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Atmospheric hydrogen peroxide and Eoarchean iron formations
Author(s) -
Pecoits E.,
Smith M. L.,
Catling D. C.,
Philippot P.,
Kappler A.,
Konhauser K. O.
Publication year - 2015
Publication title -
geobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.859
H-Index - 72
eISSN - 1472-4669
pISSN - 1472-4677
DOI - 10.1111/gbi.12116
Subject(s) - chemistry , atmosphere (unit) , ferrous , hydrogen peroxide , flux (metallurgy) , environmental chemistry , precipitation , deposition (geology) , photosynthesis , photochemistry , geology , sediment , biochemistry , physics , organic chemistry , thermodynamics , paleontology , meteorology
It is widely accepted that photosynthetic bacteria played a crucial role in Fe( II ) oxidation and the precipitation of iron formations ( IF ) during the Late Archean–Early Paleoproterozoic (2.7–2.4 Ga). It is less clear whether microbes similarly caused the deposition of the oldest IF at ca . 3.8 Ga, which would imply photosynthesis having already evolved by that time. Abiological alternatives, such as the direct oxidation of dissolved Fe( II ) by ultraviolet radiation may have occurred, but its importance has been discounted in environments where the injection of high concentrations of dissolved iron directly into the photic zone led to chemical precipitation reactions that overwhelmed photooxidation rates. However, an outstanding possibility remains with respect to photochemical reactions occurring in the atmosphere that might generate hydrogen peroxide (H 2 O 2 ), a recognized strong oxidant for ferrous iron. Here, we modeled the amount of H 2 O 2 that could be produced in an Eoarchean atmosphere using updated solar fluxes and plausible CO 2 , O 2 , and CH 4 mixing ratios. Irrespective of the atmospheric simulations, the upper limit of H 2 O 2 rainout was calculated to be <10 6 molecules cm −2 s −1 . Using conservative Fe( III ) sedimentation rates predicted for submarine hydrothermal settings in the Eoarchean, we demonstrate that the flux of H 2 O 2 was insufficient by several orders of magnitude to account for IF deposition (requiring ~10 11 H 2 O 2 molecules cm −2 s −1 ). This finding further constrains the plausible Fe( II ) oxidation mechanisms in Eoarchean seawater, leaving, in our opinion, anoxygenic phototrophic Fe( II )‐oxidizing micro‐organisms the most likely mechanism responsible for Earth's oldest IF .