Early Earth may have had breathable atmosphere

[Junghalli]

Geologists discover evidence that oxygen-rich atmosphere may have existed 3.46 billion years ago.

This may have interesting implications for the search for other habitable worlds. Sure, very young planets probably wouldn't have advanced aliens, but they'd probably be great for future human colonization; oxygen-rich atmospheres and no strong toxic indigenous ecology to worry about.

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Red jasper cored from layers 3.46 billion years old suggests that not only did the oceans contain abundant oxygen then, but that the atmosphere was as oxygen rich as it is today, according to geologists.

This jasper or hematite-rich chert formed in ways similar to the way this rock forms around hydrothermal vents in the deep oceans today.

"Many people have assumed that the hematite in ancient rocks formed by the oxidation of siderite in the modern atmosphere," said Hiroshi Ohmoto, professor of geochemistry, Penn State. "That is why we wanted to drill deeper, below the water table and recover unweathered rocks."

The researchers drilled diagonally into the base of a hill in the Pilbara Craton in northwest Western Australia to obtain samples of jasper that could not have been exposed to the atmosphere or water. These jaspers could be dated to 3.46 billion years ago.

"Everyone agrees that this jasper is 3.46 billion years old," said Ohmoto. "If hematite were formed by the oxidation of siderite at any time, the hematite would be found on the outside of the siderite, but it is found inside," he reported in a recent issue of Nature Geoscience.

The next step was to determine if the hematite formed near the water's surface or in the depths. Iron compounds exposed to ultra violet light can form ferric hydroxide, which can sink to the bottom as tiny particles and then converted to hematite at temperatures of at least 140 degrees Fahrenheit.

"There are a number of cases around the world where hematite is formed in this way," says Ohmoto. "So just because there is hematite, there is not necessarily oxygen in the water or the atmosphere."

The key to determining if ultra violet light or oxygen formed the hematite is the crystalline structure of the hematite itself. If the precursors of hematite were formed at the surface, the crystalline structure of the rock would have formed from small particles aggregating producing large crystals with lots of empty spaces between. Using transmission electron microscopy, the researchers did not find that crystalline structure.

"We found that the hematite from this core was made of a single crystal and therefore was not hematite made by ultra violet radiation," said Ohmoto.

This could only happen if the deep ocean contained oxygen and the iron rich fluids came into contact at high temperatures. Ohmoto and his team believe that this specific layer of hematite formed when a plume of heated water, like those found today at hydrothermal vents, converted the iron compounds into hematite using oxygen dissolved in the deep ocean water.

"This explains why this hematite is only found in areas with active submarine volcanism," said Ohmoto. "It also means that there was oxygen in the atmosphere 3.46 billion years ago, because the only mechanism for oxygen to exist in the deep oceans is for there to be oxygen in the atmosphere."

In fact, the researchers suggest that to have sufficient oxygen at depth, there had to be as much oxygen in the atmosphere 3.46 billion years ago as there is in today's atmosphere. To have this amount of oxygen, the Earth must have had oxygen producing organisms like cyanobacteria actively producing it, placing these organisms much earlier in Earth's history than previously thought.

"Usually, we look at the remnant of what we think is biological activity to understand the Earth's biology," said Ohmoto. "Our approach is unique because we look at the mineral ferric oxide to decipher biological activity."

Ohmoto suggests that this approach eliminates the problems trying to decide if carbon residues found in sediments were biologically created or simply chemical artifacts.

[Samuel]

Sure, very young planets probably wouldn't have advanced aliens, but they'd probably be great for future human colonization; oxygen-rich atmospheres and no strong toxic indigenous ecology to worry about.

I don't think that is going to help us- aren't all the nearby stars and hence their worlds about the same age?

[Junghalli]

I don't think that is going to help us- aren't all the nearby stars and hence their worlds about the same age?

The nearby sunlike stars are more-or-less all over the place in age. This certainly expands the list of potential candidates.

[Ariphaos]

I don't think that is going to help us- aren't all the nearby stars and hence their worlds about the same age?

Delta Pavonis and Alpha Centauri are a bit older. Tau Ceti and 61 Cygni are ancient. Epsilon Eridani and Indi are younguns.

Or, in short: The age range of surrounding, nearby sunlike stars extends across a span of some nine billion years or more.

[speaker-to-trolls]

Wait a second, does this mean there was no Oxygenation Apocalypse? This has some interesting ramifications for biology.

And yes, I suppose also for space colonisation, assuming that by the time we're colonising other stars we are still concerned by trivia like the amount of free O2 in an atmosphere.

[Lord of the Abyss]

Wait a second, does this mean there was no Oxygenation Apocalypse?

To have this amount of oxygen, the Earth must have had oxygen producing organisms like cyanobacteria actively producing it, placing these organisms much earlier in Earth's history than previously thought.

This just seems to imply that the oxygenation of the atmosphere happened earlier; not that it was filled with oxygen from the beginning. So there'd just be an earlier Apocalypse.