Study links Earth’s oxygen rise to cooling mantle and plate tectonics
New analysis published in PNAS finds that as Earth’s mantle cooled, colder subduction zones trapped more carbon and sulfur in the interior, preventing them from reacting with oxygen and allowing levels to rise during key geological eras.
A study led by Wei Shi of the Chengdu University of Technology proposes that the evolution of Earth’s oxygen-rich atmosphere is inextricably linked to changes in plate tectonics. Published in the Proceedings of the National Academy of Sciences (PNAS), the research suggests that as the planet cooled over billions of years, subduction zones became colder. This shift allowed for the efficient transport of carbon and sulfur into the Earth’s mantle rather than their release into the atmosphere via volcanoes, thereby reducing the amount of oxygen-binding gases available to scavenge atmospheric oxygen.
The research team compiled a history of subduction by comparing temperature and pressure information derived from minerals in subducted rock samples. The data indicates that lower-temperature subduction occurred between 2.2 and 1.8 billion years ago, aligning with the Great Oxygenation Event, and dominated the last 800 million years, coinciding with subsequent rises in oxygen levels. This pattern contrasts with the early Earth, where a hotter mantle led to subduction dynamics that released more carbon and sulfur back into the atmosphere.
The study links the initial oxygen jump to the assembly of the supercontinent Columbia, which provided land for erosion and nutrients for photosynthetic cyanobacteria. The subsequent breakup of Columbia facilitated deeper subduction of organic carbon. This was followed by the “Boring Billion,” a period between 2.0 billion and 800 million years ago when oxygen levels stalled. During this time, mantle convection and tectonic plate movement appear to have been sluggish.
Subsequent rises in oxygen occurred between 800 and 500 million years ago, and again between 450 and 250 million years ago, reaching modern levels. These increases align with the formation and breakup of the supercontinents Gondwana and Pangaea, which established tectonic plate boundaries resembling the present-day map. The researchers note that once low-temperature subduction became common, the balance of Earth’s oxygen was able to tilt more toward the atmosphere.
Running this history of subduction through a basic chemical model, the researchers found they could roughly reproduce the timeline of oxygenation. The study concludes that while biological and geological factors played significant roles, these processes operated on top of a baseline defined by the net flux of carbon and sulfur between Earth’s interior and exterior, which was controlled by the evolving efficiency of cold subduction on a cooling Earth.
