The role of the solid earth in regulating atmospheric O₂ levels

Abstract

Solid-earth processes act as both sources and sinks for atmospheric O₂. They act as sinks by introducing reduced minerals and gases to the earth's surface that can remove O₂ from the atmosphere and ocean. They act as sources by exporting organic carbon and sedimentary pyrite to the mantle via subduction. Here we examine the relative sizes of igneous source and sinks of O₂ for the modern earth to determine their magnitudes and if they are in balance today. We find that igneous sinks for O₂ remove 1.83×10¹² mol O₂/yr (±0.43, 1σ) while subduction indirectly releases 1.56×10¹² mol/O₂ yr (±0.33, 1σ). This indicates that today igneous O₂ sinks are balanced by solid earth sources. We propose this balance is achieved by negative feedbacks associated with either low-temperature hydrothermal sinks for O₂, which are sensitive to deep-ocean O₂ concentrations, or the amount of organic carbon and pyrite buried in sediments and subducted, which are sensitive to dissolved O₂ concentrations. We also explore how igneous sinks for O₂ may have varied in the Neoproterozoic when atmospheric O₂ concentrations are thought to have been lower and the deep ocean anoxic. We find that despite these changes, the igneous O₂ sink was essentially the same as today: 1.78×10¹² (±0.43, 1σ) mol O₂/yr. We explore how this sink would change as the deep ocean accumulated sulfate, became oxygenated, and began oxidizing oceanic crust such that there was an increase in the subduction flux of oxidants to the subarc mantle. We propose that significant changes to the O₂ cycle, both in terms of positive and negative feedbacks could occur during these transitions. For example, accumulation of sulfate in the deep ocean would increase the oxidation state of high-temperature hydrothermal fluids, decreasing the size of this O₂ sink and thus promoting an increase in atmospheric O₂. In contrast, the oxygenation of the deep ocean would have allowed hydrothermally derived H₂S to react with and consume O₂ instead of being titrated out via reactions with dissolved Fe²⁺. Additionally, deep-ocean oxygenation would have initiated the oxidation of oceanic crust at low temperatures, creating new sinks for O₂. Finally, the oxidation of the subarc mantle via subduction of newly oxidized sediments and altered oceanic crust would have increased the oxygen fugacity of arc volcanic gases, decreasing their overall demand for O₂, allowing yet more O₂ to accumulate in the atmosphere. We place these changes into a conceptual framework and discuss their potential impact on the history of atmospheric and marine O₂ concentrations from the Neoproterozoic to late Paleozoic.