Atmospheric hydrogen peroxide and Eoarchean iron formations

Area of Science:

• Geochemistry
• Astrobiology
• Early Earth Science

Background:

• Photosynthetic bacteria are known to have formed iron formations (IF) in the Late Archean-Early Paleoproterozoic (2.7-2.4 Ga).
• The mechanism behind the deposition of the oldest IF around 3.8 Ga remains unclear, with implications for the early evolution of photosynthesis.
• Abiological iron oxidation by UV radiation and atmospheric hydrogen peroxide (H2O2) have been proposed but lack conclusive evidence.

Purpose of the Study:

• To investigate the plausibility of atmospheric photochemical reactions producing hydrogen peroxide (H2O2) as a mechanism for early Archean iron formation (IF) deposition.
• To quantify the potential flux of H2O2 from the Eoarchean atmosphere to the oceans.
• To compare this H2O2 flux with the required rates for IF deposition and assess its sufficiency.

Main Methods:

• Modeled H2O2 production in an Eoarchean atmosphere using updated solar fluxes and estimated gas concentrations (CO2, O2, CH4).
• Calculated the upper limit of H2O2 rainout into the oceans.
• Compared the calculated H2O2 flux with estimated Fe(III) sedimentation rates for IF deposition.

Main Results:

• The maximum calculated H2O2 rainout flux was less than 10^6 molecules cm^-2 s^-1.
• This H2O2 flux is several orders of magnitude lower than the estimated 10^11 molecules cm^-2 s^-1 required for IF deposition.
• Abiological iron oxidation via atmospheric H2O2 is insufficient to explain the deposition of the oldest iron formations.

Conclusions:

• Atmospheric photochemical production of H2O2 could not have been the primary driver for Eoarchean iron formation deposition.
• The findings constrain plausible mechanisms for iron oxidation in the early Archean oceans.
• Anoxygenic phototrophic Fe(II)-oxidizing microorganisms are the most likely agents responsible for Earth's oldest iron formations.