Redox constraints on a Cenozoic imbalance in the organic carbon cycle

Abstract

Over geological timescales, variations in atmospheric O2 are typically attributed to the imbalance between the weathering of organic carbon (OC) and reduced sulfur on land, the major sink terms for atmospheric O2, and the burial of OC and reduced sulfur in marine sediments, the major source terms of O2 to the atmosphere. But the Fe cycle matters too. Using a compilation of C, Fe, S and H fluxes between the Earth's exosphere, continents, and mantle reservoirs, I demonstrate that hydrothermal weathering of the oceanic lithosphere and volcanic degassing of SO2 have acted as net sinks of O2, amounting to ca. 2.7 ± 1.1 Tmol O2/y, over the Cenozoic. Near constancy of atmospheric oxygen concentrations over the same interval of time suggests that this igneous sink is compensated by the sedimentary cycles of C, S and Fe. The net subduction and accretion of OC likely made the dominant contribution and, therefore, operated as a net source of atmospheric O2 over the last 50-60 Myr. This result implies that redox steady-state in the Cenozoic is dynamically maintained by a net input of solar (photosynthetic) energy, ∼ 0.8 to 2 EJ/y, into the lithospheric cycles of C, S and Fe.