This article is republished from Eos, read the original article.
Micrometeorites, unlike their larger brethren, rarely get a spotlight at museums. But there’s plenty to learn from these extraterrestrial particles, despite the largest of them measuring just millimeters across.
Nearly 50 tons of extraterrestrial material fall on Earth every day, and the majority of that cosmic detritus is minuscule. Micrometeorites are, by definition, smaller than 2 millimeters in diameter, and they’re ubiquitous, said Fabian Zahnow, an isotope geochemist at Ruhr-Universität Bochum in Germany. “You can basically find them everywhere.”
Researchers recently analyzed fossilized micrometeorites that fell to Earth millions of years ago. They extracted whiffs of atmospheric oxygen incorporated into the particles and showed that carbon dioxide (CO2) levels during the Miocene and Cretaceous did not differ wildly from modern-day values. The results were published in Communications Earth and Environment.
Newly fallen micrometeorites can be swept from rooftops and dredged from the bottoms of lakes.
Zahnow and his collaborators, however, opted to turn back the clock: The team analyzed a cadre of micrometeorites that fell to Earth millions of years ago and have since been fossilized. The team sifted through more than a hundred kilograms of sedimentary rocks, mostly unearthed in Europe, to discover 92 micrometeorites rich in iron. They added eight other iron-dominated micrometeorites from personal collections to bring their sample to 100 specimens.
Metal-rich micrometeorites such as these are special, said Zahnow, because they function like atmospheric time capsules. As they hurtle through the upper atmosphere on their way to Earth, they melt and oxidize, meaning that atmospheric oxygen gets incorporated into their otherwise oxygen-free makeup.
“When we extract them from the rock record, we have our oxygen, in the best case, purely from the Earth’s atmosphere,” said Zahnow.
Ancient carbon dioxide levels
And that oxygen holds secrets about the past. It turns out that atmospheric oxygen isotope ratios—that is, the relative concentrations of the three isotopes of oxygen, 16O, 17O, and 18O—correlate with the amount of photosynthesis occurring and how much CO2 is present at the time. That fact, paired with model simulations of ancient photosynthesis, allowed Zahnow and his colleagues to infer long-ago atmospheric CO2 concentrations.
Reconstructing Earth’s atmosphere as it was millions of years ago is important because atmospheric gases affect our planet so fundamentally, said Matt Genge, a planetary scientist at Imperial College London not involved in the work. “The story of the atmosphere is the story of life on Earth.”
But Zahnow and his collaborators first had to make sure the oxygen in their micrometeorites hadn’t been contaminated. Terrestrial water, with its own unique oxygen isotope ratios, can seep into micrometeorites that would otherwise reflect atmospheric oxygen isotope ratios from long ago. That’s a common problem, said Zahnow, given the ubiquity of water on Earth. “There’s always some water present.”
The team found that the presence of manganese in their micrometeorites was a tip-off that contamination had occurred. “Extraterrestrial metal has basically no manganese,” said Zahnow. “Manganese is really a tracer for alteration.”
Unfortunately, the vast majority of the researchers’ micrometeorites contained measurable quantities of manganese. In the end, Zahnow and his collaborators deemed that only four of their micrometeorites were uncontaminated.
Those micrometeorites, which fell to Earth during the Miocene (9 million years ago) and the Late Cretaceous (87 million years ago), suggested that CO2 levels during those time periods were, on average, roughly 250–300 parts per million. That’s a bit lower than modern-day levels, which hover around 420 parts per million.
The team’s findings are consistent with values suggested previously, said Genge, but unfortunately, the team’s numbers just aren’t precise enough to conclude anything meaningful. “You have a really huge uncertainty,” he said.
The team’s methods are solid, however, said Genge, and the researchers made a valiant effort to measure what are truly faint whiffs of ancient oxygen. “It’s a brave attempt.”
In the future, it would be valuable to collect a larger number of pristine micrometeorites dating to time periods when model reconstructions suggest anomalously high CO2 levels, said Zahnow. “What we really hoped for was to get pristine micrometeorites from periods where the reconstructions say really high concentrations.”
Confirming, with data, whether such time periods, such as the Triassic, truly had off-the-charts CO2 levels would be valuable for understanding how life on Earth responded to such an abundance of CO2.
