Could a secular increase in organic burial explain the rise of oxygen? Insights from a geological carbon cycle model constrained by the carbon isotope record

The stable isotopes of carbon in marine sedimentary rock provide a window into the evolution of the Earth system. Conventionally, a relatively constant carbon isotope ratio in marine sedimentary rocks has been interpreted as implying constant organic carbon burial relative to total carbon burial. Because organic carbon burial corresponds to net oxygen production from photosynthesis, it follows that secular changes in the oxygen source flux cannot explain the dramatic rise of oxygen over Earth history. Instead, secular declines in oxygen sink fluxes are often invoked as causes for the rise of oxygen. However, constant fractional organic burial is difficult to reconcile with tentative evidence for low phosphate concentrations in the Archean ocean, which would imply lower marine productivity and—all else being equal—less organic carbon burial than today. The conventional interpretation of the carbon isotope record rests on the untested assumption that the isotopic ratio of carbon inputs into the ocean reflect mantle isotopic values throughout Earth history. In practice, differing rates of carbonate and organic weathering will allow for changes in isotopic inputs, as suggested by [1] and [2]. However, these inputs can not vary freely because large changes in isotopic inputs would induce secular trends in carbon reservoirs, which are not observed in the isotope record. We apply a geological carbon cycle model to all Earth history, tracking carbon isotopes in crustal, mantle, and ocean reservoirs. Our model is constrained by the carbon isotope record such that we can determine the extent to which large changes in organic burial are permitted. We find both constant organic burial and 3-5 fold increases in organic burial since 4.0 Ga can be reconciled with the carbon isotope record. Changes in the oxygen source flux thus need to be reconsidered as a possible contributor to Earth's oxygenation. [1] L. A. Derry, Organic carbon cycling and the lithosphere, in Treatise on Geochemistry (2nd. Ed.), H. D. Holland, K. K. Turekian, Eds. (Elsevier, Oxford, 2014), 239-249. [2] S. J. Daines, B. J. W. Mills, and T. M. Lenton, Atmospheric oxygen regulation at low Proterozoic levels by incomplete oxidative weathering of sedimentary organic carbon, Nat. Commun. 8 (2017): 14379.