When the largest volcanic eruption in the last 70,000 years spewed giant clouds of ash and debris into the air, millions of tiny microorganisms got caught up in the blast and hitchhiked hundreds of miles to new locations, researchers have found. The first record of microbes being distributed by volcano, these diatoms can help scientists figure out the volcanic source of ancient ash deposits, which offers a new, more reliable way to unlock the mysteries of Earth’s past.
The most common way to identify layers of volcanic material has been carbon-14 dating, which estimates the age of non-living substances using the decay rate of radioactive carbon atoms, but that measurement is notoriously finicky, according to Alexa Van Eaton of the U.S Geological Survey. “It’s much easier to identify a diatom than volcanic matter,” Van Eaton says, adding, that this approach “is something people haven’t thought about before.”
Van Eaton and her team excavated the prehistoric remains of the Oruanui eruption of New Zealand’s Taupo volcano, which produced 530 cubic kilometers of magma when it erupted about 24,000 years ago (by comparison, the 1980 eruption of Mt. St. Helens produced a paltry half a cubic kilometer). They took samples of volcanic matter from eleven different locations around Taupo, and found that they all contained several species of diatoms – microscopic, photosynthetic algae that live in freshwater lakes and streams.
One of the three most abundant diatom species identified, Cyclostephanos novaezeelandiae, is found only in deep freshwater lakes in the volcanic region where Taupo is located, but it was discovered in samples as far away as the Chatham Islands, 850 km south of New Zealand. Because the profile of diatoms in those samples matched the species found in the fossilized lake bed of Lake Huka, which covered Taupo at the time of the eruption, the scientists were able to conclude that the Oruanui explosion blew the diatoms from the lake into the air, where they rode the wave of volcanic debris to new locations that they would not have been able to reach by other means.
Given the size of the Oruanui eruption, Van Eaton says it’s “very likely that [the diatoms] got to [other places], we just haven’t found them yet.” Dr. Andrew Knoll of Harvard University agrees that diatoms could be found in other sites, but “you’d have to get the stratigraphy very precise” in order to conclude that the diatoms in the fossil record came from that particular eruption…you want to be able to make the claim that an endemic diatom shows up at the same time the volcano goes off,” and that it wasn’t present in a given location before the ash layer fell.
Some diatoms are classified as “cosmopolitan” and are known to exist across multiple locations, which could interfere with future studies. However, there are plenty of local diatoms like Cyclostephanos which are found in only one geographic region, and can act as a kind of unique fingerprint to trace volcanic debris back to its source eruption. According to Van Eaton, these types of species are a new “tool in the toolbox” that scientists can use to more easily and accurately identify fossilized ash layers, leading to a more complete picture of Earth’s geological history.