Have you ever been to a dripstone cave? A cave mesmerizingly filled with the most tantalizing rock formations and all that made by water. Minerals dissolved in the water bring these rocks to life; they grow over very long timescales of thousands of years. Something similar happens on the ocean floor in the form of ferromanganese crusts on much longer timescales.

Ferromanganese (FeMn) crusts are beautiful

These metallic ores form on the bottom of all major oceans by mineral precipitation. Their richness in cobalt and rare-Earth elements, critical resources for technology, in particular batteries, made them well-known to industry and politics.  However, their growth takes millions of years for only a few millimetres. This makes them an ideal and precious archive to monitor Earth's environment over millions of years and requires preservation from excessive deep-sea mining.

The dating of these FeMn crusts can be accomplished through the radioactive isotope Be-10 (¹⁰Be). This isotope is produced by galactic cosmic rays striking Earth's atmosphere and breaking nitrogen and oxygen apart. It settles down to Earth within years and finds its way into the oceans and into FeMn crusts. The radioactive decay of ¹⁰Be with a half-life of 1.4 million years starts the clock required for dating. The only thing to do: Measure the amount of ¹⁰Be in different layers of the crust.

We accomplished this task by single-atom counting using a  technique called accelerator mass spectrometry (AMS). This ultra-sensitive method allowed us to measure ¹⁰Be concentrations in several Pacific ferromanganese crusts and date them accurately over more than 10 million years. A typical ¹⁰Be profile in a ferromanganese crust is characterized by a smooth decline of ¹⁰Be concentration with depth.

Something unexpected happened

The smooth decline stopped around 10 million years ago in one of our ferromanganese crusts which means there is more ¹⁰Be than expected. At first we were puzzled and thought of mundane reasons; a FeMn crust though is still a natural sample; deviations might be explained by changing growth rates, sediment coverage or cracks. But this turned out to be much more than that. The first FeMn crust we dated was from the Central Pacific; south-east of Hawaii. A new sample far away was needed to rule out sample specific reasons. By a lucky coincidence, a different FeMn crust from the Northern Pacific, north of Hawaii and almost 3000 kilometres away from the initial crust, was examined a few years prior for interstellar traces. An unprocessed profile was still available. We saw our chance coming.  After analysing this new FeMn crust in the relevant time interval, it became clear: This anomaly is present in all of them. Further searches in previously studied crusts revealed that another crust from the Pacific also shows this anomaly and no data so far disagrees with the anomaly. This exciting discovery was the start of more work and a dive into oceans, Earth, the Sun,  exploding stars and interstellar clouds.

What is the reason for this anomaly?

There are two options in general: Either the production of ¹⁰Be stays the same and the distribution on Earth changes, or the distribution stays the same but more is produced. Well-known anomalies seen in tree rings and ice cores were related to the Sun's activity and Earth's magnetic field. But this time, this is rather unlikely. The discovered ¹⁰Be anomaly lasts for too long and is too intense to be attributed to the Sun or Earth's magnetic field.

So we turned towards the ocean. What if the global ocean currents changed and transported more ¹⁰Be to the Pacific? This radical idea was at first rather unappealing to us, but then fate took a turn. Early 2024, a research team found evidence for the onset and ramp-up of the Antarctic Circular Current, a driver for global ocean circulation, around 12-10 Myr ago. A striking concurrency. This hypothesis could be tested by analysing more marine archives from different locations on Earth. A changing ocean current pattern should result in characteristic and distinct imprints.

But similarly stunning, another research group delivered in 2024 unwittingly an alternative explanation for the ¹⁰Be anomaly -  the encounter of the solar system with a dense interstellar cloud. Such a collision would lead to a compression of the heliosphere, the protective shell of our solar system. Earth would lie bare and unprotected against the harsh galactic cosmic radiation. This would result in an increase of cosmic radiation and therefore production of ¹⁰Be. Something similar would be achieved by a near-Earth supernova, an exploding star.

How to use this anomaly?

The question of the origin of the anomaly requires further investigations of marine archives. One key feature of this anomaly, however, is that, if present in more samples, it could serve as an independent time marker for the  transition of the Middle Miocene to the Late Miocene. The start of the anomaly was dated to around 11.5 million years and would therefore be perfectly suitable to date this transition. Even a more pronounced but less time-extended anomaly, possible due to a potential widening of the signal in the ferromanganese crust, would still be an independent marker for the Late Miocene.

Future investigations will clarify the origin of this anomaly and hopefully lead to a wide application in paleoceanography when dating marine archives.