Mercury abundance and isotopic composition indicate subaerial volcanism prior to the end-Archean whiff of oxygen

Earth's early atmosphere witnessed multiple transient episodes of oxygenation before the Great Oxidation Event 2.4 billion years ago (Ga) [e.g., A. D. Anbar et al., Science 317, 1903-1906 (2007); M. C. Koehler, R. Buick, M. E. Barley, Precambrian Res. 320, 281-290 (2019)], but the triggers for these short-lived events are so far unknown. Here, we use mercury (Hg) abundance and stable isotope composition to investigate atmospheric evolution and its driving mechanisms across the well-studied whiff of O-2 recorded in the similar to 2.5-Ga Mt. McRae Shale from the Pilbara Craton in Western Australia [A. D. Anbar et al., Science 317, 1903-1906 (2007)]. Our data from the oxygenated interval show strong Hg enrichment paired with slightly negative Delta Hg-199 and near-zero Delta Hg-200, suggestive of increased oxidative weathering. In contrast, slightly older beds, which were evidently deposited under an anoxic atmosphere in ferruginous waters [C. T. Reinhard, R. Raiswell, C. Scott, A. D. Anbar, T. W. Lyons, Science 326, 713-716 (2009)], show Hg enrichment coupled with positive Delta Hg-199 and slightly negative Delta Hg-200 values. This pattern is consistent with photochemical reactions associated with subaerial volcanism under intense UV radiation. Our results therefore suggest that the whiff of O-2 was preceded by subaerial volcanism. The transient interval of O-2 accumulation may thus have been triggered by diminished volcanic O-2 sinks, followed by enhanced nutrient supply to the ocean from weathering of volcanic rocks causing increased biological productivity.

Automated summary

Multiple transient episodes of oxygenation occurred in Earth's early atmosphere before the Great Oxidation Event 2.4 billion years ago. Research suggests that the whiff of O-2 was preceded by subaerial volcanism, leading to temporary accumulation of oxygen. These events were triggered by diminished volcanic O-2 sinks and enhanced nutrient supply to the ocean from weathering of volcanic rocks.