Both of these things can’t be right: Part 2
In my last post, I discussed evidence that oxygen first appeared in the atmosphere almost exactly 2.33 billion years ago (http://bit.ly/1sMwXTK). This tiny, just discovered grain, was used to argue that oxygen must have been present in Earth’s upper atmosphere over 2.7 billion years ago…and both of these studies can’t be right.
This is a tiny sphere of iron, about a tenth the diameter of a human hair. It was one of a handful discovered by dissolving a 2.7 billion year old limestone found in the Pilbara region of western Australia. Some of these tiny bits of iron contain crystals of iron-nickel metal at their core, a signature of them being meteoritic in origin. These are therefore the oldest micrometeorites humans have yet found on this planet.
When a micrometeorite hits Earth’s atmosphere, it heats up rapidly and melts, often producing a shooting star. That grain melts quickly and then cools off within seconds to form a solid sphere, allowing a tiny bit of time for chemistry to occur.
Iron metal, when exposed to oxygen, will react rapidly and rust, forming minerals like magnetite and hematite. These reactions can take years at Earth’s surface temperature, but in the atmosphere when the sample is heated to thousands of degrees they can complete in seconds. This reaction is happening all the time today with lots of oxygen in the atmosphere, but there’s no reason why this reaction should happen at all if there was no oxygen in the atmosphere.
These grains, therefore, suggest that there must have been oxygen, in fact a lot of oxygen, in the atmosphere 2.7 billion years ago. There is good evidence that life figured out photosynthesis before this time, so it makes sense that oxygen could be present that early. The only problem is…this makes no sense with the data presented in the last post.
In the last post, I outlined that sulfur isotope changes require a lack of oxygen in the atmosphere until 2.33 billion years ago, 400 million years after these grains formed. How can these two data sets be put together?
The scientists who discovered these micrometeorites suggested one hypothesis and it has been outlined in a number of press reports. Their idea is that the Earth had oxygen in its upper atmosphere and a methane-rich haze in the lower atmosphere. The oxygen in the upper atmosphere could react with these meteorites to rust them, the lower atmosphere could still have very little oxygen, and a temperature boundary created by the haze layer could keep the two parts of the atmosphere from mixing much.
That hypothesis was presented in their paper, but there’s one thing that hasn’t been published yet in the press reports; this mechanism actually can’t work with the sulfur isotope signature!
The sulfur isotope changes I talked about in the last post don’t happen today because oxygen in the atmosphere absorbs light at the same wavelengths required to ionize sulfur. If there is oxygen in the upper atmosphere, above the sulfur, it will actually block the light from getting to the lower atmosphere.
It might well be possible to have a layered atmosphere, with oxygen in the upper atmosphere and methane in the lower atmosphere, as these scientists suggest. However, this mechanism would still shut off the sulfur isotope signal – if the oxygen is higher up in the atmosphere than the sulfur, it will block the light that is thought to cause the sulfur isotope signal!
This is complicated geology. We’re interpreting isotopes in one case and small grains in another case, both of them over 2 billion years old. Both of them tell interesting stories about the early Earth’s atmosphere. The problem is…the stories don’t add together. Both stories cannot be true at the same time - the mechanism for sulfur changes that I talked about in the last post requires there to be no oxygen in the way, while these meteorites were used to argue for oxygen in the way.
There’s something fundamentally missing from our interpretation of this puzzle. One proposal by researchers evaluating this new work is that it’s actually all the sun; the sun could be ionizing both sulfate and water in the upper atmosphere, creating free oxygen that could bond with these meteorites as they enter, but that may not be enough oxygen. This is now an interesting puzzle for future geologists and atmospheric scientists to understand.
Image credit: Tomkins et al. http://rdcu.be/ioti
Reference: http://bit.ly/1Tsy6Ih http://bit.ly/27e8FBt
