The Earth's oxygen-rich atmosphere is a fascinating story, one that scientists are still unraveling. While life is a key player, with photosynthetic organisms pumping out oxygen, the chemistry of the solid Earth also plays a crucial role. A new study led by Wei Shi of the Chengdu University of Technology suggests that changes in the subduction of tectonic plates might be a significant part of this story. The research team's hypothesis is that variations in subduction could have influenced atmospheric oxygen levels by controlling the amount of carbon and sulfur being carried into the Earth's interior. These elements, which love to bond with oxygen, are released into the shallow mantle when the mantle is hotter and can return to the atmosphere via volcanoes. Conversely, when a plate dives into a cooler mantle, it retains more of its sulfur and carbon. This process is supported by evidence from subduction sites, where the minerals and subtle chemistry of the rock tell us about the temperatures and pressures experienced along their journey. By comparing this temperature and pressure information, the team compiled a broad picture of the history of subduction. The data seems to line up with the timing of jumps in oxygen levels. Lower-temperature subduction is observed between 2.2 and 1.8 billion years ago, coinciding with the initial Great Oxygenation Event. This period is followed by a break, and then lower-temperature subduction dominates for the last 800 million years, covering the second and third jumps in oxygen levels. Running this history of subduction through a basic chemical model, the researchers found they could roughly reproduce the timeline of oxygenation. The story begins with the assembly of an early 'supercontinent' called Columbia, which had a significant amount of land above sea level. This allowed for the delivery of nutrients to the oceans, supporting a large amount of photosynthetic cyanobacteria. The breakup of Columbia aligns with the first signs of lower-temperature subduction, enabling more organic carbon and carbonate to be subducted deep into the mantle. The 'Boring Billion' period, when mantle convection and tectonic plate movement were sluggish, follows. However, after this period, the formation and breakup of supercontinents Gondwana and Pangaea led to a map of tectonic plate boundaries resembling our present world, with lots of low-temperature subduction. The 'Ring of Fire' around the Pacific Ocean today is a prime example of this, continuously carrying carbon and sulfur-rich sediments deep into the mantle. The researchers argue that 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. This study highlights the intricate relationship between the Earth's geology and its atmosphere, suggesting that tectonic shifts have played a significant role in the oxygenation of our planet. While there is still much to uncover, this research provides a fascinating insight into the complex history of our oxygen-rich atmosphere.
