Sulfur and mercury MIF suggest volcanic contributions to Earth’s atmosphere at 2.7 Ga

The Archean eon is associated with large-scale changes in Earth’s geosphere and bio-sphere, including the onset of plate tectonics and the expansion of oxygenic photo-synthesis, although the full impacts of these changes on the atmosphere remainunclear. Here we present coupled records of mass independent fractionation of sulfur (S-MIF) and mercury (Hg-MIF) isotopes from well preserved sediments of the ∼2.7 billion year old (Ga) Manjeri Formation, Belingwe Greenstone Belt, Zimbabwe. These palaeoatmospheric proxies record different trends for S-MIF and odd number Hg-MIF versus even number Hg-MIF, providing novel constraints on atmospheric chemistry during this time. S-MIF and odd number Hg-MIF values are muted in comparison to values preserved in later Archean sediments, representing a combination of enhanced volcanic input and local mixing. Even number Hg-MIF is absent from these sediments, consistent with complete photo-oxidation of gaseous Hg0, which could have been driven by increased halogen emissions from arc volcanism. When considered within a global geo-dynamic context, these MIF data suggest an important role for subduction zone-related volcanism associated with early platetectonics in modulating the ∼2.7 Ga atmosphere.