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 biosphere, including the onset of plate tectonics and the expansion of oxygenic photosynthesis, although the full impacts of these changes on the atmosphere remain unclear. Here we present coupled records of mass independent fractionation of sulfur (S-MIF) and mercury (Hg-MIF) isotopes from well preserved sediments of the similar to 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 Hg-0, which could have been driven by increased halogen emissions from arc volcanism. When considered within a global geodynamic context, these MIF data suggest an important role for subduction zone-related volcanism associated with early plate tectonics in modulating the similar to 2.7 Ga atmosphere.

Automated summary

The Archean eon was marked by significant changes in Earth's geosphere and biosphere, such as the emergence of plate tectonics and the expansion of oxygenic photosynthesis. Analysis of sulfur and mercury isotopes from sediments dating back to around 2.7 billion years ago provides insights into atmospheric chemistry during this time, highlighting the impact of volcanic input and local mixing on the muted S-MIF and odd number Hg-MIF values. The absence of even number Hg-MIF in these sediments suggests complete photo-oxidation of gaseous Hg-0, possibly influenced by increased halogen emissions from arc volcanism, indicating the important role of early plate tectonics-related volcanism in shaping the atmosphere around 2.7 Ga.