For approximately the first 2 billion years of Earth history, atmospheric oxygen levels were extremely low. It wasn't until at least half a billion years after the evolution of oxygenic photosynthesis, perhaps as early as 3 billion years ago, that oxygen rose to appreciable levels during the Great Oxidation event. Shortly after, marine carbonates experienced a large positive spike in carbon isotope ratios known as the Lomagundi event. The mechanisms responsible for the Great Oxidation and Lomagundi events remain debated. Using a carbon-oxygen box model which tracks surface and interior C fluxes and reservoirs while also tracking C isotopes and atmospheric oxygen levels we demonstrate that about 2.5 billion years ago a tectonic transition resulting in increased volcanic CO 2 emissions could have led to increased deposition of both carbonates and organic carbon via enhanced weathering and nutrient delivery to oceans. Increased burial of carbonates and organic carbon would have allowed accumulation of atmospheric oxygen while also increasing delivery of carbon to subduction zones. Coupled with preferential release of Users may view, print, copy, and download text and data-mine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use:
doi: 10.7185/geochemlet.1823The redox state of mantle lithologies, based on xenoliths from continental lithospheric mantle, has been shown to decrease with depth and reach oxygen fugacities ( fO 2 ) at which graphite/diamond will be the stable form of carbon at pressures greater than about 3-4 GPa (e.g., Frost and McCammon, 2008). On the other hand, the depth-fO 2 profile of the convecting mantle remains poorly known. We compare the CO 2 -Ba and CO 2 -Nb systematics of natural oceanic basalts to the CO 2 -trace element concentrations that can be generated via contributions from depleted peridotite partial melts and graphite-saturated partial melts of subducted lithologies. Results suggest that to produce the CO 2 enrichments relative to the depleted end member observed in natural oceanic basalts, subducted lithologies cannot be graphite-saturated at the onset of melting or must undergo oxidative transformation below the respective volatile-free solidi. Therefore, the oxygen fugacity profile of the continental lithospheric mantle may not be applicable to the deep convecting upper mantle, with the convecting upper mantle to at least 150 km depth being more oxidised than the carbonate vs. graphite/diamond buffer.
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