Fossil micrometeorites and the air of the ancient Earth
Micrometeorites trapped in ancient rocks are tiny probes of the past. Iron spherules 2.7 billion years old have been used to test what the upper atmosphere was like long before animals existed.
When an iron micrometeorite melts in the air, oxygen reacts with it and turns metal into iron oxide. How much it changes depends on the air it fell through. Spherules preserved in Australian limestone from 2.7 billion years ago are heavily oxidised. One team read that as a sign of oxygen high in the ancient atmosphere; another argued that lots of carbon dioxide could explain it instead. Either way, a speck of space dust has become a sample of air from billions of years ago.
Key facts
- Oldest known
- About 2.7 billion years old, in limestone from the Pilbara region of Western Australia
- What they show
- Strong oxidation of iron while falling through the upper atmosphere
- Competing readings
- Oxygen-rich upper air (2016) or carbon dioxide-rich air (2020)
- Other records
- Dust pulses from asteroid break-ups about 466 and 8 million years ago
Space dust in old rocks
Micrometeorites have been landing for as long as the Earth has existed, and some are buried in sediments that later turned to rock. Most are destroyed by water and heat, but in a few kinds of rock they survive: limestone, salt deposits, slowly formed hard layers on the ancient seabed, and clays. Old spherules are usually altered, with their original minerals replaced, but their shapes and textures can still be recognised, and sometimes their chemistry too.
Spherules from 2.7 billion years ago
In the Pilbara region of Western Australia, layers of limestone formed about 2.7 billion years ago, in the Archaean eon, when life consisted only of microbes and the air near the ground had almost no free oxygen. In 2016, a team led by Andrew Tomkins of Monash University reported 60 iron micrometeorites from these rocks, each about the width of a hair. They had begun as particles of iron metal, but most had been turned into iron oxides during entry, the same change seen in modern I-type spherules.
For metal to oxidise like that, the particles needed a source of oxygen at the height where they melted, about 75 to 90 km up. The team concluded that the upper atmosphere then contained free oxygen at a concentration close to today's. They suggested the ancient air was layered: oxygen produced by sunlight splitting molecules high up, separated from oxygen-poor air near the ground by a haze of methane.
Or was it carbon dioxide?
In 2020, a team at the University of Washington, including the micrometeorite pioneer Donald Brownlee, offered another explanation. Carbon dioxide can also oxidise molten iron. Their computer model of falling particles found that an atmosphere containing roughly 6 per cent to more than 70 per cent carbon dioxide, far more than today's 0.04 per cent, could produce the same oxidation without any free oxygen.
That fits other evidence that the Archaean air was rich in carbon dioxide, which would have kept the Earth warm while the young Sun shone about a fifth less brightly than today. It also avoids conflict with the widely accepted view that oxygen built up in the atmosphere only later, in the Great Oxidation Event that began about 2.4 billion years ago. The debate continues, and more samples of different ages are needed to settle it.
The great dust shower 466 million years ago
Around 466 million years ago, in the Ordovician period, a large asteroid in the asteroid belt was shattered. Its fragments are the L chondrites, still the second most common meteorites to fall today. In Swedish limestone of that age, geologists have found more than a hundred fossil meteorites, along with a huge increase in tiny grains of chromite, a durable mineral that survives when the rest of a particle has dissolved.
Analyses of helium in the limestone suggest the dust supply rose by many times for perhaps two million years. Some researchers have proposed that the extra dust in space screened out a little sunlight and helped cool the climate, contributing to an ice age of that time, though this remains debated. Chromite grains from just before the break-up also show that the mix of meteorite types reaching the Earth was then very different, with many more of the rare primitive achondrites.
More recent pulses
Deep-sea sediment cores record dust through helium-3. One clear spike began about 8.2 million years ago and faded over about 1.5 million years. It has been matched to the break-up of a large asteroid that formed the Veritas family, whose fragments still share similar orbits. Another rise, about 36 million years ago, has been linked to a shower of comets or to an asteroid break-up, at around the time two large impact craters formed, Popigai in Siberia and Chesapeake Bay in North America.
Why fossil dust matters
Fossil micrometeorites are among the few direct samples of the upper atmosphere of the distant past, and of the asteroid belt as it was millions or billions of years ago. Each new find adds a point to a timeline that connects the Earth's air, its climate and the collisions happening far out in space.
Did you know?
The 2.7-billion-year-old Pilbara spherules are the oldest micrometeorites known, more than half as old as the Earth.
Fossil meteorites from the Ordovician dust shower are found in Swedish limestone quarries, where quarry workers have spotted them as dark patches in slabs of stone.
Helium-3 is useful because the Earth's crust has almost none of it, so even a trace in sediment stands out as coming from space dust.