HE IS more or less over it now, but Birger Schmitz once had an odd habit. He would visit a train station or an airport – any public building with a large expanse of stone floor would do – and shuffle around on his hands and knees, eyes glued to the ground. “I have had problems with security guards,” he admits. “If you start crawling around in the dark corners of an airport, people become suspicious.”
Schmitz is no terrorist. He is one of the world’s foremost hunters of fossil meteorites, ancient extra-terrestrial stones. It so happens that limestone floor tiles are an excellent place to look for them. Others prefer remote Australian deserts or Antarctic ice. But whether your searching ground is mundane or exotic, it is an inestimably difficult task.
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Only about two olive-sized meteorites fall on an area the size of Wales each year. Your odds of finding one aren’t good even if you know what to look for. Now imagine looking for a meteorite that fell millions of years agobefore being entombed in solid rock like the bones of a dinosaur. It is so difficult that it is almost laughable.
But it isn’t impossible. That much has become apparent over the past few years, as fossil meteorite hunters have unearthed them, first in dribs and drabs, then in huge numbers. It turns out they have a unique story to tell: contemporary meteorites tell us about how the solar system grew into its current form. Fossil meteorites, on the other hand, hold information about the conditions on Earth during its deep history that we can’t get any other way.
Distinguishing a meteorite from an Earth rock isn’t easy. Fresh falls often have a characteristic crust, their surfaces burnished to a shine during their passage through the atmosphere. Some are truly beautiful. But many, let’s face it, just look like rocks. The best way to decisively identify a meteorite is to delve into its chemistry. Large amounts of elements that are rare on Earth, such as nickel or iridium, are the surest sign of extraterrestrial origins.
Ancient impacts
Schmitz’s interest in meteorites took hold as a young scientist when he spent time working in the US with Luis and Walter Alvarez. The father and son hypothesised that a huge asteroid had smashed into Earth 66 million years ago, snuffing out the dinosaurs and nearly all other life. They never found any fragments of the meteorite, just a layer of iridium distributed throughout rocks of this age the world over, presumably leftover from when the meteorite vaporised on impact. But what really caught Schmitz’s attention was the idea that events in space could affect the course of Earth’s history. The Alvarez duo had identified one point of influence. Were there others?
To find out, Schmitz want…ed to discover some proper fossil meteorites. When he returned home to Sweden in the early 1990s, there were barely any known to science. Then one day Schmitz, who now works at Lund University, read a newspaper report about an amateur geologist named Mario Tassinari, who had found a few fossil meteorites in a quarry on the southern shore of Lake Vänern, Sweden. He and Tassinari soon agreed to collaborate on a study of the quarry. It wasn’t difficult – the quarry workers cut out slabs of rock systematically to turn them into floor tiles. Schmitz and Tassinari simply asked the team to call them any time they came across a slab with an imperfection. About four times a year for the next few years, the workers would find a black smudge in the rock that turned out to be an ancient meteorite.
It seems that the limestone in this particular quarry was created in a way that was uniquely suited to preserving ancient meteorites. It formed very slowly, allowing time for meteorites to accumulate and be locked away. Schmitz investigated other quarries, but he hasn’t found any others quite like this one. Eventually, he got the idea of going hunting in places where tiles from this quarry had been used to make floors. This is what led him to crawl around on his knees in public buildings across Europe. But, for a long time, his finds all came from freshly quarried rock.
By the early 2000s, Schmitz had more than 40 fossil meteorites. He began to think that this was a lot. When he did the sums, it seemed like there must have been a significantly greater influx of meteorites hitting Earth when this rock formed than there is today, which seemed odd. But with a sample size of just a few dozen meteorites, it was hard to be sure. He needed more – and he had a plan to get them.
Schmitz’s proposal relied on the fact that, while large meteorites are incredibly rare, they become increasingly common the smaller they are. When you get down to tiny particles of space dust, there are a huge number of them. We estimate that nowadays Earth is sprinkled with about 100 tonnes of micrometeorites – extraterrestrial dust particles less than 1 millimetre wide – every year.
