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By Yvette Naudé, Gretel van Rooyen and Egmont Rohwer
Although highly visible in grasslands, fairy circles are not exclusive to vegetated areas of the region, the north-west of Namibia. Just as powder will reveal a fingerprint, so grasslands may serve to expose the presence of fairy circles. Gretel’s keen eye discovered that these nearly circular depressions continue even in the absence of vegetation! Many decades of research by a number of scientists produced fascinating clues to the puzzle that we know as the fairy circles:
• Fairy circles debut as nearly circular zones with severely stressed (yellowing/greying) plants.
• The stressed plants do not survive and eventually a bare soil patch develops (mature fairy circle).
• After rain, seedlings appear in a fairy patch, fail to thrive and ultimately do not survive.
• Fairy circles range in size, from small to super-sized structures (even up to a kilometre), and in shape, e.g. circular, elliptical, or elongated pockmarks.
• Fairy circles are often surrounded by a halo of flourishing vegetation.
• Fairy circles may contain different plant species (e.g. annuals or shorter lived species), while perennials usually grow between the circles.
• Fairy circles have more moisture.
• It is no surprise that insects, amongst others, are attracted to fairy circles, given that circles are a source of moisture and the surrounding halo of relatively lush vegetation provides food and shelter.
• Fairy circles have unusual microbial population patterns.
• Fairy circles contain a significantly higher density of microbial organisms that thrive in an oxygen poor environment than outside the circles. Mycorrhiza, which are considered to be oxygen loving organisms, are mainly absent in the roots of plants growing inside fairy circles, whereas they are generally present in the roots of plants growing outside the circles.
• Fairy circles are zones with altered soil chemistry: even in the laboratory, plants show poor growth and eventually do not survive when planted in soil collected from fairy circles. This stress response is not observed for plants growing in soils collected from outside the circles.
• Fairy circles are zones with a subsurface seep factor: plants growing in circle soil placed in containers with sealed bottoms do survive. However, once the bottom of the container is removed the plants wither and die.
• A mathematical study shows that the space-filling nature of fairy circles’ distribution is either random, or it shows self-organising features, or it is spatially independent.
• Baby circles appear, as if by magic, when it rains. Often these satellite circles appear close to a parent (bare soil) circle.
Dr Harald van der Werff, an expert on geological remote sensing in the faculty of Geo-Information Science and Earth Observation (ITC) of the University of Twente in The Netherlands, has published extensively on circular geo-botanical phenomena (called halos) associated with gas and hydrocarbon seepage.
On being asked if these halos of stressed vegetation and bare soil spots (resembling “our” fairy circles) are unusual, he replied thus:
“Halos caused by gas are a wide-spread phenomenon. I came across a book of a Shell exploration geologist, written in 1947. This man already noted the circles as a well-known indicator for seepage in Indonesia (then a Dutch colony), and this was mentioned in a way that reflected it was actually public knowledge. More to the point, I have confirmations of halos found in the UK (carbon dioxide), Italy (hydrocarbons) and California (hydrocarbons).”
Describing halos in Hungary: “In any way, the gas simply follows the easiest route up. At the top, the recent sediments (now soils) cover the older faults, leading the gas to rise more dispersed, creating a tulip-like structure which is seen at the surface as a circle.”
Describing gas/hydrocarbon seep circles in general: “Seeps can bring up water – a reason why you can not only find stressed but also green vegetation, sometimes at the same time. A small amount of hydrocarbon might also act as a fertilizer…So, it’s complicated … I studied seepages that result from geothermal and seismic pressure. These are bound to seismically active regions, typically plate boundaries. Depending on source and the structure of the substratum, the migration mechanism and resulting surface patterns differ. Microseepage can also occur completely diffuse, where an area of several km2 might be affected as if it was one big ‘circle’ anomaly. The seeps I studied all had a central vent, and are thus not diffuse, apart from dispersion in the few meters of soil close to the surface. Depending on soil condition, and the substratum, the sizes can thus differ enormously – the very reason why remote sensing of these phenomena is difficult, as each seep may look different.”
So, the fairy circle phenomenon is not unique to southern Africa. Any of several geobotanical anomalies can appear in an area of hydrocarbon microseepage. These can take the form of vegetation sparseness, stress effects, and species differences. We propose a brand new consideration to the perplexing puzzle: natural gas venting from below is the fairy circle maker. We discovered that periodic degassing from below takes place in fairy circles. We also detected low volatility hydrocarbon complex mixtures and fossil fuel biomarkers, pristane and phytane, in the soils of baby fairy circles.
This discovery supports our hypothesis of natural gas from below causing chemically altered zones, and triggering stunted growth of vegetation, resulting in fairy circles. Since the fairy circles bear an uncanny resemblance to offshore gas pockmark fields, it is perhaps no coincidence then that the region, both onshore and offshore, is extensively covered by gas and hydrocarbon prospecting licences.
A geochemical origin of the Namibian fairy circles seems to tick all the boxes.