The effect of widespread early aerobic marine ecosystems on methane cycling

6 The balance of evidence suggests that oxygenic photosynthesis had evolved by 3.0–2.7 Ga, several 7 hundred million years prior to the Great Oxidation ≈2.4 Ga. Previous work has shown that if oxygenic 8 photosynthesis spread globally prior to the Great Oxidation, this could have supported widespread 9 aerobic ecosystems in the surface ocean, without oxidizing the atmosphere. Here we use a suite of 10 models to explore the implications for carbon cycling and the Great Oxidation. We find that recycling 11 of oxygen and carbon within early aerobic marine ecosystems would have restricted the balanced 12 fluxes of methane and oxygen escaping from the ocean, lowering the atmospheric concentration of 13 methane in the Great Oxidation transition and its aftermath. This in turn would have minimised any 14 bi-stability of atmospheric oxygen, by weakening a stabilising feedback on oxygen from hydrogen 15 escape to space. The result would have been a more reversible and probably episodic rise of oxygen 16 at the Great Oxidation transition, consistent with existing geochemical evidence. The resulting drop 17 in methane levels to ≈10 ppm is consistent with climate cooling at the time but adds to the puzzle of 18 what kept the rest of the Proterozoic warm. A key test of the scenario of abundant methanotrophy 19 in oxygen oases before the Great Oxidation is its predicted effects on the organic carbon isotope 20 (Corg) record. Our open ocean general circulation model predicts  Corg ≈ -30 to -45 ‰ consistent 21 with most data from 2.65–2.45 Ga. However, values of Corg ≈ -50 ‰ require an extreme scenario 22 such as concentrated methanotroph production where shelf-slope upwelling of methane-rich water 23 met oxic shelf water. 24