Rock Bands Spin an Oxygen Record
The first half of Earth's history was devoid of oxygen, but it was far from lifeless. There is ongoing debate over who the main biological players were in this pre-oxygen world, but researchers are digging up clues in some of the oldest sedimentary rocks on the planet.
Most scientists believe the amount of atmospheric oxygen was insignificant up until about 2.4 billion years ago when the Great Oxidation Event (GOE) occurred. This seemingly sudden jump in oxygen levels was almost certainly due to cyanobacteria – photosynthesizing microbes that exhale oxygen.
"We don't know what is cause and what is consequence," says Dominic Papineau of the Carnegie Institution of Washington. "Several things happened at the same time, so the story still isn't clear."
To help sort out the geologic plotline, Papineau is studying banded iron formations (BIFs), sedimentary rocks that formed at the bottom of ancient seas.
Papineau's research, which is supported by the NASA Exobiology and Evolutionary Biology Program, is focusing on specific minerals in the BIFs that may be tied to the life (and death) of ancient microbes.
The iron minerals within BIFs make up the world's largest source of iron ore. However, these rocks are valuable for more than just making steel. Geologists mine them for their rich historical record that spans from 3.8 billion to 0.8 billion years ago.
The origin of the oldest BIFs is, however, a bit of a mystery. Iron will only form sediments in the ocean when it is oxidized. In the current epoch that's not a problem, since there's plenty of oxygen that can react with the iron (this is why there is so little iron in modern seawater). But BIFs that formed prior to the GOE had to have gotten their oxidized iron from somewhere else.
Although non-biological mechanisms exist, the pre-GOE iron was most likely oxidized by organisms. These simple single-celled sea creatures didn't leave us any bones or shells to pick through, but Papineau thinks there could still be mineral or geochemical fossils in the BIFs.
He and his colleagues have found carbonaceous material in BIFs associated with apatite, a phosphate mineral that is sometimes tied to biology. The implication is that the BIF builders were entombed in their own handiwork.
To verify this, Papineau's team will be studying the mineralogical structure, trace element abundances and isotopic ratios in the BIF carbon. They will compare these results to other carbonaceous-mineral associations known to be of non-biological origin, including minerals found in a Martian meteorite.
"This work has the potential to show that microbial biomass was associated and deposited together with the iron minerals," says Andreas Kappler from the
Iron-eaters vs. oxygen-exhalers
Kappler is not sure Papineau's results by themselves will be able to identify what sort of microbes, if any, built the BIFs. The most likely candidate is iron-oxidizing bacteria, or more specifically "photoferrotrophs," which use a combination of iron and sunlight to reduce carbon dioxide for making organic compounds. Modern-day versions of these bacteria (found in iron stromatolites) inhabit a wide range of marine and freshwater environments.
"Due to their simple metabolism and the fact that plenty of iron, light and CO2 were around on early Earth, the presence of photoferrotrophs as early as 3.8 billion years ago would not be surprising to me," remarks Kappler.
Photoferrotrophs, however, are not the only organisms that could have played a part in BIF construction. Alternatively, oxygen from cyanobacteria could have caused iron oxidation in the pre-GOE ocean.
But if the cyanobacteria appeared long before the GOE, why did it take several hundreds of millions of years for their oxygen exhalations to build up in the atmosphere?
Papineau and his colleagues may have found part of the answer in a complex interplay of biology and geology.
The early oxygen from cyanobacteria may have been destroyed by a preponderance of methane. The two gases react with each other to produce carbon dioxide and water.
"Oxygen can't accumulate in a methane-rich environment," Papineau says.
The methane is believed to have come from microbes called methanogens that spew out methane as a result of consuming carbon dioxide and hydrogen.
In this scenario, the methanogens and cyanobacteria shared the ancient ocean, but the methanogens had the upper hand – their methane emissions kept oxygen at bay, and also warmed the planet through a greenhouse effect. But then around the time of the GOE, these organisms went into decline, and the resulting methane-depleted atmosphere began to fill with oxygen from cyanobacteria.
No nickel to spare
Connecting the GOE to a methanogen decline has been done before, but there has been little evidence to support this hypothesis. Recently, however, Papineau and his collaborators reported in the journal Nature a significantly higher nickel-to-iron ratio in the very oldest BIFs, which they claim favored methanogen production.
The researchers reason that the higher nickel-to-iron ratio in the Earth's distant past was due to a hotter mantle, and correspondingly higher nickel content in volcanic eruptions. But around 2.7 billion years ago, the mantle cooled enough to freeze the nickel-rich rocks, and consequently the nickel abundance in the ocean fell by 50 percent.
This is significant because methanogens rely on nickel: it is a central ingredient to the metabolic enzymes involved in their methane production. When the nickel levels dropped, the methanogens presumably starved.
The nickel-famine scenario makes a pre-GOE evolution of cyanobacteria more plausible, but confirming this will take more evidence.
Kappler believes that studying the origin of the oldest BIFs could tell us when life evolved the ability to breathe out oxygen and thereby change the world forever.