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. Author manuscript; available in PMC: 2019 Jul 1.
Published in final edited form as: Nat Geosci. 2019;12:186–191. doi: 10.1038/s41561-019-0309-7

Figure 1. Illustration of a possible well-oxygenated marine margin before the GOE.

Figure 1.

Evidence exists for sufficient O2 accumulation in an ancient water column between 2.6 and 2.5 Ga to weakly oxygenate underlying sediments3 (“suboxic”; Panel A). However, O2 penetration into these sediments was not sufficient to promote Mn oxide burial11,12. If settings capable of burying Mn oxides were present in ancient oceans (“oxic”; Panel B) over a large seafloor area, then seawater Tl and Mo isotope compositions would have decreased and increased, respectively. The Mt. McRae Shale was deposited under locally euxinic conditions37 and should therefore have captured these changes in seawater isotope signatures18,28. Sedimentary redox structure is modified from previous work50.