Abstract
Abundant geologic evidence shows that atmospheric oxygen levels were negligible until the Great Oxidation Event (GOE) at 2.4–2.1 Ga. The burial of organic matter is balanced by the release of oxygen, and if the release rate exceeds efficient oxygen sinks, atmospheric oxygen can accumulate until limited by oxidative weathering. The organic burial rate relative to the total carbon burial rate can be inferred from the carbon isotope record in sedimentary carbonates and organic matter, which provides a proxy for the oxygen source flux through time. Because there are no large secular trends in the carbon isotope record over time, it is commonly assumed that the oxygen source flux changed only modestly. Therefore, declines in oxygen sinks have been used to explain the GOE. However, the average isotopic value of carbon fluxes into the atmosphere–ocean system can evolve due to changing proportions of weathering and outgassing inputs. If so, large secular changes in organic burial would be possible despite unchanging carbon isotope values in sedimentary rocks. Here, we present an inverse analysis using a self‐consistent carbon cycle model to determine the maximum change in organic burial since ~4 Ga allowed by the carbon isotope record and other geological proxies. We find that fractional organic burial may have increased by 2–5 times since the Archean. This happens because O2‐dependent continental weathering of 13C‐depleted organics changes carbon isotope inputs to the atmosphere–ocean system. This increase in relative organic burial is consistent with an anoxic‐to‐oxic atmospheric transition around 2.4 Ga without declining oxygen sinks, although these likely contributed. Moreover, our inverse analysis suggests that the Archean absolute organic burial flux was comparable to modern, implying high organic burial efficiency and ruling out very low Archean primary productivity.
Highlights
Understanding how Earth's atmosphere became O2-rich is among the most important unanswered questions about deep time
Explanations for Earth's atmospheric oxygen accumulation can be broadly divided into increases in oxygen sources or decreases in oxygen sinks: either oxygen sources increased over Earth history or efficient oxygen sinks declined to trigger transitions between anoxic, low oxygen, and near-modern levels of atmospheric oxygen (Catling & Claire, 2005; Catling & Kasting, 2017, Ch. 10; Holland, 2002)
Self-consistent carbon cycle modeling of the mantle, crust, and surface reservoirs shows that the 13C of carbon inputs into the atmosphere and oceans need not equal mantle values over Earth history. This implies that fractional organic burial may not be straightforwardly reflected in the carbon isotope record
Summary
Understanding how Earth's atmosphere became O2-rich is among the most important unanswered questions about deep time. Daines et al (2017) further quantified this effect by incorporating O2-dependent organic burial using the oxidative weathering model of Bolton et al (2006) into a carbon–oxygen cycle model They showed that changes in organic burial may not be reflected in the carbon isotope record because of the oxygen dependence of carbon inputs leading to changes in 13Cinputs. Oxygen-dependent organic weathering inputs were incorporated into biogeochemical cycle modeling to help explain large excursions in the carbon isotope record without invoking extreme redox imbalances (Miyazaki et al, 2018) These studies establish that fractional organic burial may not track the carbon isotope record in a straightforward way due to variations in 13Cinputs. We evaluate whether the carbon isotope record is consistent with sufficiently large changes in fractional organic burial over Earth history to explain the rise of oxygen without recourse to declining O2 sinks
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