To paraphrase Jon Stewart, the United States after the presidential election is the same as it was before. However, the Earth is certainly not the same since it was showered by the flotsam of numerous supernova explosions around 2 million years ago. In this post, which is a followup to this one, we review a paper which purports to present evidence of those long ago supernovae as recorded in microfossils at the bottom of the ocean. The paper’s abstract is at the end of this blog post.
First, what are microfossils? In the present case, they are the remains of bacteria which populate the ocean floor. Usually, the remains left behind by bacteria are anomalous chemical signals, such as the banded iron formations made by photosynthesizing bacteria. Since bacteria are single celled creatures, they rarely leave fossils of their actual body parts (organelles). However, some bacteria do produce hard internal structures that can get left behind. One incredible type of bacteria does leave actual fossils are called Magnetotactic Bacteria (MTB). These bacteria ingest iron from ocean water and manufacture tiny magnets within their single-celled bodies, which they can then use to orient to the Earth’s magnetic field. When the bacteria die, these little magnets, called magnetosomes, remain as microscopic chains of magnetite. Unambiguous fossils of these little guys can be found going back to almost a billion years.
Ludwig, et al., perform analyses on MTB and other iron microfossils from two oceanic drill cores from the Pacific Ocean. Most of the iron within the microfossils is made up of typical 58Fe, but the analysis presented here demonstrates a spike in 60Fe within these magnets around 2-2.5 Million years ago (Ma). As stated before, 60Fe is most likely produced by supernova explosions, but it can also be delivered to the Earth by meteorites. Ludwig, et al., isolate supernova-specific 60Fe via a careful chemical leaching technique which draws out only secondary iron oxides and microscopic grains, thus leaving any potential micrometeorite particles in the discarded residue. The leached material was then analyzed by accelerator mass spectrometry. They found a clear spike in the ratio 60Fe/Fe around 1.8-2.6 Ma in both samples, and attributed the spike to the debris of multiple supernova explosions.
As noted previously, the supernovae were probably associated with the formation of the so-called Local Bubble. According to the authors:
The Local Bubble is a low-density cavity ~150 parsecs (pc; 1 pc = 3.09×1016 m) in diameter, within the interstellar medium of our galactic arm, in which the solar system presently finds itself. It has been carved out by a succession of ~20 [supernovae] over the course of the last ~ 10 Ma, likely having originated from progenitors in the Scorpius-Centaurus OB star assocation, a gravitationally unbound cluster of stars ~50 pc in radius.
A future Stone Telescope post will discuss the possible relationship between the Local Bubble supernovae and the Pliocene-Pleistocene geologic boundary, but for now let’s consider two aspects of this story: 1) the use of geology for historical astronomy, and 2) the plight of the magnetotactic bacteria.
Geology as a temporal telescope
Typical presentations of astronomy compare looking through a telescope to traveling in a time machine. Lightspeed is finite, and the closest star to our solar system is a few light years away. Just like Han Solo’s 12 parsec Kessell Run is strange, since a parsec is a unit of distance and not a unit of time, a light year is a unit of distance, not time – it is how far an object would travel in one year if traveling at the speed of light. Say something happens 100 light years (ly) away from the Earth. The absolute soonest that we would know anything about that is 100 years after it happened, when its radiation finally reaches our planet. Therefore, looking through a telescope, we see objects as they were long ago.
However, when you see an object through a telescope, you are not looking at a stop-motion picture. What you see is changing. For example, Johannes Kepler saw a supernova in 1609. Astronomers have located the remnant of this supernova, which is now a cloud of plasma much larger than the original star. It’s about 13,000 ly away, which means the actual supernova occurred about 13,407 years ago. In other words, when we look at the remnant today, we are seeing it as it was 407 years after Kepler observed the explosion. To see what Kepler saw, we need to read his famous book on the subject. There is no other physical evidence to see through the telescope.
In geology, the physical evidence is still there! We can, in a sense, pick chunks of that astronomical event up off of our planet’s crust. Our planet is a net that captures the stuff of cosmic phenomena, and preserves it for future scientists to study. In the present case, those little magnetotactic bacteria caught pieces of supernovae and incorporated them into their tiny bodies, which are preserved to this day for us to find.
Those little bacteria
But, what do the bacteria care? They were just huffing up iron ions they found in the sludge at the bottom of the ocean. Could they tell the difference between the usual 58Fe and that rare delicacy 60Fe? Maybe they couldn’t, but maybe they could.
Vladimir Vernadsky famously emphasized that different organisms are characterized by different atomic weights of specific elements within their bodies. The calcium in a horse would have a different atomic weight than the calcium in a mushroom, which means a different ratio of calcium isotopes. Vernadsky believed that organisms sought out and selected specific isotopes of elements with which to build their bodies. That as just a brief indication, maybe the magnetotactic bacteria could tell the difference between the usual fare and the exotic 60Fe.
Maybe the 60Fe made slightly better magnets? Biological functions have been shown to respond slightly due to isotope variations, for example in ATP synthesis. If there was some type of advantage in taking in 60Fe, perhaps this lent an evolutionary advantage to those bacteria that could tell the difference?
But, even if they couldn’t tell the difference, they were organisms that tasted of the supernovae. Perhaps other organisms felt the effect of those supernovae as well. But that’s an investigation for next time.
Massive stars (M≳10 M⊙), which terminate their evolution as core-collapse supernovae, are theoretically predicted to eject >10−5M⊙ of the radioisotope 60Fe (half-life 2.61 Ma). If such an event occurs sufficiently close to our solar system, traces of the supernova debris could be deposited on Earth. Herein, we report a time-resolved 60Fe signal residing, at least partially, in a biogenic reservoir. Using accelerator mass spectrometry, this signal was found through the direct detection of live 60Fe atoms contained within secondary iron oxides, among which are magnetofossils, the fossilized chains of magnetite crystals produced by magnetotactic bacteria. The magnetofossils were chemically extracted from two Pacific Ocean sediment drill cores. Our results show that the 60Fe signal onset occurs around 2.6 Ma to 2.8 Ma, near the lower Pleistocene boundary, terminates around 1.7 Ma, and peaks at about 2.2 Ma.