The loss of mass‐independent fractionation in sulfur due to a Palaeoproterozoic collapse of atmospheric methane

Abstract

We use a 1‐D numerical model to study the atmospheric photochemistry of oxygen, methane, and sulfur after the advent of oxygenic photosynthesis. We assume that mass‐independent fractionation (MIF) of sulfur isotopes – characteristic of the Archean – was best preserved in sediments when insoluble elemental sulfur (S8) was an important product of atmospheric photochemistry. Efficient S8 production requires three things: (i) very low levels of tropospheric O2; (ii) a source of sulfur gases to the atmosphere at least as large as the volcanic SO2 source today; and (iii) a sufficiently high abundance of methane or other reduced gas. All three requirements must be met. We suggest that the disappearance of a strong MIF sulfur signature at the beginning of the Proterozoic is better explained by the collapse of atmospheric methane, rather than by a failure of volcanism or the rise of oxygen. The photochemical models are consistent in demanding that methane decline before O2 can rise (although they are silent as to how quickly), and the collapse of a methane greenhouse effect is consistent with the onset of major ice ages immediately following the disappearance of MIF sulfur. We attribute the decline of methane to the growth of the oceanic sulfate pool as indicated by the widening envelope of mass‐dependent sulfur fractionation through the Archean. We find that a given level of biological forcing can support either oxic or anoxic atmospheres, and that the transition between the anoxic state and the oxic state is inhibited by high levels of atmospheric methane. Transition from an oxygen‐poor to an oxygen‐rich atmosphere occurs most easily when methane levels are low, which suggests that the collapse of methane not only caused the end of MIF S and major ice ages, but it may also have enabled the rise of O2. In this story the early Proterozoic ice ages were ended by the establishment of a stable oxic atmosphere, which protected a renewed methane greenhouse with an ozone shield.

Geological sulfur isotopes indicate elevated OCS in the Archean atmosphere, solving faint young sun paradox

“…SO 2 photolysis is the most important reaction controlling the isotopic composition of sulfate aerosols, which mainly reflects that of residual SO 2 after the photolysis. These conclusions are consistent with previous model studies without OCS chemistry (3)(4)(5). In the reducing atmosphere, however, sulfur entering the atmosphere would be largely converted into OCS.…”

“…An alternative explanation, considering the sun's mass loss history, is that the early sun was brighter than has previously been thought, although the period of mass loss is likely shorter than what is required to resolve the paradox (21). Greenhouse gases that have been suggested include NH 3 (20,22), CO 2 (18,23), and CH 4 (4,5,24).…”

“…In these Tables S1-S3). We introduced OCS photochemistry into the model, which was not included in the previous models of Archean atmosphere (3)(4)(5)18). The model calculation gives the concentrations of various sulfur compounds and the isotopic composition of product sulfate and elemental sulfur aerosols at a given time after injecting 10 ppm of SO 2 into the model atmosphere.…”

“…Moreover, it would have blocked UV sunlight inhibiting the supposed photochemical process(es) producing the isotope anomaly. In contrast, given reducing conditions, two isotopically distinct sulfur species such as elemental sulfur (polysulfur) and sulfate could be formed photochemically and precipitate at the surface, where the MIF signal could be preserved in sediment (3)(4)(5). Sulfur MIF in geological samples is regarded as indicating an anoxic atmosphere.…”

Iron formations: A global record of Neoarchaean to Palaeoproterozoic environmental history

“…It is thus possible that a Ni famine eventually led to a cascade of events that began with reduced methane production, the expansion of cyanobacteria into deep-water settings previously occupied by methanogens, and ultimately increased oxygenic photosynthesis that tipped the atmospheric redox balance in favour of oxygen, leading to the GOE. At the heart of this model is the simple idea that a decreased flux of methane led to greater oxygen concentrations in the atmosphere even if the marine O 2 flux to the atmosphere remained constant (Zahnle et al, 2006).…”