The evolution of the oceanic redox state before the Great Oxidation Event traced by stable tungsten isotope analyses in iron formations

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Free oxygen represents an essential basis for the evolution of complex life forms on the modern habitable Earth. However, in earliest Earth’s history oxygen concentrations were more than 100.000 times lower compared to present levels. The essential increase in environmental oxygen concentrations occurred during several major oxygenation steps and was tightly linked to the evolution of simple life forms and primary production by photosynthetic bacteria. We plan to trace earliest increases in oxygen concentrations by stable tungsten isotope measurements in iron formations deposited between 3.2 to 2.4 billion years ago. The deposition of these iron formations requires the oxidation of dissolved Fe2+ and the subsequent formation of poorly soluble Fe3+ hydroxides. Thereby, isotopically light tungsten is preferentially adsorbed onto such Fe3+ hydroxides leaving behind isotopically heavy seawater. In times of globally increased iron formation deposition the global tungsten isotopic composition of seawater and sediments will shift towards heavier values due to the homogeneous distribution of tungsten in the early Earth’s oceans. Thus, tungsten stable isotopes might be the first qualitative tracer for the global extend of iron formation deposition at a specific time. In comparison to other geochemical proxies such as Mo isotopes, W isotope variation is already expected as a result of slight increases in the oceanic redox state. Therefore, W isotope measurements allow for a more distinct investigation of earliest and only marginal changes in the marine redox state. Moreover, the deposition of iron formations in oxygen-rich shallow marine water habitats is closely related to biologic activity (i.e. photosynthetic cyanobacteria). Accordingly, tungsten isotopes represent a novel geochemical approach, which can also potentially trace the earliest increases in oxygen production and the very beginning of photosynthetic activity, an essential process for the evolution of complex life forms.