The Pacific seafloor laid down around 10 million years ago is dramatically enriched in beryllium-10, compared to older and younger layers. The geologists who discovered this spike propose it could be the result of a major shift in ocean currents, which reshaped the climate for a period. Alternatively, it could indicate a nearby supernova we were not previously aware of.
Beryllium is a relatively rare element most people only know about because it occurs so near the start of the periodic table. However, geologists have a reason to love beryllium-10, as it can help date deposits too ancient to be measurable by traditional radiocarbon dating. That’s because beryllium-9 is stable, while beryllium-10 has a half-life of 1.4 million years.
The older a sedimentary sample is, therefore, the lower the ratio of 10Be:9Be, or at least that would be the case if the amount of beryllium-10 deposited in the first place was consistent. Beryllium-10 atoms are formed in the upper atmosphere when cosmic rays strike oxygen molecules, and fall to Earth within a year or two. Consequently, an upsurge in cosmic radiation could produce an increase in beryllium-10 in the sky, and soon after on land or in the deep ocean.
That’s one explanation for the spike in beryllium-10 abundance found by Dr Dominik Koll of Helmholtz-Zentrum Dresden-Rossendorf and colleagues in samples of ferromanganese crusts deposited around 10.1 million years ago in the central and north Pacific Ocean.
Such sampling needs to allow for Be-10’s decay rate. Less than 1 percent of the beryllium-10 deposited at that time would still be present. Koll said in a statement, “At around 10 million years, we found almost twice as much 10Be as we had anticipated.” Naturally the first task was to check for contamination from more recent sources where less beryllium-10 has decayed.
Once the team had tested enough samples to assure themselves the spike was real, they set about looking for explanations. A burst of cosmic rays from a supernova roughly 70 light-years away, such as produced more recent iron-60 spikes, is the most obvious and dramatic one, but others are possible.
The heliosphere protects the Earth from most of the cosmic rays astronauts would experience if they ventured far enough into space. A temporary weakening of the heliosphere, for example from passage through an interstellar cloud, would cause a beryllium-10 spike like the one the team found.
Alternatively, the explanation might be oceanic, rather than astrophysical. The Antarctic Circumpolar Current is known to have dramatically increased in strength 12-10 million years ago. “This could have caused 10Be to be unevenly distributed across the Earth for a period of time,” Koll said. “As a result, 10Be could have become particularly concentrated in the Pacific Ocean.”
Other possibilities, such as a pulse of beryllium released during a major ice-melting event, or changes to the Earth’s magnetic field, were also considered, but rejected as too small or too brief.
The ocean current explanation is relatively easy to test – if samples worldwide show the same rise, it would be refuted. On the other hand, if in other ocean basins beryllium-10 levels fell at the time, so the global average was fairly constant, currents would almost certainly be the reason.
Schematic showing how beyrellium-10 is formed in the upper atmosphere before being incorporated into sediments, and a time-adjusted chart of the amount deposited with time.
Image credit: HZDR / blrck.de
The deep ocean isn’t easy to sample, but the team are seeking to obtain cores to conduct further tests, as well as hoping other teams will do their own investigations. If the increase is global, it could be the start of a long search to work out which astrophysical explanation is correct.
Solving the question will set off plenty of new research avenues, including what the consequences for life were from either exposure to increased cosmic rays or a major redistribution of oceanic warmth.
Geologists would greatly benefit from confirmation of a global spike. Distinctive global events are needed to align the timelines for samples from different locations. Radioisotopes from nuclear testing provide these benchmarks for recent events, and solar storms known as Miyake Events serve the same purpose for thousands of years before.
Going further back, Laschamp Events, when the Earth’s poles flip, are used to synchronize timelines. “For periods spanning millions of years, such cosmogenic time markers do not yet exist,” Koll said. “However, this beryllium anomaly has the potential to serve as such a marker.”
The study is published open access in Nature Communications.
