It is generally accepted today that a mass extinction of species – notably but not exclusively the dinosaurs – happened about 65 million years ago. However, that is only one of several occasions on which the development of life on Earth was thrown out of kilter by a devastating event that was almost literally earthshattering. These events appear to have been caused either by the impact of a large extra-terrestrial body, such as a comet or an asteroid, or a massive volcanic event. Sometimes both seem to have happened at roughly the same time.
About 250 million years ago a vast amount of lava poured on to the Earth’s surface in what is now Siberia. The flows occurred over a period of around a million years, covered an area of more than 2.75 million square miles, and amounted to more than half a million cubic miles of material. This flow marked the end of the Permian period of geological history and ushered in the Triassic.
It is also conjectured that a large piece of space rock, probably several kilometres across, hit Earth at around this time in what is now Antarctica. This formed what is known as the Wilkes Land crater, which can only be seen by analyzing seismic and gravity data due to being buried deep under the Antarctic ice cap.
It has also been suggested that the asteroid impact could have caused the lava output by, in effect, cracking open the Earth’s crust. However, this theory is highly speculative and is not universally accepted.
The net result was that the planet suddenly became a very difficult place on which to live, due to the vast quantities of noxious volcanic gas that would have been belched out into the atmosphere. An asteroid impact would also have produced so much dust that the Sun would have been shielded for many years leading to plant life being severely affected. It is thought that some 96% of marine species and 70% of land vertebrate species became extinct. It could have taken as much as 10 million years for animal life to recover.
But what has all this to do with the prevalence of cancer in China?
The volcanic gases mentioned above would have caused the rain that fell to be highly acidic. This in turn would have worn away the rock it fell on in the same way that acid rain today wears away the stonework of buildings. The rainwater would have become very rich in silica, which comes from the grains of sand worn from the bedrock.
In what is now Xuanwei County in China’s Yunnan province the silica-rich water would have become the groundwater in an area of lush humid forest, which was doomed to decline due to the difficulties mentioned above. The forest would in any case have been sitting on a deep layer of peaty soil formed from many generations of growth. This peat would now be impregnated with large quantities of silica.
After millions of years of consolidation and pressure peat turns to coal, which now underlies Xuanwei County. It is this coal that is mined and burned by the local people, but it is coal that is unusually rich in silica, and the silica is blamed for causing the extra cancers.
How it does this is a matter for debate. It could be that the silica is released into the air that people breathe, and silica is a known carcinogen. Or it could be that coal fumes, which are also carcinogenic, could be made more dangerous by the silica. Or possibly both factors are at play.
Coal is widely used as heating and cooking fuel throughout China, in exactly the same way that it is in Xuanwei County, so something must account for the fact that there are 20 times more cancer cases there than elsewhere, and that applies to both smokers and non-smokers.
So could what happened 250 million years ago have such dramatic consequences today? The jury is out, but it does seem possible.
© John Welford
No comments:
Post a Comment