A Himba myth of a dragon, living in a crack deep under the earth whose poisonous breath, when it rises to the surface, poisons all plants and causes “fairy circles” in the Namib Desert may well prove to be based on scientific fact.
Recent research suggests these mysterious circles may be caused by natural gas bubbles, bubbling up from below.
Fairy circles, which are circular depressions devoid of any vegetation but often surrounded by a fringe of taller growth, occur in a belt that stretches from southern Angola to the Northern Cape in a belt between 60km and 120km from the Atlantic coastline.
For decades the mysterious manner in which they have baffled scientists, with various theories advanced: spots of concentrated radio-activity, poisonous gas generated by termites, or by toxins released by the Euphorbia damarana plant.
But in a little-noticed article published last year May by the Journal of Arid Environments, a team of University of Pretoria scientists proposed a new hypothesis that the fairy circles are caused by a geo-chemical rather than a biological factor.
Led by botanist Professor Gretel van Rooyen from the department of botany, Yvette Naude and Professor Egmont Rohwer of the chemistry department set out to test their suspicion that circles are formed by degassing.
Bare soil circles, newly-formed circles and in between the circles were tested for carbon monoxide emissions as an indicator of a natural gas microseep.
Although they were not able to measure specifically for methane (the portable gas detector lacked a methane sensor), laboratory tests of the soil using gas chromatography-mass spectrometry revealed geochemical biomarkers (low volatility alkanes) typically associated with fossil hydrocarbon seeps. Thus, a geochemical origin of the carbon monoxide gas was confirmed.
Van Rooyen, however, had over the past 12 years exhaustively tested various other theories advanced for the circles, eliminating them one by one. The gas seep theory was not new: it was previously believed gas originating from fermented grass seeds collected by termites were causing these dead zones within the circles.
In general, gas seepage changes the soil atmosphere. Van Rooyen’s work showed the roots of plants growing in the circles did not contain any of the aerobic mycorrhizal fungi, whereas the roots of plants growing between the circles usually contained these oxygen-loving organisms.
Tests conducted in conjunction with the Polytech of Namibia’s Dr Willem Jankowitz also showed plants grown in pots shriveled and died off when placed with open bottoms in the fairy circles, suggesting the presence of a gaseous substance. Plants in pots not open to the soil below showed no ill effect from being in the circle, she said.
But Naude is careful to draw too many conclusions from the results thus far. To be absolutely sure, they would have to conduct further tests to identify the mixture of gases that appears to be emerging from within the fairy circles, she said.
Natural gas consists of mostly of methane.The geochemical biomarkers point to natural gas originating from fossil material, she said.
In addition, vegetation on the edges of the circles grow better than adjoining areas, suggesting that nutrients are also being carried to the surface by the rising gas bubbles.
The researchers also discovered that magnetic absorptive soil stirrers used to test the soil accumulated a tar-like substance rich in magnetite (a magnetised form of iron oxide). Shallow magnetic formations are known to be associated with hydro-carbon seepage elsewhere in the world, as are anaerobic soil conditions in areas associated with oil and gas fields.
Naude cautioned against conclusion that this was to be seen as proof of an underlying gas or oil field: “Although hydrocarbon seepage is more common than is generally believed, it is relatively rare for seeps to overlie major oil or gas fields,” she and her two co-authors noted in their paper on the subject.
But until they had final proof, she also preferred the Himba myth as an explanation, Naude quipped.
A previous version of this article appeared in the March 16 to 22 2012 edition of the newspaper