Apatite trapped in 3.4-billion-year-old zircon shows the mantle responding to surface oxygen

The "billion years" in the headline depends on which end of the signal you measure from. The Great Oxidation Event sits at about 2.3 billion years ago ^[9]. The mantle signal rose between 3.4 and 3 billion years ago ^[12]. Counted from 3.4 the lead is 1.1 billion years, and from 3.0 it is 0.7 billion ^[16][17]. At the other end, the signal reached today's levels by around 2.1 billion years ago ^[13], at most 0.2 billion years after the Great Oxidation Event ^[18].

In the study's account, surface oxygen comes first, in small patches, and the mantle responds: the sulfur shows the deep mantle was already reacting to oxygen from the surface ^[11], and scientists have long suspected brief local oxygen oases between 3 and 3.5 billion years ago ^[10]. The mantle signal rises inside that window. The account says this suggests some form of subduction was already carrying oxidized material downward ^[14].

Zircon is one of the toughest minerals on Earth and survives largely unchanged for billions of years ^[2]. Some zircons enclose apatite, sealed in and protected from change, so the apatite is the same age as its host ^[3]. Moreira, who led the study, said of such grains: "Because they're so well protected, we can analyze their chemical fingerprints to get an untouched snapshot of a moment billions of years in the past." ^[15]

The team read the apatite's sulfur. Its form shows how much oxygen was present when the crystal formed, at the point where the outer shell is dragged down and recycled in subduction ^[6]. The grains are small. Co-author Marine Cotte, the scientist in charge of the ID21 beamline, said the crystals are often narrower than a human hair and hold only small amounts of sulfur ^[7]. She said the team therefore needed "an X-ray beam that was very fine, very sensitive and fast enough to measure lots of crystals. That way, we could be confident the results reflect the bigger picture and not just one or two samples." ^[8]

The curve covers only part of the mantle. The signal is for the mantle layers affected by recycling ^[12], and the sulfur reads oxygen at the subduction site ^[6]. On that design, mantle that never met subducted material is outside what the crystals sample.

Cotte's point is that many crystals were measured. The account does not say how many crystals or rock samples stand behind each time window. We think the shape of the curve is the sturdy result: a rise after 3.4 billion years ago, persistence, and modern values by 2.1 billion years ago. The step to subduction is a further inference, and the account itself words it as a suggestion.