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Life invented photosynthesis from 3.5 billion years ago ...
:: 01 January, 2002
The atmosphere of our planet contains the oxygen in large quantity from 2.5 billion years. At least what we thought until the recent publication in Nature of a new analysis of rocks in the Pilbara region in Australia. Some geochemists, the atmosphere of the Earth might be like today hundreds of millions of years ago!
The photosynthesis appeared in some primitive organisms between 2800 and -2400 million years if we are to believe certain geological land. But some make up much earlier on the assumption that the stromatolites which are found in even the oldest layers are the product of biological activity. Currently, we are confident that for those from -2724 million years but stromatolites existed on Earth there are 3.5 billion years.
Anyway, one thing remains on the huge deposits of iron Hamersley Basin in Australia. They date from the era of Siderien while the surface of the continents had become large enough to form shallow sea surrounded by wide continental shelves. The conditions were satisfied for large carpet bacteria build stromatolites in large quantities and releases en masse oxygen through photosynthesis.
This gas was then able to oxidize the iron in solution in the oceans and cause its precipitation as iron hydroxide, carbonate of iron, silicate and sulfide according to changes in acidity and the degree of redox of seawater
This is called the Great Oxidation or Catastrophe of oxygen.
To -1900 million years, almost all of the iron in the oceans had precipitated and was found today in large deposits of minerals such as global Hamersley. Oxygen production by stromatolites did not stop and if the oceans have begun to release this gas into the atmosphere, which quickly reaches a rate of 15% of the current value.
At least the standard scenario accepted by the community of geochemists. A group of geologists with among them the geochemist Hiroshi Ohmoto of Penn State University believe that we must review this case, just as they explain in an article in Nature Geoscience.
A very thick oxide layer and deep ...
They are based on analysis of recent filings red jasper found in the region of the Pilbara craton of Australia. This mineral is a sedimentary rock containing 90 to 95% silica, often ranked with quartz, microcrystalline, and sometimes made up of radiolarian calcedonieux caught in a cement of chalcedony. Drilling in these layers have shown that they were associated with hydrothermal vents similar to those we know today at the bottom of oceans such as the City lost.
Because it is rich in hematite, a mineral composed of iron oxide III formula Fe 2 O 3, it was thought that this jasper from a subsequent oxidation of the rocks exposed to the air free or at least installed under layers of deep water while the Earth's atmosphere became rich in oxygen. Pilbara rocks dating -3.46 indeed billions of years long before the Great Oxidation.
A first problem arose when the researchers realized that drilling showed excessive thickness of red jasper to be the product of an altered surface. But today we see that compounds rich in iron floating on the ocean surface are transformed into oxides under the action of the rays UV and thus not necessarily mean that water or air rich in oxygen.
When they sank to the bottom, in the form of small particles, it is sufficient for these oxides are heated by hydrothermal vents more than 100 ° C for the hematite that appears. It could explain the presence of a large number of layers in a continuous process of deposition of these particles. But this hypothesis should be tested.
It is an observation by electron microscopy to see whether we are in the presence of aggregates of small grains of hematite or whether large crystals having precipitated in water rich in oxygen. The second hypothesis proved to be good and it has fascinating implications ...
Nobody doubts that the layers have 3.46 billion years. This means that, at that time, they were formed from contact with fluids rich in iron and with warm sea water charged with oxygen .... And all at great depth!
We can not as easily escape the conclusion that not only the oceans of the entire planet were themselves rich in oxygen, but probably also to saturation, which implies an important degassing and an atmosphere containing oxygen in large quantities there are 3.46 billion years.
The discovery of photosynthesis by life would be old and this conclusion fits perfectly with the age of the oldest known stromatolites, remember, are dated 3.5 billion years. Going even further, this suggests that the traces of carbon residue that some interpret as resulting from biological activity is -3.8 billion years, because of their unusual enrichment in 12 C, are evidence of the presence of life and photosynthesis at that time postponed.
