Oxygen Oases Before and After the GOE: Insights From Metals and Models

Abstract:

The evolution of oxygenic photosynthesis fundamentally changed the structure of the marine biosphere and the chemistry of Earth’s ocean-atmosphere system. Atmospheric oxygenation, however, was decoupled from the onset of biological O₂ production—possibly lagging by as much as half a billion years—and O₂ remained low for two billion years following initial O₂ accumulation. Although uncertainties remain regarding the fate of biogenic O₂ during the Precambrian, it is becoming clear that the consequences of oxygenesis were both spatially and temporally variable. Several lines of evidence support the existence of aerobic ecosystems associated with O₂ oases within an otherwise anoxic Archean ocean; however—with notable exceptions—atmospheric O₂ remained low enough to severely curtail oxidative weathering processes on long-term average throughout the Archean. During the subsequent Great Oxidation Event (GOE) in the early Paleoproterozoic, atmospheric O₂ irreversibly increased above the sensitivity thresholds of several well-established proxies, but the level at which O₂ eventually stabilized remains unclear. Consequently, the dynamics of O₂ cycling are poorly characterized both before and after the GOE. Nevertheless, recent analytical and numerical results suggest exceptionally low O₂ levels that may have favored Archean-style O₂ oases in the mid-Proterozoic.

We used Fe speciation and trace metal records from Precambrian shales, including data from two new cores that target the 2.7 Ga Roy Hill Shale, to investigate pre- and post- GOE redox heterogeneity in Earth’s surface environments. Fe speciation supports the reconstruction of local marine redox conditions, and, in this context, trace metals can allow glimpses of redox conditions beyond the local environment, which may have throttled the supply of key redox-sensitive trace metals to the ocean. Then, using O₂ constraints derived from these inorganic proxies, we use an Earth System model to explore C, O, and nutrient cycling in the late Archean and into the mid-Proterozoic. Although our results allow profound perturbation to several biogeochemical cycles and the climate system as a result of the GOE, we find that the GOE may have had only minor significance for the long-term average O₂ content of typical surface seawater in the Proterozoic.