Want an oxygen-rich atmosphere? Stuff oxygen’s friends in the mantle.
Source: Ars Technica
Getting carbon and sulfur into Earth’s interior may be part of oxygen’s story.
Planet Earth has some pretty great qualities going for it. (Negative reviews mostly revolve around the staff and clientele.) Pretty high on the list of positives is a richly oxygenated atmosphere. But that’s something that evolved and built up over a couple billion years, only eventually resulting in a world conducive to animal life like us.
Scientists have many ideas about what could have caused oxygen to increase, and it seems that a number of them are probably correct — no single factor in isolation can explain it. Life is part of the story, with photosynthetic organisms pumping out oxygen. The chemistry of the solid Earth also played a role, both by supporting photosynthetic life and by shuttling oxygen between the atmosphere and rocks deep inside the planet.
A new study led by Wei Shi of the Chengdu University of Technology suggests that evidence of changes in the subduction of tectonic plates—the process by which they disappear down into Earth’s interior—lines up with the timing of jumps in oxygen levels.
Reference: The complicated history of how the Earth’s atmosphere became breathable (Ars Technica, May 2023)
Cooling off
The Earth has gradually cooled over time, and the scant remnants of its earliest history show us that major geologic processes evolved quite a bit as a result. Early in its history, cold, dense surface rock would have sunk through hot mantle rock in ways that bear little resemblance to modern plate tectonics. The continents we see today are the product of a 4.5‑billion‑year‑long construction project, so imagination is required to picture what was present early on.
It wasn’t a smooth, linear evolution—there appear to be transition points in that geologic history. The oxygenation of Earth’s atmosphere wasn’t linear, either. It started with a jump during the Great Oxygenation Event about 2.4–2.0 billion years ago, stalled, then resumed between 800 and 500 million years ago, and finally increased again between 450 and 250 million years ago to reach modern levels.
Hypothesis
The research team proposed that changes in subduction might have influenced atmospheric oxygen by controlling how much carbon and sulfur—both of which readily bond with oxygen—were carried into the deep interior of the Earth.
- When the mantle is hotter, carbon and sulfur don’t travel far down with the subducted rock. They’re released into the shallow mantle and can soon return to the atmosphere via volcanoes, where they scavenge oxygen.
- Conversely, a plate diving into a cooler mantle retains more of its sulfur and carbon.
At sites where subducted rock resurfaces, the minerals and subtle chemistry inside them record the temperatures and pressures experienced during their journey. By comparing these temperature‑pressure data, the team compiled a broad picture of subduction history. If the hypothesis holds, lower‑temperature subduction should coincide with increases in atmospheric oxygen.
Findings
The data do appear to line up:
- 2.2–1.8 billion years ago: Lower‑temperature subduction, matching the initial Great Oxygenation Event.
- After a hiatus (the “Boring Billion”): Low‑temperature subduction dominates for the last 800 million years, covering the second and third oxygen jumps.
Thus, periods of cooler subduction correlate with the major rises in atmospheric oxygen, supporting the idea that deep‑Earth processes helped regulate Earth’s habitability.
Tectonic Shifts
Running this history of subduction through a basic chemical model, the researchers found they could roughly reproduce the timeline of oxygenation.
The beginning of the story, they say, could be the assembly of an early “supercontinent” (think Pangaea) called Columbia. With an appreciable amount of land above sea level, erosion could deliver enough nutrients to the oceans to support a large amount of photosynthetic cyanobacteria. We can see the evidence of this in seafloor sedimentary rocks rich in organic carbon.
The breakup of Columbia aligns with the first signs of lower‑temperature subduction. That would have enabled more of this organic carbon—and carbonate accumulating in shallow water around Columbia—to be subducted deep into the mantle.
Then comes the Boring Billion, when even mantle convection and tectonic plate movement seem to have been sluggish. After that, the formation and breakup of the supercontinents Gondwana and Pangaea move us toward a map of tectonic plate boundaries that looks like our present world, with lots of low‑temperature subduction.
The “Ring of Fire” around the Pacific Ocean today, for example, marks a huge zone of subduction that continuously carries carbon‑ and sulfur‑rich sediments deep into the mantle. Once this sort of subduction became common, the balance of Earth’s oxygen was able to tilt more toward the atmosphere.
There is certainly a lot more to the story, both in terms of biology and geology. Our oxygen‑rich atmosphere is the product of a rich set of interactions. But, the researchers write, “These processes all operated on top of the baseline defined by the net flux of carbon (and sulfur) between Earth’s interior and exterior, which we argue was controlled by the evolving efficiency of cold subduction on a cooling Earth.”
PNAS, 2026. DOI: 10.1073/pnas.2534056123 – About DOIs
Author
Scott K. Johnson – Scott has written about geoscience and energy at Ars Technica as a freelancer since 2011.
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