Photochemistry and climate on the Archean earth

The purpose of my thesis was to explore the consequences of reduced levels of oxygen and reduced solar luminosity, both of which should have affected atmospheric composition and climate.; Recent studies of Archean sediments discovered evidence of mass-independent sulfur isotope fractionation in sulfides and sulfates. This type of fractionation pattern cannot be produced by aqueous-phase chemical or biochemical reactions. Mass independent fractionation (MIF) is observed, however, in atmospheric photochemical reactions (photolysis of SO₂, H₂S). Using laboratory data on SO₂ photolysis, I calculated how sulfur isotopic fractionation propagates to other sulfur-bearing species (HA S, SO, H ₂SO₄) by means of atmospheric chemical reactions. I have been able to estimate what types of isotopic separation must have occurred in order to produce the observed fractionation pattern in Archean sediments.; Although mass-independent fractionation continued to occur in the post-Archean atmosphere, its signature was lost before being preserved in sediments because all sulfur-bearing species were re-homogenized as oceanic sulfate. To transfer the mass-independent fractionation pattern from the atmosphere into sediments, the Archean atmosphere must have contained virtually no free oxygen (<10 −5 PAL [times the Present Atmospheric Level]).; 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 (in an anoxic Archean atmosphere) 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.; Moreover, biological CH₄ production should have led to CO ₂ drawdown by way of a negative feedback loop involving the carbonate-silicate geochemical cycle. I suggest here that the atmospheric CH₄/CO ₂ ratio approached the value of ∼1 needed to trigger the formation of Titan-like organic haze.; Finally, some evidence may exist to link my models of the Archean atmosphere to the geologic record. The hydrocarbon 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. (Abstract shortened by UMI.)...