Echoes of Ancient Cataclysm Heard Through Ocean Rock
Although I should be joining the angst against our newly elected president, this paper fell into my lap. It precisely represents the purpose of my blog, and so reviewing it is probably a better way to spend my time than yelling at idiots on facebook.
A neat paper just went up on arxiv.org: The Link Between the Local Bubble and Radioisotopic Signatures on Earth. It came to my attention via the Astrobiology Web, where they published the abstract. I’ll throw up the abstract below, after a quick summary of the paper and its predecessor.
The iron isotope 60Fe is quite rare in nature. It’s only created during cataclysmic events – yes, more cataclysmic than the 2016 presidential election. Think supernovae, or worse. It has a half-life of about 2.6 million years (Myr), which means that none of the 60Fe generated 4-5 Million years ago (Ma) during the creation of our solar system is left. We know it existed, though, because its echo is preserved in its daughter isotope, 60Ni. This isotope of nickel is also quite rare, is found in super old Earth crust, and is created only as the decay product of 60Fe. Thus, any 60Fe we find in the crust today must have salted the Earth only a few million years ago.
An earlier paper by Wallner, et al., Recent near-Earth supernovae probed by global deposition of interstellar radioactive 60Fe, presented mass spectrometer analysis of eight portions of ancient ocean crust scattered around the world, each containing tiny amounts of <sup>60</sup>Fe. This 60Fe was most likely produced by supernova explosions within the last 10 Myr or so. It just so happens that astronomers have found possible evidence of these explosions, in the bodies of stars and matter within the Scorpius Centaurus Association, which stretches from Antares in Scorpius all the way down to include the Southern Cross. These supernovae likely inflated a cavity within the local Orion Spur, a dense part of the Milky Way’s interstellar medium. In fact, evidence of this cavity has been explored for the past few decades, and it has the name Local Bubble – our solar system has been traveling inside this cavity for at least the past 10 Myr. Hence, the 60Fe found by Wallner, et al., sounds like an echo of this supernova chorus that sang out so near our planet a few million years ago.
The authors go further in the arxiv.org paper: Feige, et al.. Here, several stars from the Scorpius Centaurus Association are identified as potential precursors to the Local Bubble supernovae. Tracing the stellar trajectories back in time, Feige et al. create a synthetic history of the Local Bubble to model what Earth’s environment may have encountered. They find that, according to the model, the best fit to the 60Fe signal found in the Earth’s crust occurs at about the time our solar system passed through the shell of the expanding bubble, 2-3 Ma. They note that the oceanic drill cores also indicate a supernova signal around 6.5-8.7 Ma, which they will attempt to model in the future.
It should be noted that Wallner, et al., are not the first to identify live 60Fe in the Earth’s rock. Other researchers have found such deposits in the single domain magnet chain fossils of magnetotactic bacteria, again, in the same rough date range (~3 Ma). The transition from Pliocene to Pleistocene also occurred around this time, and may have had some direct causal relationship with the supernovae that peppered the Earth with 60Fe. Maybe we’ll investigate some of these loose ends in future posts!
Traces of 2-3 Myr old 60Fe were recently discovered in a manganese crust and in lunar samples. We have found that this signal is extended in time and is present in globally distributed deep-sea archives. A second 6.5-8.7 Myr old signature was revealed in a manganese crust. The existence of the Local Bubble hints to a recent nearby supernova-activity starting 13 Myr ago. With analytical and numerical models generating the Local Bubble, we explain the younger 60Fe-signature and thus link the evolution of the solar neighborhood to terrestrial anomalies.