SNOWBALLS, AIR-SEA GAS EXCHANGE, AND THE RISE OF ATMOSPHERIC OXYGEN

A canonical view of the evolution of atmospheric oxygen describes three plateaus in oxygen partial pressure: extremely low in the Archean, higher - perhaps 1% of modern values - throughout most of the Proterozoic, and then roughly modern values from the late Neoproterozoic to the present. It has also been noted that both the rises in atmospheric oxygen are broadly associated with the suggestion of global glaciation - or Snowball Earths - although the exact sequence of events is still debated. Previous work (Laakso and Schrag, 2013) proposed a mechanism to stabilize atmospheric oxygen at multiple equilibrium states, consistent with the geologic evidence. The hypothesis involves an "iron trap" that sequesters phosphorous, reducing primary productivity and organic burial and sustaining oxygen at Proterozoic values. The iron trap could be terminated if there were an abrupt rise in atmospheric oxygen, leading to more modern levels. I present here a new hypothesis for what may have created this jump in atmospheric oxygen, and discuss how it may apply to glaciations in the Neoproterozoic. The critical feature of the hypothesis is that extensive ice cover over most (>99%) of the ocean surface stifles air-sea gas exchange, disabling the strong negative feedback of organic matter burial, which would otherwise prevent any large change in atmospheric oxygen. Snowball conditions allow for accumulation of oxygen in the atmosphere, consistent with the association of banded iron deposits and barite layers in the glacial terminations. If this hypothesis is correct, it implies that the glaciations were critical to the rise of atmospheric oxygen in the Neoproterozoic. The temporal association of glaciations in the Paleoproterozoic with the first rise in atmospheric oxygen is intriguing but remains an enigma.