Co-evolution of primitive methane-cycling ecosystems and early Earth’s atmosphere and climate

Abstract

The history of the Earth has been marked by major ecological transitions, driven by metabolic innovation, that radically reshaped the composition of the oceans and atmosphere. The nature and magnitude of the earliest transitions, hundreds of million years before photosynthesis evolved, remain poorly understood. Using a novel ecosystem-planetary model, we find that pre-photosynthetic methane-cycling microbial ecosystems are much less productive than previously thought. In spite of their low productivity, the evolution of methanogenic metabolisms strongly modifies the atmospheric composition, leading to a warmer but less resilient climate. As the abiotic carbon cycle responds, further metabolic evolution (anaerobic methanotrophy) may feed back to the atmosphere and destabilize the climate, triggering a transient global glaciation. Although early metabolic evolution may cause strong climatic instability, a low CO:CH 4 atmospheric ratio emerges as a robust signature of simple methane-cycling ecosystems on a globally reduced planet such as the late Hadean/early Archean Earth.

The possible occurrence of iron-dependent anaerobic methane oxidation in an Archean Ocean analogue

“…A widespread occurrence of AOM coupled to Fe oxidation offers an alternative explanation that would have led to the decrease of dissolved CH 4 in the ocean and consequently the emission of CH 4 to the atmosphere, thus going in the direction of the study of Riedinger et al 39 . This is consistent with the revision of the putative composition of the Archean atmosphere 55 that suggests that the amount of greenhouse warming by CH 4 was more limited than previously thought 49 , 56 .…”

“…However, geological archives do not provide constraints on the magnitude of the CH 4 concentrations, and large CH 4 fluxes are calculated with quantitative models that limit the existence of Fe-dependent CH 4 oxidation and assume a negligible role of SO 4 2− driven CH 4 oxidizers. This paradigm has recently been challenged by Olson et al 48 and Sauterey et al 49 who showed that the combined effects of competition between methanogens and SO 4 2− reducers and occurrence of SO 4 2− driven CH 4 oxidation would have effectively reduced the CH 4 fluxes from the ocean even at modest SO 4 2− concentrations, and regardless of O 2 concentrations. Similarly, we propose that strong interactions between photoferrotrophs and Fe-dependent CH 4 oxidizers might have exerted an important control on the Archean–Proterozoic CH 4 cycling.…”

“…However, geological archives do not provide constraints on the magnitude of the CH 4 concentrations, and large CH 4 fluxes are calculated with quantitative models that limit the existence of Fe-dependent CH 4 oxidation and assume a negligible role of SO 4 2− driven CH 4 oxidizers. This paradigm has recently been challenged by Olson et al 48 and Sauterey et al 49 who showed that the combined effects of competition between methanogens and SO 4 2− reducers and occurrence of SO 46 suggests that photoferrotrophy would have been high enough to potentially support the oxidation of 100% of the CH 4 flux to the early atmosphere. In Kabuno Bay, a modern analog to early Earth ocean, measured photoferrotrophy rates would have been sufficient to sustain a smaller (31 ± 18%, n = 3), but still important, part of the oxidation of the CH 4 flux measured during this study (19 ± 11 mmol m −2 d −1 , n = 3).…”

“…www.nature.com/scientificreports/ Finally, the great oxygenation event was shortly afterwards followed by a low latitude glaciation that would be associated to lower atmospheric CH 4 concentrations. However, the trigger of the collapse of CH 4 has not been clearly identified, some authors arguing that CH 4 decreased as a consequence of rising O 2 58 , while others proposed that low CH 4 level preceded the transition to an O 2 -richer atmosphere 49,51 . Co-occurrence of photoferrotrophy and Fe-dependent CH 4 oxidation in the Archean would support the latter hypothesis and the view of Olson et al 48 who recently proposed an alternative mechanism for the initiation of low-latitude glaciation with low baseline atmospheric CH 4 levels.…”

Chance played a role in determining whether Earth stayed habitable

“…Or, conversely, given that the early Earth was not frozen, if the rest of the climate system had remained unaltered then the oceans would now be boiling under today's brighter Sun. Evidently neither occurred, for reasons that are still debated [8][9][10][11][12] ; although plausible solutions exist, in particular when combining multiple processes 13 , as yet there is no firm consensus as to how in reality the 'Faint Young Sun Paradox' was overcome 14 .…”