Aerobic bacterial pyrite oxidation and acid rock drainage during the Great Oxidation Event

Metal-respiring bacteria as atmospheric oxidation agents Free oxygen appeared in Earth's atmosphere for the first time around 2.5 billion years ago, in what is known as the Great Oxidation Event, which resulted in profound changes to biogeochemical cycling. Konhauser et al . examine rocks from this time and find that chromium was largely immobile on land until around the Great Oxidation Event, but that within the following 160 million years, it was solubilized on a large scale. The authors suggest that this mobilization was possible only through the action of aerobic, bacterial respiration on abundant supplies of pyrite. This early exploitation of atmospheric oxygen also represents the first record of acid rock drainage. The enrichment of redox-sensitive trace metals in ancient marine sedimentary rocks has been used to determine the timing of the oxidation of the Earth’s land surface 1 , 2 . Chromium (Cr) is among the emerging proxies for tracking the effects of atmospheric oxygenation on continental weathering; this is because its supply to the oceans is dominated by terrestrial processes that can be recorded in the Cr isotope composition of Precambrian iron formations 3 . However, the factors controlling past and present seawater Cr isotope composition are poorly understood. Here we provide an independent and complementary record of marine Cr supply, in the form of Cr concentrations and authigenic enrichment in iron-rich sedimentary rocks. Our data suggest that Cr was largely immobile on land until around 2.48 Gyr ago, but within the 160 Myr that followed—and synchronous with independent evidence for oxygenation associated with the Great Oxidation Event (see, for example, refs 4–6 )—marked excursions in Cr content and Cr/Ti ratios indicate that Cr was solubilized at a scale unrivalled in history. As Cr isotope fractionations at that time were muted, Cr must have been mobilized predominantly in reduced, Cr( iii ), form. We demonstrate that only the oxidation of an abundant and previously stable crustal pyrite reservoir by aerobic-respiring, chemolithoautotrophic bacteria could have generated the degree of acidity required to solubilize Cr( iii ) from ultramafic source rocks and residual soils 7 . This profound shift in weathering regimes beginning at 2.48 Gyr ago constitutes the earliest known geochemical evidence for acidophilic aerobes and the resulting acid rock drainage, and accounts for independent evidence of an increased supply of dissolved sulphat