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Gas that bubbles out of the floor in a deep mine has a chemical composition that can provide the food source for microbes living in deep ancient fluids underground.Credit: J Telling

Locked deep in the impermeable granite of the Canadian Shield lies a reservoir of water so ancient it may have been cut off from the surface for more than half the age of Earth itself.

The water, found bubbling up from boreholes in a mine 2.4 kilometres below Timmins, Ont., appears to bear traces of a long-vanished ocean and an atmosphere as it was just as oxygen-breathing life was coming into being around 2 1/2 billion years ago.

"It's exciting on about five different levels," said Barbara Sherwood Lollar, a professor of earth sciences at the University of Toronto who was part of the team that made the discovery. "It's like finding another world."

An analysis of the water, published today in the journal Nature, shows that it has a chemistry similar to that found near deep sea vents. That suggests it could support a microbial ecosystem that is completely isolated from the surface. In that case, the find would signal the existence of a "Galapagos of the deep," Prof. Sherwood Lollar said – a subterranean realm that has been free to follow an independent evolutionary path.

Among the key ingredients found in the water, clear and intensely salty to the taste, are isotopes of xenon gas, which are produced by the radioactive decay of uranium in the rock. The gas is chemically inert, so its accumulation can be used to measure how long the water has been sealed off.

The xenon measurement places this cutoff time at no less than 1.5 billion years ago. However, the water also contains some lighter xenon isotopes that are not produced in radioactive decay and must have come from a relatively xenon-rich atmosphere before the water was isolated. The amount of those isotopes suggests the water was last in contact with the atmosphere when its xenon content was higher than it is now, corresponding to a time that is closer to the age of the rock itself – about 2.7 billion years.

At that time, the rock was forming on an ancient sea floor, like lava that oozes out of the ocean floor near mid-ocean ridges. It's conceivable that the water was encapsulated then, Prof. Sherwood Lollar said.

The presence of salt water in deep interconnected pockets has long been known to hard-rock miners in Canada and elsewhere, but its ancient heritage has only recently come to light. In 2006, Prof. Sherwood Lollar and others determined that water found in South Africa's Mponeng gold mine has been isolated for millions of years and yet still harbours microscopic life that is able to survive on chemical energy.

Such finds are seen to bolster the theory that life could be flourishing below the surface of Mars or on other distant worlds where conditions at the surface are inhospitable.

The volcanic rock within which the Canadian Shield water was found is "very similar to Martian rocks," said Greg Holland, a geochemist at Manchester University in Britain who helped analyze the chemical signature of the water. "When Mars was much more habitable, three or four billion years ago, maybe life there went underground and has carried on living happily ever since."

Bacteria isolated for 250 million years have also previously been found in tiny inclusions of water in salt crystals, but the long isolation of the Canadian Shield water means it has the potential to open a window onto a far earlier era in the history of life on Earth.

"The only trick in this is what was the ambient temperature at the time the system sealed up," said Tullis Onstott, a geomicrobiologist at Princeton University who was involved in the South African find. If the water was too hot when it was isolated, he added, it could be sterile.

Even if life is present in the Canadian Shield water, it may take months to spot it, Prof. Onstott said. Because of the extreme conditions, the life would be sparse and most likely detected through fragments of DNA floating in the water. It could take many tens of thousands of litres flowing through a filter for months to gather enough DNA to produce a signature.

But, he added, "the potential for not only getting one type of organism but maybe multiple types of organisms, and sequencing their DNA to look at the evolution of these bacterial genomes … would be a tremendous discovery – the first of its kind."