High concentrations of greenhouse gases would have been required to offset low solar luminosity early in Earth's history. Enhanced CO2 levels are probably at least part of the solution, but CH4 may have played a significant role as well, particularly during the Late Archean era, 2.5–3.0 Ga, when methanogenic bacteria were almost certainly present. Indeed, biological CH4 production should have led to CO2 drawdown by way of a negative feedback loop involving the carbonate-silicate geochemical cycle. We suggest here that the atmospheric CH4/CO2 ratio approached the value of ∼1 needed to trigger formation of Titan-like organic haze. This haze was strongly depleted in 13C relative to 12C and was produced at a rate comparable to the modern rate of organic carbon burial in marine sediments. Therefore, it could provide a novel explanation for the presence of extremely low-13C kerogens in Late Archean sediments.

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