Constraining the redox landscape of Mesoproterozoic mat grounds: A possible oxygen oasis in the ‘Boring Billion’ seafloor

Abstract

The Earth’s middle age or the ‘Boring Billion’ (∼1.8 to ∼ 0.8 Ga, billion years ago) represents one of the most enigmatic intervals in Earth’s history, characterized by the absence of significant carbonate carbon isotope excursions and the sluggish evolution of eukaryotes. It is widely accepted that the atmospheric O2 level was low (<1% PAL or < 10% PAL) and the ocean remained predominantly anoxic with the development of sulfidic (H2S rich) continental margins. It is suggested that 2–10% of seafloor euxinia was sufficient to deplete some micro-nutrients in the ocean inventory, such as Mo and Cu, which are essential for nitrogen-fixation for eukaryotes. However, recent studies report multicellular fossils and episodic/sporadic inception of oceanic oxidation in Mesoproterozoic. These findings imply the possible occurrences of oxygen oasis that provided habitable niches for eukaryote evolution. Such oxygen oasis likely developed at shallow marine seafloor covered with microbial mat, where O2 produced by benthic cyanobacteria resulted in the oxygenation of seafloor. Therefore, in order to constrain the habitability of Mesoproterozoic oxygen oasis, it is essential to reconstruct the O2 fugacity in the seafloor. In this study, we analyzed pyrite sulfur isotopes and pyrite contents of the Mesoproterozoic Wumishan Formation (∼1.4 Ga). The Wumishan Formation is composed of cyclic deposition of, in a shoaling upward sequence, the subtidal calcareous shale, massive thrombolitic dolostone, and microbial laminated dolostone. Both microbial laminated dolostone and thrombolytic dolostone precipitation involved with microbial activities, and thus might record the redox condition of putative oxygen oasis in the Mesoproterozoic oceans. We apply the One-Dimensional Diffusion-Advection-Reaction (1D-DAR) model to simulate diagenetic pyrite formation in sediments. Sedimentation rate can be well constrained by the depositional cycles of the Wumishan Formation. The modelling results indicate that more than 60 ∼ 80% of H2S that was generated in microbial sulfate reduction (MSR) was reoxidized, and that organic matter supply, both from surface water and seafloor, was limited. Thus, our study indicates that the seafloor could be substantially oxygenated in Mesoproterozoic, even when the atmospheric O2 level was extremely low. Shallow marine seafloor covered with microbial mat may function as the oxygen oasis, providing habitable niches for the evolution of eukaryotes.