Reconfiguring oxygenation at ∼1.4 Ga: New constraints as informed by the ancient oceanic sulfur cycle

Abstract

The extent of atmospheric and oceanic oxygenation during the Mesoproterozoic remains an area of active debate. Here, we report major and trace elements, organic sulfur (OS) speciation, sulfur isotopes in multiple phases [pyrite (py), OS, and carbonate-associated sulfate (CAS)], and a numerical model of sulfate concentrations from one of the Mesoproterozoic's best-preserved geochemical archives, the Xiamaling Formation (ca. 1.4 Ga) in the Yanliao Basin. These data reveal a previously unrecognized ocean-atmosphere oxygenation process. Starting with Unit 3, intense sulfurization increased organic carbon burial and thus oxygen release, resulting in a deep-water oxidation event within the Yanliao Basin that could be regarded as a transient restricted oxygen oasis based on low pyrite content, elevated kerogen S:C ratios, higher δ34Spy than δ34SOS (Δδ34SOS-py < 0), and a low calculated sulfate concentration of ∼0.07–0.14 mM. During Unit 2, the basin was reconnected to the open ocean with the deposition of organic-rich shale across northern China and northern Australia, accompanied by a shift in Δδ34SOS-py to 5.0‰ and a calculated seawater sulfate concentration of ∼0.1–1.14 mM. Such large-scale burial of organic matter released oxygen into the atmosphere, accompanied by significant sulfate delivery to the basin and euxinic conditions characterized by Mo concentrations and 34S-depleted pyrite at the topmost of Unit 2. A subsequent globally synchronized atmospheric and deep ocean oxygenation event may have occurred during Unit 1. This event was recorded by a progressive increase in Δδ34SOS-py (up to +9.6‰), increased oxidized OS species, and positive Ce anomalies (attributed to the reductive dissolution of the Ce-enriched Mn oxides), all of which were caused by increased sulfate concentrations (calculated to be ∼1.15–2.31 mM) and a spatial descent of the redox interface. The combination of these lines of evidence illustrates the sensitivity of the oceanic sulfur cycle to the ocean-atmosphere system and provides novel constraints on oxygenation at ∼1.4 Ga.