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., 317, 1903-1906 (2007); M. C. Koehler, R. Buick, M. E. Barley, 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 recorded in the ∼2.5-Ga Mt. McRae Shale from the Pilbara Craton in Western Australia [A. D. Anbar et al., 317, 1903-1906 (2007)]. Our data from the oxygenated interval show strong Hg enrichment paired with slightly negative ∆Hg and near-zero ∆Hg, 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, 326, 713-716 (2009)], show Hg enrichment coupled with positive ∆Hg and slightly negative ∆Hg 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 was preceded by subaerial volcanism. The transient interval of O accumulation may thus have been triggered by diminished volcanic O sinks, followed by enhanced nutrient supply to the ocean from weathering of volcanic rocks causing increased biological productivity.