05 Origin of oxygen: the largest extinction on earth's history

In connection to an iron ore introduced in the previous essay, I have focused on oxygen this time, which played a vital role in the formation of an iron ore. The origin of oxygen is also intriguing. The evidence that oxygen had been generated huge in mass in a short term could be found in far north of Canada.

We inhale air unconsciously every day. By taking in air, we absorb oxygen and discharge carbon dioxide in the process of generating energy. It is a physiological phenomenon, called respiration. Most organisms on earth breathe. Then, how is oxygen generated and where does carbon dioxide go?

A school textbook taught us that photosynthetic organisms discharge oxygen in exchange of absorbing carbon dioxide. Photosynthetic organisms decompose carbon dioxide in order to absorb carbon, which becomes a component of their bodies.

If oxygen is solely generated from photosynthetic organisms, it means that the birth of oxygen originates in the birth of the organisms, and until then the world had been the place without oxygen. In other words, the generation of oxygen and the birth of photosynthetic organisms on earth are identical.

When did the first photosynthetic organisms appear on earth? The answer can be found in fossils and remains of the organisms.

Fossils of cyanobacteria, the first photosynthetic organism appeared on earth, have been discovered from the stratum of approximately 3.2 billion years ago. There is another theory, which states photosynthetic organism existed even before that, but it fails to get consensus from scientists. The dominant theory is that the first photosynthetic organisms appeared on earth approx. 3.2 billion years ago and from the fact that huge amount of cyanobacteria fossils were discovered concentrated in the stratum of approx. 2.7 billion years ago, the organisms have irrupted at that time.

The fossils were discovered from rocks called "stromatolite". In the cross section, a stromatolite is a concentric circle of a few score centimeters diameter and about 1m high. Numerous layers at a space of as small as a few millimeters can be found in the vertical section of a stromatolite, altogether forming a mushroom-like pattern. This duplicate layer system of a stromatolite is made of numerous clusters of minute cyanobacterias.

Great many stromatolites were discovered from the stratum of approx. 2 billion years ago in the eastern shore of Great Slave Lake in Northern Territory of Canada. Of all the stromatolites I have ever seen, landscape there was the most overwhelming.

Numerous stromatolites are stretched all along the lakeside. Just imagine very big mushrooms are tightly placed as far as you can see. Even up from the sky on a hydroairplane, I could confirm the magnitude of the scale.

I have mentioned in the previous essay that the sudden increase of oxygen in seawater led to the deposit of iron ore in the sea bottom. The sudden increase of oxygen was caused by the irruption of cyanobacteria. Huge amount of iron ores and huge amount of stromatolites had been formed almost coincidently with a strong correlation.

A cyanobacterium is the first organism that generated oxygen on earth. What would the world have been like if there had been no oxygen at all? Atmosphere must have been mainly consisted of carbon dioxide and nitrogen.

Oxygen changed the world upside down for creatures on earth. Almost all the creatures on earth at present can intoxicate oxygen and utilize it, through mitochondria in cells.

Organisms, which had lived on earth without oxygen, could no longer exist ever since oxygen was added in the atmosphere. Because once oxygen intrudes in cells of their body, all the components start to dissolve by oxidization. In other words, for creatures, which did not have mitochondria, oxygen worked as a highly toxic substance.

More than 2 billion years ago, sudden generation of oxygen caused by irruption of cyanobacteria triggered an extensive destruction to earth's environment. Global scaled pollution by oxygen resulted in massive extinction of species. Although the actual facts about the extinction have not been revealed, there is no doubt that the scale had been the largest of all the extinctions on earth ever experienced in its history.

Environmental problems on earth are raised as points of issue in various occasions now. But earth had lived through by far the larger catastrophe. Wonderfully enough, some organisms had survived such catastrophe, and others had learned to make use of the highly toxic substance and developed an even more efficient system in their body. There is no doubt that such organisms are the ancestors of human being as well as any other organisms living on earth at present.

May 1st 2003
Yoshiyuki Koide

ASTER image
Image - 1 A satellite image of eastern Great Slave Lake (ASTER/VNIR observed on November 2nd 2000)
For enlargement (jpeg file 2.8MB)

This is the false color image of one entire ASTER scene (60x60km), of eastern part of Great Slave Lake. The colors red, green and blue are allocated to ASTER band 3, 2 and 1 respectively. The difference of stratums is well reflected by the difference in colors.

ASTER image
Image - 2 The enlargement of image - 1
For enlargement (jpeg file 0.5MB)

This is an enlargement of 7x7km in Image - 1 centering on the stromatolite stratum. The stratum can be separated in 5 parts based on the color appear in the image, mixture of red and green (A in the image), thin stripe pattern with much green and little white (B in the image), thick stripe pattern with much white and little green (C in the image), stripe pattern with green and while and sporadical red (D in the image) and red (E in the image). It corresponds to the difference in types of rocks. Area A shows igneous rock in later age (1.6~1 billion years ago), B shows dolomite stratum containing stromatolites, C shows limestone, D shows dolomite with stromatolites older than B, and E shows shale, the oldest stratum in the area. I talked about stromatolites in D in the essay.

location map
Figure-1 Location map


Picture - 1 Stratum at the east of Great Slave Lake in Northern Territories of Canada

A cliff of stratum is found along the lakeside. Stromatolites are contained in it. This stratum continues as long as 200km in the eastern lakeside of the huge Great Slave Lake. You can imagine the magnitude of the stromatolites formed approx. 2 billion years ago.


Picture - 2 Stromatolite

One round rock in the picture is one stromatolite. You can find numerous round rocks along the lakeside in the area.


Picture - 3 Cross section of stromatolites

You can see concentric patterns in the cross section well. The word "stromatolite" means concentric rock.


Picture - 4 Vertical section of stromatolites

Pattern in vertical section is a little different from cross section. Hundreds of layers are piled up to form a mushroom like pattern.


Picture - 5 Sample of stromatolite (vertical section)

Numerous minute stripe patterns can be found in the sample of a stromatolite (92x40mm). These stripes are comparable to the rings of a tree since it was formed by the accumulation of cyanobacteria over time.

Copyright of image - 1 and - 2 belongs to JSS, while Picture - 1~ 5 and the text belong to Prof. Yoshiyuki Koide of Sapporo Gakuin University. Permission of JSS is need of their use for other purposes.

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