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," says
Hiroshi Ohmoto, professor of geochemistry, Penn State. "That is why we
wanted to drill deeper, below the water table and recover unweathered
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," says 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 reports in a recent issue ofNature Geoscience.
The next step was
to determine if the hematite formed near the water's surface or in the depths.
Iron compounds exposed to ultraviolet light can form ferric hydroxide, which
can sink to the bottom as tiny particles and then convert to hematite at
temperatures of at least 140 degrees Fahrenheit.
The key to determining if ultraviolet 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 ultraviolet
radiation," reveals 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," says 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
Coring Rock Layers Billions of Years Old
April 7, 2009