Questioning Myths of the Middle Proterozoic Time

English Abstract:

The middle age of the Earth, the so-called ‘Boring Billion’, attracted growing interest in the last decade. It is generally accepted that C isotope variations in seawater were muted in the aftermath of the ~2.22-2.1 Ga Lomagundi C isotope excursion until ~1.3 Ga. Seawater Sr composition, a proxy for hydrothermal vs. terrestrial fluxes, seems to be also invariant during that time, while it subsequently sharply rose starting at ~800 Ma. Redox history of the atmosphere and oceans in the Middle Proterozoic seems uneventful; it is generally assumed that deep oceans were anoxic and Fe-rich (ferruginous) and atmospheric oxygen was low until the late Neoproterozoic. The exact timing and trend of the Neoproterozoic oxygenation event are still uncertain, with views ranging from those favoring the Ediacaran to those arguing for pre-Sturtian oxygenation recently advocated. These patterns were broadly related to sluggish tectonics, with possible shutdown or slowdown of plate tectonics limiting nutrient fluxes and biological productivity.

We will present results of our ongoing work challenging some of these interpretations and pointing towards a more dynamic history of seawater redox and its chemical composition in the Middle Proterozoic. Specifically, our data point to a short-lived carbon isotope excursion with a large magnitude at ~2.03 Ga. Deep-oceans were likely in a suboxic state during the ‘Boring Billion’, e.g., capable to partially oxidize Fe and, possibly, Mn. Atmospheric oxygen started to rise before 1.3-1.4 Ga, reaching the peak before the ~810 Ma Bitter Springs Excursion and was highly variable in its aftermath in association with climatic changes and changes in biogeochemical C cycle. Atmospheric oxygen likely rose high and fell back several times in the Proterozoic before the emergence of Metazoans in the late Neoproterozoic.