Zircon Crystals Show Earth's Mantle Was Absorbing Oxygen Billions of Years Early

Earth's deep interior was responding to oxygen at the surface roughly a billion years before oxygen became abundant in the air, according to a new international study published in Nature Geoscience. The work, led by Dr. Hugo Moreira, a Royal Society university research fellow at the University of Portsmouth, traces the chemistry of ancient rocks back to a time when the planet looked nothing like it does today.

Moreira's team used zircon, one of the toughest minerals on Earth. Zircon crystals can survive largely unchanged for billions of years, and lasers at Portsmouth's Mass Spectrometry and Laser Ablation Lab can date them precisely. Some samples were pinned to exactly 3.4 billion years old. Certain zircons contain a smaller mineral, apatite, trapped inside like an insect in amber. Because the zircon shields it from change, the apatite is the same age as its host.

The researchers gathered ancient rock samples from Australia, Finland, South Africa, Brazil and Scotland and sent them to Portsmouth for initial analysis. They then took them to the European Synchrotron Radiation Facility in Grenoble, France, one of the most powerful X-ray facilities in the world, where a specialized technique measured the chemical state of sulfur inside each apatite crystal.

The form sulfur takes works as a sensitive fingerprint of how much oxygen was present when the crystal formed, at the point where Earth's outer shell is dragged down and recycled into the interior in a process called subduction. The crystals are often narrower than a human hair and hold very little sulfur. Marine Cotte, the scientist in charge of the beamline, said the team needed an X-ray beam that was fine, sensitive and fast enough to measure many crystals so the results would reflect the bigger picture rather than one or two samples.

For most of early Earth's history there was almost no oxygen in the air or oceans. That changed permanently around 2.3 billion years ago in the Great Oxidation Event, often described as the moment the planet started breathing. Scientists have also suspected brief, local bursts of oxygen earlier, between 3 and 3.5 billion years ago, when there was barely any life to make it.

The new data show the mantle was already picking up that signal. The chemical signature of oxygen in the mantle layers affected by recycling rose between 3.4 and 3 billion years ago and stayed present across the whole 3.4-billion-year span studied, reaching today's levels around 2.1 billion years ago. That suggests some form of subduction was carrying oxidized material downward while the atmosphere was changing above, linking the deep Earth and the surface far earlier than previously realized.

It is also the first study to use the same type of rock across the entire time span, which Moreira said gives a more robust comparison than bridging different kinds of evidence. He acknowledged the work will not change daily life soon. "It's about answering the philosophical questions that are in our nature as human beings," he said, adding that plate tectonics has likely played a major role in both the emergence of life and its continued development. The study follows earlier Nature Geoscience work from 2023 that first connected Earth's ancient atmosphere with deep-Earth chemistry across the Great Oxidation Event.