Finding these specks in ancient limestone was a huge challenge. Schmitz’s idea was to look for an extremely hardy mineral found in meteorites called chromite. He reasoned that he could use strong acids to dissolve chunks of limestone from the quarry and that practically indestructible motes of chromite would be left behind. By 2003, he had used this method to harvest just over 500 extraterrestrial chromite grains from the quarry rock that had been formed during a period called the Ordovician, which began between about 485 and 490 million years ago. He looked at the numbers of sizeable meteorites and chromite grains in this rock and found that, in one small section, the frequency of both shot up massively. It was strong evidence that, for about 1 or 2 million years, that was 100 times more intense than usual.
An epic collision
What was going on? Schmitz’s best explanation is that this uptick was caused by an incident known as the L-chondrite parent body break-up event. This was a massive collision between rocks in the asteroid belt beyond Mars probably involving one huge asteroid, which is thought to be the source of many of the meteorites that land on Earth to this day. It must have been truly epic, releasing great tides of dust and rock into the inner solar system.
At the same time, Earth was experiencing an turning point to rival the extinction of the dinosaurs: the Great Ordovician Biodiversification Event (GOBE). Just over 500 million years ago, the progress of evolution seemed to stutter before exploding again at top speed, producing a plethora of new species. Schmitz’s hypothesis was that the break-up of that massive asteroid had increased the number of meteorites hitting Earth, which could have caused localised extinctions. With ecological niches left vacant, life spilled over again to fill them. It was a wild idea, but if he could prove it, this would be a second example of events in space influencing Earth’s history.
Meanwhile, other scattered reports of fossil micrometeorites were beginning to appear in scientific papers. These were often just a few chance finds, nothing like the quantities Schmitz was dealing with. But the whispers of thesediscoveries reached Andy Tomkins at Monash University in Melbourne, Australia, and it aroused his competitive spirit. “I wanted to find the oldest ones,” he says. He is based just a short flight away from Pilbara, home of Earth’s oldest known rocks, on the other side of Australia. It was surely worth a try.
“These meteorites represent a part of the atmosphere we would otherwise have no way to look at – it’s lost to time”
In 2014, Tomkins and his colleague Lara Bowlt, then also at Monash University, flew to Port Hedland on the coast of Western Australia, rented a four-wheel-drive vehicle and drove south into the desert. Eventually, the road ran out and they motored along a dry riverbed, then hiked the last few kilometres to the Tumbiana Formation, an area of rock that formed about 2.7 billion years ago. “It’s beautiful, rugged country,” says Tomkins.
These rocks are known for their pristine stromatolites, formations produced when layers of ancient bacteria grew. They are the oldest evidence we have for living microorganisms. “Any micrometeorites would have been raining down on an ancient, shallow sea, with the stromatolites getting all these extra nutrients from space added to them,” says Tomkins. There was no guarantee they would find anything. But Tomkins and Bowlt dug out about 10 kilograms of limestone blocks and had them shipped back to Melbourne.
There, they cut the rocks into cubes, dissolved them in acid and sifted through the insoluble remnants with magnets and sieves. iron micrometeorites. That these were extraterrestrial was apparent from the presence of an iron oxide-based mineral called wüstite, which can only form at the extreme temperatures produced when these rocks sear through the atmosphere.
These were, by far, the oldest meteorites ever found – and Tomkins realised they were remarkable for more than just their age. Picture a shooting star streaking across the sky. It flashes hot and bright at first as it smashes into Earth’s atmosphere at top speed, but then quickly slows. Tomkins realised the iron oxide in these meteorites must have formed in that brief period of initial heating, and so it captured oxygen atoms then and there. “It was the first time anyone had thought of a way to sample the upper atmosphere back in time,” says Tomkins.
