Sulfur speciation and isotope analysis of the 2.7 Ga shallow- and deep-facies black shales from Pilbara, Western Australia

It has been widely believed that Great Oxidation Event (GOE: Holland, 1994) occurred at ~2.4-2.2 Ga ago. However, some previous studies have found evidence for oxic ocean and atmosphere from earlier rock records (e.g., Hoashi et al., 2009). In order to explore if such oxic environment was local or global and if there was redox heterogeneity in a sedimentary basin before the inferred GOE, using the 2.7 Ga pyrite-bearing drillcore black shales (deep-facies WRL1 and shallow-facies RHDH2A drillcores) from Pilbara, Western Australia, we separately quantified abundance of S-bearing species (S_AVS (acid-volatile sulfide), S_py (pyrite), S_SO4 (sulfate), S_org (organic-S), and S⁰ (elemental S) and Fe-bearing species (Fe_carb, Fe_ox, and Fe_mag) by using sequential extraction methods. These samples were previously used by Brocks et al. (1999), Yamaguchi (2002), Yamaguchi et al. (2005), and Eigenbrode and Freeman (2006).

The shallow samples have high S contents and are interpreted to have deposited in relatively anoxic environment, but most of deep samples with elevated Fe contents deposited in relatively oxic environment. The DOP values and δ³⁴S_py values are relatively higher in shallow samples, suggesting active bacterial sulfate reduction in reducing environment created due to near-complete consumption of dissolved O₂by decomposition of organic matter produced by photosynthesizers living in the surface ocean.

All of these observations consistently suggest that the shallower part was anoxic and deeper part was oxic in the 2.7 Ga ocean. The surface ocean would have been oxygenated due to activity of oxygenic photosynthesis. Such redox stratification of the ocean, i.e., development of mid-depth (shallow) OMZ in an essentially oxic ocean, is typically seen in highly productive regions in the modern ocean. Modern-style oceanic redox structure could have existed as far back as 2.7 Ga ago, much earlier than the inferred GOE at ~2.4-2.2 Ga.

Brocks et al. (1999) Science 285, 1033-1036; Eigenbrode & Freeman (2006) PNAS 103, 15759-15764; Hoashi et al. (2009) Nature Geosc. 2, 301-306; Holland (1994) Early Life on Earth, Columbia Univ. Press; Yamaguchi (2002) Ph.D. dissertation, Penn State Univ.; Yamaguchi et al. (2005) Chem. Geol. 218, 135- 169.