ELEVATING ATMOSPHERIC OXYGEN: RAMP, STEPS, OR ROLLER COASTER?

The evolution of atmospheric oxygen has been a subject of considerable debate since the 1960's. Universal agreement exists that the early Earth's atmosphere had vanishingly low levels of oxygen, and that today's oxygen level is 21%. The details of the path from 0 to 21% are largely unknown with any certainty, although various constraints have been brought to bear to elaborate the course of atmospheric oxygen evolution.

For most investigators, the abrupt cessation of substantial mass-independent fractionation (MIF) of sulfur isotopes during the Archean-Proterozoic transition represents a step-change in the oxygenation of the atmosphere, with levels rising above ~0.001% of the present atmospheric level. But loss of MIF provides no upper limit to atmospheric oxygen. Apparent transient suppression of the range of variability of MIF during intervals of the Archean has led others to suggest that the path to atmospheric oxygenation may have been more of a roller coaster ride than a step climb. The exciting ride apparently ended in the Proterozoic; any oscillations in the oxygen level during the post-Archean were somehow prevented from dipping below the threshold for the re-appearance of MIF in the sulfur system.

Stepwise models of atmospheric oxygen evolution call on a second rise near the end of the Precambrian, one that finally allowed for the explosion of metazoan diversity. Phanerozoic oxygen levels likely fluctuated, but the roller coaster didn't dip to Archean lows. Nevertheless, the heightened sensitivity of a biosphere adapted to abundant oxygen may have played a role in Phanerozoic mass extinction.

Numerical modeling has demonstrated that even the wind-mixed surface ocean beneath an oxygenated atmosphere can be anoxic or sulfidic if basinal waters are euxinic. Future work on oxygen proxies should be focused on the terrestrial record, given the difficulty of deconvolving changes in ocean redox state brought on by climate change from similar changes tied to variations in atmospheric oxygen levels themselves.