Cosmic Bombardment of the Earth ca 2.2 Million Years Ago?

There are bacteria that use Iron (and other elements) to make tiny magnets that they carry around so they don't get lost. (I anthropomorphize slightly.) There are isotopes of Iron that are not of the Earth, but are found only elsewhere in the universe.

Suppose an event happened elsewhere and spewed some of that cosmic Iron isotope, say Fe-60, onto the earth, and the bacteria who were busy making their tiny compasses at that time used some of it. Then the bacteria died and were trapped inlayers in seafloor sediment and later examined by scientists looking for ... well, looking for evidence of cosmic events trapped in bacterial compasses!

Well, that happened.

A bit of sea floor was found to have Iron-60 in it a few years back. Iron-60 is radioactive and decays into Cobalt-60, with a known (but only recently known as it turns out) decay rate. That bit of rock was taken as possible evidence of an ancient supernova. The event was tied, conjecturally, to human evolution as all things must be whenever even remotely possible:

Cosmic fallout from an exploding star dusted the Earth about 2.8 million years ago, and may have triggered a change in climate that affected the course of human evolution. The evidence comes from an unusual form of iron that was blasted through space by a supernova before eventually settling into the rocky crust beneath the Pacific Ocean.

...

The team has now analysed a ... piece of ocean crust, where the supernova detritus is concentrated into a clear band of rock that can be accurately dated. The researchers found small but significant amounts of an isotope called iron-60 in the rock, which could only have come from a supernova.

"We've looked at all the possibilities and we can't find anything else that could produce such quantities," Korschinek says.

The human evolution impact idea comes from a possible cooling effect the exploding star would have had on the earth. Back in 2004 it was estimated that the earth would have been bathed in extra cosmic rays for about 100,000 years which would have, it was said, created condensation in the atmosphere which would have cooled the earth. There was a cooling event around that time (but quite possibly well after this date, so don't hang any hats on this) so I suppose this could be. But, I'm not going to assume that the cooling effects of cosmic rays are a thing at this point. I do know that people have gotten the effects of upper level vapor wrong a few times so I'm going to avoid making any assumptions about that here.

Anyway, last April, a paper was given at the American Physical Society conference giving preliminary findings related to some follow up research. Shawn Bishop and his team obtained a core from the Pacific dating to between 1.7 and 3. 3 million years ago. They sampled it at 100K intervals and extracted and separated out Iron in a way that would show Iron-60 if there was any. And ...

“It looks like there’s something there,” Bishop told reporters at the Denver meeting. The levels of iron-60 are minuscule, but the only place they seem to appear is in layers dated to around 2.2 million years ago.

And, the iron was concentrated in the target layers by the action of compass-using bacteria.

Notice the change in date from 2.8 to 2.2. This is, I think, because the half life of Iron-60 was refigured based on some intervening research. Now, the date is probably too late for a significant cooling event. But really, there were a whole bunch of cooling events from somewhere over 5 million years ago to about 2 point something million years ago, and there is a long list of candidates for what caused them, including numerous big volcanoes, continental movements, and now, a supernova.

I don't think anyone is claiming to know what star exploded.

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