The scientific consensus has long been that Earth’s atmosphere essentially zero oxygen until 2.4 billion years ago. But Tomkins and Bowlt’s fossil meteorites seemed to show that this wasn’t true. To account for the formation of the iron oxide, he and his team calculated that the upper atmosphere must have been about 25 per cent oxygen. There could have been a layer of haze that prevented the upper and lower parts of the atmosphere from mixing, meaning that any early oxygen wouldn’t necessarily have been available to life on the ground. Still, it was a startling discovery.
Rebecca Payne at Pennsylvania State University agreed that Tomkins and Bowlt found genuine fossil meteorites, but she wondered if it was really oxygen that had been responsible for creating the iron oxide. Couldn’t the oxygen have come from other sources, like carbon dioxide? Payne and her colleagues used computer modelling and experiments to investigate, and showed that if the upper atmosphere contained about 25 per cent CO2, it could have oxidised the molten iron in the way Tomkins thought oxygen had.
Tomkins now accepts that Payne is probably right. But that doesn’t diminish what he is excited about, which is using fossil meteorites as a means to probe the past. Payne is on the same page. “These meteorites represent a part of the atmosphere that we would otherwise have no way to look at – it’s lost to time,” she says. “It’s now a question of finding more.”
That is exactly what Martin Suttle wanted to do. In 2014, Suttle was studying for his PhD and decided to try fossil meteorite hunting in his spare time. One day, he found some lumps of exposed limestone on a roadside near his parents’ home in Surrey, UK. The rock turned out to be 87 million years old. If there were meteorites inside, they would have fallen during the Cretaceous, the last days of the dinosaurs.
Suttle treated the rock in a similar way to Tomkins and retrieved . Yet these tiny grains were very different from those found by Schmitz or Tomkins. They contained the sorts of iron-based minerals you would expect in a meteorite, but where the tell-tale nickel should have been, there was manganese instead.
Element swap
This was weird – except, when he thought about it, Suttle realised it made sense. The process of fossilisation often changes the composition of materials. Take dinosaur bones, which often aren’t bones at all but minerals deposited in cavities where the bone used to be. Suttle realised that fossilisation could have swapped the nickel in the meteorites for manganese.
This was a game changer. Anyone looking through ancient sediments who came across particles like the ones Suttle found wouldn’t have thought them to be extraterrestial because of the lack of nickel. “We have probably not recognised fossil micrometeorites that might have been found by other people in the past because this fossilisation process alters their chemistry,” says Suttle. “We predict that they are quite a lot more common than is appreciated.” Suttle is now leading a much wider study of fossil micrometeorites from ocean sediments in the collections of the Natural History Museum in London, and plans to use these to help reconstruct Earth’s past climate.
For his part, Schmitz hasn’t been idle. After his 2003 study, he built an industrial limestone-dissolving facility and had his team work its way through more than 1500 kilograms of the stuff, eventually amassing 900 chromite grains. His group also looked at 3000 micrometeorites found in ancient Antarctic sediments. Using both lines of evidence, he could dust flux to a specific period in the Ordovician.
Schmitz now thinks the effects of this increased flux went beyond the GOBE. He says the increasing amount of incoming dust in the atmosphere could have trigged, or at least exacerbated, the ice age known to have taken place at this time. “I think they’ve done a really good job of proving that there was a massive influx of extraterrestrial material around about that time,” says cosmochemist at Arizona State University. “This certainly could have exacerbated the situation if there was already some cooling going on, in a runaway effect.”
You might be wondering if Schmitz’s search for fossil meteorites in floor tiles ever paid off. The answer is yes: in all his years of searching, he found precisely one. He won’t reveal where it is, for fear of prompting too many visitors. All he will say is that it is in a small railway station in southern Sweden, hidden under a staircase. “The people who placed it obviously thought: “This tile is ugly’,” says Schmitz. “To them, it’s a second-grade rock.”
