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Volcanic sulfur dioxide (SO₂) is central to one explanation for sulfur-isotope clues preserved in ancient rocks, and researchers are also studying whether its ultraviolet photochemistry could have produced some molecular oxygen. Those are related but distinct questions: the isotope evidence is commonly interpreted as a record of an oxygen-poor atmosphere, while a possible oxygen-producing reaction does not show that SO₂ drove the Great Oxidation Event (GOE). The rise of atmospheric oxygen depended on the balance between production and geological and atmospheric sinks.
What the sulfur in ancient rocks can—and cannot—tell us
Archean rocks preserve unusual relationships among sulfur isotopes, known as sulfur mass-independent fractionation (S-MIF). Sulfur has several stable isotopes, and some ancient sulfide and sulfate minerals show isotope patterns that do not follow ordinary mass-dependent fractionation. Because ultraviolet light can drive distinctive sulfur chemistry in the atmosphere, these minerals provide clues about atmospheric conditions even though they are not direct measurements of ancient air.
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The prevailing interpretation treats S-MIF as strong evidence for distinctive atmospheric photochemistry under very low oxygen conditions. Shuhei Ono’s 2017 review identifies SO₂ photochemistry as a primary focus in explanations of how the isotope pattern formed. The inference is powerful, but it is still an interpretation of a geological proxy—not a direct reading of atmospheric oxygen. Ono’s review of SO₂ photochemistry and S-MIF
How ultraviolet light changes sulfur dioxide
When SO₂ absorbs ultraviolet light, it can undergo photochemical reactions that separate sulfur isotopes to different degrees. Ono’s review reports large-magnitude S-MIF from SO₂ photolysis at wavelengths shorter than 220 nanometres and from photoexcitation at 240–340 nanometres.
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Researchers have discussed several physical explanations for the isotope effects, including isotopologue-dependent self-shielding, differences in absorption cross-sections, and vibronic coupling during intersystem crossing. These are proposed mechanisms in an active field, not a single settled account of every step. Their significance is that SO₂ exposed to ultraviolet light offers a plausible way to generate isotope patterns like those preserved in Archean rocks.
Could SO₂ photochemistry also have made oxygen?
Yes, a possible oxygen-producing pathway is under study. A 2023 Chemical Science paper examines vacuum-ultraviolet photodissociation of SO₂ and its implications for molecular oxygen production in the early atmosphere. That makes SO₂ relevant to both sulfur-isotope chemistry and possible oxygen chemistry, but the two implications should not be conflated. The 2023 study of vacuum-ultraviolet SO₂ photodissociation
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A reaction that produces O₂ is not by itself proof that oxygen accumulated in the atmosphere. Net accumulation depends on how oxygen production compares with processes that consume it. The 2023 study therefore points to a possible photochemical contribution; it does not establish that volcanic SO₂ was the sole cause of the GOE or explain the planet-wide rise on its own.
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What the Great Oxidation Event means for the timeline
The GOE is commonly placed in the early Paleoproterozoic, around 2.4 billion years ago. Its precise start, pace, and degree of global synchrony depend on which rock sequences and proxies are considered. There is no single direct atmospheric measurement that timestamps the transition everywhere on Earth.
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A South African record places a rapid transition at 2.33 billion years ago
Luo and colleagues used continuous multiple-sulfur-isotope profiles from three coeval South African drill cores. Their 2016 study assigned the transition from mass-independent to mass-dependent sulfur-isotope signals to 2.33 billion years ago and inferred rapid oxygenation within 1 to 10 million years, followed by a slower increase in ocean sulfate. Those are findings from that set of cores, not a universal measure of the transition’s duration across the whole planet. Luo and colleagues’ South African drill-core study
Other records challenge a single globally synchronous marker
A 2018 Nature Communications study argues that sulfur-isotope transitions were asynchronous across records from South Africa, North America, and Australia. It also describes how weathering and recycling can preserve older sulfur signatures after oxygenation begins, complicating estimates based on the last appearance of S-MIF. On this interpretation, the familiar South African marker should not be treated as a precise global timestamp for the GOE. The study of globally asynchronous sulfur-isotope signals
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Different interpretations address different parts of the evidence. The conventional account links S-MIF to ultraviolet atmospheric chemistry under very low oxygen; an alternative proposal challenges how strongly that isotope signal requires a wholly anoxic atmosphere. Separately, models and experiments address how oxygen might have been produced or allowed to accumulate. These lines of inquiry are related, but none should be treated as a complete explanation of the others.
- Conventional S-MIF interpretation: Ono’s 2017 review presents SO₂ photochemistry as a leading explanation for the isotope anomalies and describes them as strong evidence for an early anoxic atmosphere.
- Alternative sulfur-cycle proposal: Hiroshi Ohmoto’s 2020 paper proposes that much early volcanic sulfur was emitted mainly as SO₂ by subaerial eruptions of oxidized, arc-like magmas since at least 3.5 billion years ago. It further proposes that some MIF-S could form through ultraviolet reactions involving SO₂ and O₂ in a stratosphere above an oxygen-rich troposphere, or through high-temperature reactions in the ocean. This challenges the conventional inference from S-MIF; it is a proposal, not a replacement established by the isotope record alone. Ohmoto’s 2020 alternative interpretation of early volcanic sulfur
- Changing oxygen sources and sinks: A 2020 modeling study reports that declining reducing power in volcanic gases could have helped oxygen accumulate only around 2.5 billion years ago. This is one proposed source-and-sink explanation, not a settled verdict on the GOE’s cause. The modeling study of volcanic-gas chemistry and oxygen accumulation
These accounts differ in what they explain: sulfur-isotope patterns, atmospheric conditions inferred from those patterns, a possible molecular pathway to O₂, or the conditions for net oxygen accumulation. Keeping those questions separate makes the evidence easier to interpret.
So, did volcanic sulfur dioxide cause Earth’s oxygen rise?
The evidence supports a qualified answer. SO₂ ultraviolet photochemistry is a leading explanation for S-MIF in ancient rocks, and recent work investigates whether SO₂ photodissociation could also contribute molecular oxygen. But S-MIF is a proxy whose atmospheric meaning is debated, and a possible oxygen-producing reaction does not demonstrate net global accumulation. The GOE is best understood through multiple lines of evidence and the balance between oxygen sources and sinks, with its timing varying according to the geological record used.
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