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.