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Bizarre Lost City vent is truly ancient


Lost City hydrothermal vent

An active vent on the side of a 25-m high chimney in the Lost City field on the Altantic seabed (ETH Zurich)

The Lost City, a bizarre hydrothermal vent at the bottom of the Atlantic Ocean, has been active for at least 30,000 years, scientists have been surprised to discover.

Insights into the chemical process that build these towering chimney-like structures above the vent suggest that systems like it could be active for hundreds of thousands - and possibly millions - of years, according to a report in the journal, Science.

Carbon dating of the 30 or so structures - some of which are no longer active, and therefore have ceased to grow - show that "many of the sediments sampled to date were deposited just before the last glacial maximum 20,000 years ago," wrote the team led by Dr Gretchen Früh-Green of the Swiss Federal Institute of Technology in Zurich.

The Lost City was first discovered by chance in December 2000 during studies of a giant seafloor mountain known as the Atlantis Massif, in the mid-Atlantic.

Hot springs with temperatures of 70°C that mark the vent are found on a terrace of the mountain about 850 m below the sea surface and the array of chimneys rises up to a record 60 m above them. Scientists are intrigued by the vent because it is unlike any other known.

The heat that drives it is generated by chemical reactions between seawater and elements in the Earth's crust. All others discovered to date are the result of high-temperature volcanic or tectonic activity, resulting in so-called black-smokers - with temperatures up to 370°C - that build darkly mottled structures featuring mainly sulphide minerals.

The Lost City chimneys are mainly made of carbonates, like stalagmites in limestone caves, and range from a clean white colour to grey. Some have been colonised by corals, and microbes are believed to feed off gases such as methane that are released from them.

The international team of scientists estimate that the mantle rocks immediately below the vent are relatively young - about 1.5 million years old - but no active volcanic or tectonic process are taking place there today.

But like other hydrothermal vents, the researchers said the Lost City may serve as a natural laboratory that yields clues into the nature of the Earth's early ecology, into the origins of life itself and perhaps into the conditions that might foster life on other planets.

The team used chemical data to infer the rate at which seawater penetrates to a depth of a few kilometres into the mantle, which is heavily fractured and cracked in that region. When the seawater enters these cracks, it hydrates the rocks to form the mineral serpentine; in doing so, it releases heat that drives the high-temperature venting.

The team's calculations imply that the process has been continuously active for at least 25,000 years - the age of the oldest sediments - and the researchers note that even older vent debris lies beneath them in piles up to 20 m deep.

They calculate that the process could keep on producing serpentine and releasing heat for many tens of thousands of years: about 180 cubic km of the massif have already been converted to serpentine.

Tags: environment, marine-biology, ecology