Dynamic model constraints on oxygen-17 depletion in atmospheric O2 after a snowball Earth

A large perturbation in atmospheric CO2 and O-2 or bioproductivity will result in a drastic pulse of O-17 change in atmospheric O-2, as seen in the Marinoan Oxygen-17 Depletion (MOSD) event in the immediate aftermath of a global deglaciation 635 Mya. The exact nature of the perturbation, however, is debated. Here we constructed a coupled, four-box, and quick-response biosphere-atmosphere model to examine both the steady state and dynamics of the MOSD event. Our model shows that the ultra-high CO2 concentrations proposed by the snowball' Earth hypothesis produce a typical MOSD duration of less than 10(6) y and a magnitude of O-17 depletion reaching approximately -35 parts per thousand. Both numbers are in remarkable agreement with geological constraints from South China and Svalbard. Moderate CO2 and low O-2 concentration (e. g., 3,200 parts per million by volume and 0.01 bar, respectively) could produce distinct sulfate O-17 depletion only if postglacial marine bioproductivity was impossibly low. Our dynamic model also suggests that a snowball in which the ocean is isolated from the atmosphere by a continuous ice cover may be distinguished from one in which cracks in the ice permit ocean-atmosphere exchange only if partial pressure of atmospheric O-2 is larger than 0.02 bar during the snowball period and records of weathering-derived sulfate are available for the very first few tens of thousands of years after the onset of the meltdown. In any case, a snowball Earth is a precondition for the observed MOSD event.