MODELING THE REDOX CHEMISTRY OF MID-PROTEROZOIC ATMOSPHERE-OCEAN SYSTEM

During the Mesoproterozoic Eon (~1.6 to ~1.0 Ga), oceanic interior below euphotic layer had been kept in pervasive anoxic (ferruginous/sulfidic) condition. Such reducing condition is considered as a corollary of a weakly oxidized atmosphere at that time. However, the atmospheric oxygen level (pO₂) in the Proterozoic has not been well constrained, and we need quantitative study to understand how and why the redox state of atmosphere-ocean system during Earth’s middle age had been stabilized. Here we present the theoretical constraints on Proterozoic oceanic redox state and atmospheric pO₂, as well as an implication to their stabilization mechanism.

We developed a new, vertically one-dimensional marine biogeochemical cycle model in which the several biogeochemical dynamics of C, H, O, N, S, P, and Fe are included, and investigated the responses of oceanic redox state to the changes in pO₂. The sensitivity experiments demonstrate that pervasive anoxia and euxinia (i.e., “Canfield ocean”) would appear when pO₂ < 0.14 atm and < 0.11 atm, respectively. An expansion of sulfidic waters in the oceanic interior significantly promotes the sulfate reduction, enhancing the precipitation of pyrite into marine sediments. In addition, low pO₂ condition results in the low riverine input of sulfate to the ocean, giving rise to a low sulfate condition (SO₄ < 5 mM) when pO₂ < 0.11 atm. We also found that, under pO₂ < ~0.02 atm, a scarcity of sulfate results in the anoxic but non-sulfidic (namely low O₂ and H₂S) condition, indicating the ferruginous conditions.

Sensitivity experiments with respect to other factors affecting long-term oceanic redox state (e.g, riverine nutrient input, sea-level stand, settling rate of marine snow in water column) indicate that, although the critical value of pO₂ for changes in oceanic redox depends significantly on those environmental factors, the essential biogeochemical consequences are not changed. These quantitative results would provide insight into further understanding of the Earth’s redox history and its stabilization mechanism(s) from a perspective of the biogeochemical dynamics.