Vents hold key to oceans' secrets

June 6, 1997

FOR the past 20 years marine geologists may have been searching for deep vents on the ocean floor in the wrong places.

These hydrothermal vents emit plumes of chemical-rich waters from the earth's crust into the ocean. They form some of the most extreme conditions on earth, with vast pressures and temperatures, absolute dark and a deadly mixture of chemicals. Yet they are home to bacteria and larger life forms including crabs, clams, blind shrimps and snails.

The vents were first discovered on the floor of the Pacific mid- ocean ridge in the 1970s. Since then, marine geologists have searched for other vents along the volcanically active ridges that form when the earth's plates collide.

But now a research team from Southampton University's Oceanography Centre has suggested that hydrothermal vents may be far more common than once thought. They believe that the vents may form not along the centre of the ridges, but in more faulted and thinned areas up to 20 kilometres away.

Until now, hydrothermal vents have been discovered by chance along mid-ocean plate boundaries, says Southampton researcher Lindsay Parson. The pulling apart of plates allows sea water to enter the earth's crust and flow through hot rocks, dissolving minerals and chemicals as it goes.

"The sea water becomes a nasty chemical cocktail," Dr Parson said. "It discharges at a temperature in excess of 3500C through some convenient crack and hits water of 10. Much of its chemical content is frozen out, leaving hydrothermal deposits, such as copper, zincs and gold."

The creatures that gather around the vents are effectively stranded in the middle of the Atlantic ocean. "They have been protected from anthropogenic effects and light for 120 million years, developing and evolving in isolation," Dr Parson said.

The Southampton team wanted to map these vents because they play a key role in the chemical composition of the oceans, with a possible knock-on effect on the chemistry of the atmosphere.

The team chose a section of the mid-Atlantic ridge 250km long just south of the Azores. "Groups of these vents are only a few hundred metres across on a ridge which may be 10km wide. So if you just go and look for them, you have to be fairly lucky," Dr Parson said.

Instead they used a Tobi -Towed Ocean Bottom Instrument - which was developed in the UK, to detect the plumes. At a depth of between 200 and 300 metres from the ocean floor, the water plumes, still containing quantities of dissolved chemicals, become less buoyant and spread out instead of rising.

"We towed the Tobi at this depth throughout the 250km long, 10km wide section. We knew of two vents in this area, but we found seven more," Dr Parson said.

"The significant thing is they were not in the parts of the ridge which were most volcanically active. Instead they were up to 20km from the hottest areas of the ridge. We think now that as you move away from the hot sections of the ridge you come into sections which have been more faulted and thinned."

He believes it is these faults which allow high-temperature vents to surface. Researchers from Southampton are now at sea testing the theory.

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