Oxidative sulfide dissolution on the early Earth
Résumé
Recent evidence suggests that the biological production of oxygen in Earth's surface oceans may have preceded the initial accumulation of large amounts of oxygen in the atmosphere by 100 million years or more. However, the potential effects of early oxygen production on surface ocean chemistry have remained little explored, and questions persist regarding the locus of oxidation of crustal material (i.e., subaerial and/or submarine settings). Here, we revisit the notion of spatially restricted 'oxygen oases' in the Archean surface ocean by employing a simple steady-state box model of the surface ocean in a coastal upwelling system. Using pyrite as an example, we then explore the possibility that oxygenic photosynthesis in such a system could support the widespread oxidation of crustal sulfide minerals without concomitant accumulation of oxygen in the atmosphere. We find that it is possible to establish strong air-sea gas exchange disequilibrium with respect to O2. However, in marine settings there is an apparent timescale mismatch between the kinetics of oxidative dissolution and the rate at which sulfide minerals delivered physically to shallow marine sediments will be buried below the zone of oxygen penetration. Estimated dissolution timescales compare somewhat more favorably with typical timescales of soil development and physical weathering/transport in the subaerial realm, despite the much lower dissolved oxygen concentrations inferred at gas exchange equilibrium with atmospheric pO2 relative to those attainable in productive regions of the surface ocean. However, broad scale subaerial weathering of sulfides would likely have required either local O2 production at the site of weathering or transient increase in atmospheric pO2 above the most plausible levels characteristic of a pervasively reducing atmosphere.