Oxygen was as abundant 3.46 billion years ago as it is today

March 25th, 2009 - 12:36 pm ICT by ANI  

Washington, March 25 (ANI): Analysis of deep sea rocks in Australia has suggested that they date back to 3.46 billion years ago, suggesting that not only did the oceans contain abundant oxygen then, but that the atmosphere was as oxygen rich as it is today.

The researchers drilled diagonally into the base of a hill in the Pilbara Craton in northwest Western Australia to obtain samples of jasper or hematite-rich chert 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 Hiroshi Ohmoto, professor of geochemistry, Penn State.

The next step was to determine if the hematite formed near the waters 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, said Ohmoto. So, just because there is hematite, there is not necessarily oxygen in the water or the atmosphere, he added.

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, he added.

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 the present day atmosphere. (ANI)

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