The unrelenting whine of the Cessna 170 light airplane buzzing in my ear, I gaze down at the vast, stretching forests of northern Ontario struck by the sheer scale of the world’s second largest country, Canada. But my daze is suddenly interrupted and my curiosity piqued. I spot a group of almost perfectly circular, light-coloured rings in the forest below that appear to range from a few hundred metres to over a kilometre in diameter. Noticing my interest, my pilot, JR, chips in: “Ah, you’ve spotted our forest rings! Pretty cool, eh?”

Ranging from 30 metres to over 2 kilometres in diameter, these mysterious rings that spatter Ontario’s forests and farmland, are so subtle on the ground that people simply stroll right through them without noticing anything amiss. It was only with the advent of aerial photography in the region that these features were actually discovered. Their number is now thought to exceed 8,000. When investigated, it became clear that they represent rings of stunted tree growth. The exact cause of this phenomenon, however, remains controversial with a wide range of hypotheses; the more popular ones include the ideas that the rings lie above methane deposits, mineral deposits, and even diamond-bearing kimberlite pipes (named after the famous diamond-mining city of Kimberley in South Africa). Others favour a tree-killing fungus as responsible. Although now disregarded, one extreme hypothesis even suggested they were created by multiple meteorite impacts.

A team of Earth Scientists lead by Kéiko Hattori, of the University of Ottawa, and Stew Hamilton, of the Ontario Geological Survey, have shed light on the nature of the peculiar forest rings since beginning research into them in 1998.

They believe that the forest rings are caused by huge, naturally occurring electrochemical cells, i.e. big centres of negative charge (called reduced chimneys). These are known to geologists to often sit over mineral and methane deposits. As Elle Andra-Warner explains, it is best to “think of them as huge natural electrical batteries.” The negative charge from the ‘batteries’ radiates outwards and where it meets positively charged oxygen in the soil the interaction of the opposite charges creates acidic conditions. This then eats away at the carbonate soil covering the region, resulting in a circular depression around the natural battery and stunted tree growth.

But what is producing these electrical fields and why in northern Ontario in particular?

Hamilton believes there is a clue in the age of the natural gas that is found within the rings. Its isotopic signature suggests that it is geologically very young and is likely “still being produced today” rather than representing ancient gas deposits. It appears that Ontario’s glaciated landscape has a roll to play in the ongoing creation of this gas and thus the mysterious electrical fields themselves. Until about 10,000 years ago glaciers blanketed the entire region and as the ice retreated it left behind the youngest and most extensive glacial clay deposits in the world. The theory is that dead plankton and other organic matter left in the clay is now being consumed by large volumes of methane-producing bacteria.

This would explain why northern Ontario is home to the highest concentration of forest rings anywhere on Earth although they have also been reported in Quebec, the Yukon, and even as far afield as Russia and Australia.

Hamilton believes that 80-85 % of the rings in Ontario are a result of these methane plumes and that the rest can be attributed to other features such as kimberlites or the nickel, copper, and zinc deposits for which the region is known.

The case is by no means closed however; the exact origin of the huge electrical field within the rings remains a major sticking point. The team’s hypothesis, according to Stew Hamilton, is that “millions of [methane-producing] micro-organisms are creating a massive, low-voltage electrical field that causes their food, the chemicals, to come forward to them. The bacteria don’t have to move – the food keeps coming to them along the electrical field they have created.”

It’s a fantastic idea but remains to be tested. This is not the last we’ve heard of Ontario’s wonderful Os.

Gavin Kenny

Ph.D. student, Department of Geology, Trinity College Dublin

@GavinGKenny

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