WHY DID THE RISE OF OXYGEN PRECEDE ITS "CAUSE"?

A remarkable feature of the geochemical history of the Paleoproterozoic is the shift toward markedly positive carbon isotope compositions of marine carbonates. If globally representative, this excursion, which spanned a 200 million year interval from ~2200-2000 Ma, is the most extreme, sustained perturbation of the carbon cycle in Earth history.

The conventional explanation of the excursion has been that it marks a period of high rates of marine organic carbon burial and oxygen production, and this interpretation was consistent with other geologic indicators for the "rise" of atmospheric oxygen. However, improved age constraints, and the more definitive atmospheric oxygen proxy provided by mass-independent fractionation of sulfur isotopes that occurs only in an anoxic atmosphere, clearly show that the rise of oxygen preceded its apparent cause by at least 100 million years.

The timing of the rise of oxygen has previously been argued to be linked to a stepwise increase in the oxidation state of the upper mantle volcanic source region during the widespread emplacement of igneous provinces at 2450 Ma. The subsequent positive carbon isotope excursion had little to do with this event, instead being the result of low rates of silicate weathering of a Paleoproterozoic supercontinent, allowing proportionally higher rates of organic matter burial (enhanced by efficient P recycling in an anoxic ocean). Oxygen production rates during this interval may not have been anomalously large.