PHOTOFERROTROPHY, CONTINENTAL GROWTH, AND THE HAZY NEOARCHEAN ATMOSPHERE (Invited Presentation)

Atmospheric methane likely played a key role in maintaining relatively clement climates throughout Earth’s early history when solar luminosity was much lower than today. Photolysis in the atmosphere would have continually destroyed methane driving hydrogen loss to space and the permanent oxidation of the Earth. This strong methane sink thus implies a continuous source of methane to the atmosphere from the Earth’s surface to maintain greenhouse conditions. Methane fluxes from the Earth’s surface are driven by microbial methanogenesis, which is fuelled by the degradation of organic matter created through photosynthesis. Production of this organic matter is commonly ascribed to oxygenic phototrophic bacteria (ancestors to modern cyanobacteria), which paradoxically produce molecular oxygen that reacts with methane through microbially catalyzed methanotrophy strongly curtailing modern methane emissions the atmosphere. While oxygenic phototrophs likely evolved in the Archean, nutrient dynamics in Fe-rich oceans would have precluded their proliferation until later in Earth’s history when marine Fe concentrations declined. Instead, anoxygenic phototrophs that grow by oxidizing ferrous Fe, photoferrotrophs, would have driven much of the primary biomass production throughout the Archean. We thus propose that atmospheric methane in the Archean was largely supported through methanogenic degradation of biomass produced through photoferrotrophy. We developed biogeochemical models with primary production driven through photoferrotrophy and these models reveal how dynamics in hydrothermal activity, ocean circulation, and tectonics, could have conspired to modulate fluxes of methane to the atmosphere. Such modulation is well supported by geochemical proxies. Our model also reveals how continental growth and enhanced ocean circulation in the Neoarchean could have triggered atmospheric haze formation leading to the first large-scale oxygenation of the atmosphere.