A page from the Anglo-Saxon Chronicle. The entry for
774 AD refers to a “red crucifix” appearing in the sky.
Earlier this month, researchers found that Japanese trees had preserved a centuries-old spike in the atmosphere’s proportion of carbon-14, apparently caused by a burst of cosmic rays that hit earth between 774 and 775 AD. But that left a perplexing mystery: the most likely source of that excess carbon-14 would be cosmic rays emitted by a supernova. But any supernova powerful enough to generate carbon-14 should have been visible to people alive then, and there was no known record of what should’ve been a pretty notable event.
Enter Jonathon Allen, a polymath undergraduate who majors in biochemistry and also has a deep interest in history. Allen dug around in a contemporary manuscript and found a reference to a “red crucifix” appearing in the sky in 774 AD. This celestial signal may have marked a supernova that birthed the cosmic rays and created the trees’ carbon-14 peak.
A tree’s rings mark both its age and the local environmental conditions, which allows researchers to track historical changes in an ecosystem. But tree rings can also encode signals from beyond Earth: Ancient trees in the Northern Hemisphere have preserved a radioactive souvenir from a 1200-year-old burst of cosmic rays.
Using tree rings, researchers have collected 3,000 years worth of data on the presence of the radioactive carbon isotope carbon-14 in the atmosphere. When examining the ebb and flow of carbon-14 over time, Japanese researchers noticed an increase during the 8th and 9th centuries CE. They decided to look at that period in detail by studying the yearly concentrations of carbon-14 in Japanese cedar trees. The cedars revealed a 1.2 percent carbon-14 spike that lasted less than a year between 774 and 775 CE, which corresponded with similar spikes in North American and European trees. This peak, twenty times the amount of variation normally caused by the sun’s fluctuations, resulted from a short-term burst of cosmic rays. But where did those rays come from—a supernova, a solar flare, or some other source?
Hardy Antarctic moss.
Ah, Antarctica. A vast expanse of ice, interrupted by mountains, ice… and more ice (with the occasional penguin). But in the East of the continent and on the Windmill Islands near Australia’s Casey research station, bare ground can actually be seen during summer months. Here Antarctica’s endemic plants dwell: lichens, terrestrial algae, and mosses. These smatterings of bryophytes are amongst the hardiest flora in the world, providing a home for a variety of minute life. They survive being covered in snow most of the year, only growing briefly during the summer months, watered by snowmelt. Except for in-person observations made over the last two decades, little definitive was known about these oases of diversity, like their age or how they might respond to changes in climate.
But now, some of the moss’s secrets are out. A recent study in the journal Global Change Biology found that some of these plants must be more than a century old, and a few may even be thousands of years old, said researcher and study author Sharon Robinson via email. On average these mosses grow at the glacial speed of 1 millimeter per year—and some of the turfs are meters thick. That means many of these unassuming mossy carpets were there when humans first made it to the continent a century ago—and likely well before. “These mosses are effectively the old growth forests of Antarctica—in miniature,” Robinson said.