The effect of widespread early aerobic marine ecosystems on methane cycling and the Great Oxidation

The balance of evidence suggests that oxygenic photosynthesis had evolved by 3.0-2.7 Ga, several hundred million years prior to the Great Oxidation approximate to 2.4 Ga. Previous work has shown that if oxygenic photosynthesis spread globally prior to the Great Oxidation, this could have supported widespread aerobic ecosystems in the surface ocean, without oxidising the atmosphere. Here we use a suite of models to explore the implications for carbon cycling and the Great Oxidation. We find that recycling of oxygen and carbon within early aerobic marine ecosystems would have restricted the balanced fluxes of methane and oxygen escaping from the ocean, lowering the atmospheric concentration of methane in the Great Oxidation transition and its aftermath. This in turn would have minimised any hi-stability of atmospheric oxygen, by weakening a stabilising feedback on oxygen from hydrogen escape to space. The result would have been a more reversible and probably episodic rise of oxygen at the Great Oxidation transition, consistent with existing geochemical evidence. The resulting drop in methane levels to approximate to 10 ppm is consistent with climate cooling at the time but adds to the puzzle of what kept the rest of the Proterozoic warm. A key test of the scenario of abundant methanotrophy in oxygen oases before the Great Oxidation is its predicted effects on the organic carbon isotope (delta C-13(org)) record. Our open ocean general circulation model predicts delta C-13(org) approximate to -30 to -45 parts per thousand consistent with most data from 2.65 to 2.45 Ga. However, values of delta C-13(org) approximate to -50 parts per thousand require an extreme scenario such as concentrated methanotroph production where shelf-slope upwelling of methane-rich water met oxic shelf water. (C) 2015 The Authors. Published by Elsevier B.V.