EVEN Jules Verne didn鈥檛 foresee this. Down at the bottom of the Atlantic Ocean is the hottest water on Earth, in a 鈥渟upercritical鈥� state never seen before in nature.
The fluid spews from a pair of black smokers called Two Boats and Sisters Peak, and could offer a glimpse of how minerals such as gold, copper and iron are leached out of the entrails of the Earth and released into the oceans.
鈥淚t鈥檚 water, but not as we know it,鈥� says Andrea Koschinsky, a geochemist at the Jacobs University in Bremen, Germany, whose team discovered the strange substance.
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It is in a state between gas and liquid. Water normally boils as temperature rises, but if you push up both pressure and temperature beyond a certain critical point, the gas and liquid phases merge into a supercritical fluid that is denser than steam but lighter than liquid water.
No one had seen supercritical water in nature until Koschinsky and her colleagues sailed just south of the equator in 2006. As they report in this month鈥檚 Geology, they discovered a new set of hydrothermal vents on the Mid-Atlantic Ridge more than 3000 metres beneath the surface, with water temperatures higher than 407 掳C. Seawater goes supercritical above 407 掳C and 300 atmospheres.
So where does it come from? Geologists think that water seeps into cracks in the seabed, growing hotter as it gets deeper. Eventually it becomes supercritical, and being far less dense than liquid water it shoots up and out into the ocean through the vents.
Supercritical water is especially effective at leaching metals and other nutrients out of the rock. Some, such as sulphur, provide energy for the organisms that live around vents, which have no light to kick off a food chain. And iron is essential for the growth of plankton.
The details of this leaching process are unknown. 鈥淚t鈥檚 not possible to drill into active vents,鈥� Koschinsky points out. 鈥淒rilling equipment would melt.鈥� So anything they can learn about the supercritical water from the new vents will be invaluable. 鈥淲e stand to greatly improve our models of fluid circulation,鈥� says Margaret Tivey, a geochemist at the Woods Hole Oceanographic Institution in Massachusetts.
Geochemists expected to find supercritical water in the Pacific, where ridges spread faster, bringing hot magma closer to the seabed. This discovery in the South Atlantic might indicate that the crust in there is unexpectedly hot and dry, says Koschinsky鈥檚 colleague Colin Devey of the University of Kiel in Germany.
