Even Jules Verne did not foresee this one. Deep down at the very bottom of the Atlantic Ocean, geochemist has found something truly extraordinary: 鈥淚t鈥檚 water,鈥� she says, 鈥渂ut not as we know it.鈥�
At over 3 kilometres beneath the surface, sitting atop what could be a huge bubble of magma, it鈥檚 the hottest water ever found on Earth. The fluid is in a 鈥渟upercritical鈥� state that has never before been seen in nature.
The fluid spews out of two black smokers called Two Boats and Sisters Peak.
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Koschinsky, from Jacobs University in Bremen, Germany, says it is somewhere between a gas and a liquid. She thinks it could offer a first glimpse at how essential minerals and nutrients like gold, copper and iron are leached out of the entrails of the Earth and released into the oceans.
Liquids boil and evaporate as temperature and pressure rise. But push both factors beyond a critical point and something odd happens: the gas and liquid phase merge into one supercritical fluid. For water, this fluid is denser than vapour, but lighter than liquid water.
Hot 鈥榖ubble鈥�
Water and seawater have both been pushed past this critical point in labs, but until Koschinsky and her colleagues sailed to just south of the Atlantic equator in 2006, no-one had seen supercritical fluids in nature. Geochemists suspected that if they were to find them anywhere, they would be coming out of very deep hydrothermal vents.
In 2005, a team of scientists including Koschinsky visited 5掳 south, as part as a six-year project to investigate the southern end of the mid-Atlantic Ridge. There, they discovered a new set of vents, which they revisited in 2006 and 2007, lowering a thermometer into them each time.
Computer models suggest that the fluid that comes out of these black smokers initially seeps down into surrounding cracks in the seabed, gradually getting deeper and hotter as it approached the Earth鈥檚 magma. Eventually, at 407 掳C and 300 bars of pressure, the water becomes supercritical.
Because supercritical water is far less dense than liquid water, it shoots up to the seabed like a bubble and it is spat out into the ocean through vents.
Powering life
From their first visit in 2005, the team found temperatures in the vents were at least 407 掳C, and even reached 464 掳C for periods of 20 seconds.
Supercritical water leaches metals and other elements out of rock far more efficiently than liquid water or vapour. Gold, copper, iron, manganese, sulphur and many more are brought out of the Earth鈥檚 guts when the water is ejected from the black smokers.
Some, such as sulphur, provide energy to the locally adapted organisms, which have no light to power a food chain. Manganese is similarly used as an energy source by microbes higher up in the water column. Iron is essential for the growth of all phytoplankton.
Koschinsky estimates up to half the manganese and one tenth of the iron found in the oceans could come from vents. But because supercritical fluids have never been observed in nature, little is really known about how this happens.
Melting equipment
鈥淲e stand to greatly improve our models of fluid circulation and heat and mass transfer,鈥� says , a geochemist at the Woods Hole Oceanographic Institute (WHOI) in Massachusetts.
Because of the extreme conditions, computer models are the only way of understanding the processes that drag elements out of the seafloor at hot vents. 鈥淚t鈥檚 not yet possible to drill into active vents,鈥� explains Koschinsky. 鈥淭emperatures are so high, much of drilling equipment would melt and joins would not work anymore.鈥� The data from the new vents will be invaluable in testing the models.
鈥淭he findings are significant,鈥� says , also of WHOI. 鈥淭he high temperature of the venting is especially interesting as this [mid-ocean ridge] does not spread very rapidly.鈥�
The Pacific spreads faster than the Atlantic, bringing magma closer to the seabed. For this reason, geochemists expected to find supercritical seawater there too. 鈥淪o one can presume that this portion of the south mid-Atlantic ridge is in a very magmatic phase and has been for a few years,鈥� adds Fornari.
鈥楧ry as a biscuit鈥�
In the Pacific, vents tend to cool after a year or so, but it is likely that the Two Boats and Sisters Peak have been active since an earthquake shook the region in 2002. 鈥淭he magma body underneath is probably enormous,鈥� says Koschinsky.
Her colleague of the University of Kiel in Germany is not so sure. 鈥淭he explanation could be that there鈥檚 lot of magma, but after a few more years of high temperatures, it鈥檚 going to get to the point where it will be embarrassing how much magma there needs to be to maintain them for that long.鈥�
He thinks the long-standing temperatures could indicate something more fundamental. The fact that vents cool much more quickly in the Pacific could indicate the crust there is much more water-logged than it is in the Atlantic, where it could be 鈥渄ry as a biscuit鈥�.
鈥淚f that turns out to be the case then we will have taken down some very, very holy grails,鈥� says Devey.
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