issue of changes in style of tectonics at the ArcheanProterozoic boundary, and ultimately focus on the style of volcanism (Kump and Barley, 2007). This was, again, the result of Holland pointing out that in modern volcanic systems, the oxygen demand from the release of fluids, gases of subaerial volcanoes was significantly less than that from the release of fluids from submarine volcanoes. We looked at the distribution in time of subaerial versus submarine volcanism. We looked in particular at a previous compilation by Prokoph and colleagues of the large igneous province emplacement through time and hypothesized that submarine volcanism dominated the Archean (Prokoph et al., 2004). In fact, that is exactly what we found. Our story that developed was that there was a significant change in the proportion of subaerial volcanoes and an increase in the proportion of subaerial volcanoes with the stabilization of the continents. The ability to support largestrata volcanoes and volcanism in the post-Archean led to a decline in the oxygen demand and thus allowed for this state transition from an anoxic Archean to an oxic post-Archean. The details of the history of atmospheric oxygen evolution are much more complicated than that, but the sulfur isotope record, the non-mass-dependent fractionation seems to indicate that the atmosphere did not revert to an anoxic state once it became oxygenated. So for me, we crossed some sort of threshold, and we are thinking about a change in the style of tectonism driven by the progressive heat loss from the planet, leading to a change in the style of plate tectonics and the growth of stabilization of continents that led to this change in volcanism.