Sea Citadels Point to Past
A team of scientists who unexpectedly discovered the world’s tallest hydrothermal vents on the ocean bottom late last year now say the dramatic towers may turn out to be a common feature of the sea floor--and may also represent what the young Earth was like when life first evolved.
The remarkable features--dubbed the Lost City by the researchers--are on the flanks of the Atlantis Massif, a 5,000-foot mountain in the middle of the North Atlantic Ocean at about the latitude of Santa Barbara. One chimney structure is as tall as an 18-story building.
In December, oceanographers diving in the Alvin, a submersible research vehicle, were flabbergasted when they found the towers--some wide-mouthed, some needle thin and many capped with fringy deposits of minerals wafting in the warm vent water. The team from the University of Washington, the Scripps Institution of Oceanography and Duke University dropped plans to study other areas in order to concentrate on the huge field of vents, which existed in a place where vents were not thought to form.
“Just by looking at the vents, we knew they were different,” said Deborah Kelley, a marine geologist at the University of Washington who is one of only three people who have seen the vent field firsthand.
The tallest vent, dubbed Poseidon, was 180 feet tall, and 100 feet taller than the average vent. The vents were starkly white--much lighter than the “black smoker” vents found elsewhere on the sea floor. And they were not sitting on a volcanically active area but on crust that was 1.5 million years old.
“I would have said needle in a haystack. You’re never going to find that,” said Andy Fisher, a hydrogeologist at UC Santa Cruz who is an expert on hydrothermal systems on the sea floor.
Previously, hydrothermal vents were thought to form only in young rock, on areas where the sea floor is being pulled apart and magma is flowing up to fill the gap, Kelley said. Heat from the magma creates rising columns of hot water, where temperatures can top 750 degrees. The hot water picks up metal-rich minerals as it passes through the new volcanic rock. As the water cools as it moves farther from the gap, those minerals separate out and fall back on the sea floor to build up the hollow vents.
The main puzzle was how the newly discovered vents could have formed some 15 miles from where the sea floor was being ripped apart, in a place where the rock was old and relatively cold.
After months of chemical analysis, the group now thinks it has an answer. Their results were published recently in the journal Nature.
The heat that drives these vents, says Kelley, comes from a simple chemical reaction. The vents sit on tortured real estate: sea floor that has been stretched, fractured and broken apart to expose rock from deep within the Earth. When seawater hits this rock, it transforms the minerals within the rock. This transformation produces heat.
Temperatures at the vents range from 104 to 167 degrees--not nearly as hot as other vents but far warmer than the 44-degree temperature of the surrounding ocean water. While black smoker vents can spew large amounts of dark, mineral-rich water, the new structures are a kinder, gentler vent. “It’s almost like [seeing] a breeze in the ocean,” said Kelley.
Because the vents are far from unstable regions of lava and are composed of harder minerals, they are more likely to grow to large sizes. And because of the different chemistry of this environment--the vents are made of pure limestone--they are white and not black.
With all the pieces in place, said Kelley, “it hits you in the head. Everything about this place makes total sense.”
Fisher, who was not part of the research team, agreed. “My sense is that these scientists have got it sorted out,” he said, adding that the extraordinary discovery was in line with what deep sea voyagers often find on their journeys.
“Only a tiny fraction of the sea floor’s been explored. We have maps of less than 1%,” he said. “Virtually every time there’s an exploration of the sea floor we learn something new.” Indeed, some researchers now believe that vents of this new sort may cover as much as 10% of the ocean floor.
The warmer waters at the vents also serve as a kind of biological oasis. The vents were so covered with microbial life in some places, researchers could barely see the rock underneath.
First discovered two decades ago, hydrothermal vents have been a constant supply of new and strange forms of life, from giant tube worms to fields of clams and colorless crabs.
Because the chemistry of the new vents is so different--it is full of methane and hydrogen but lacks metals--Kelley anticipates discovering a suite of new organisms that may be far different from their brethren at black smoker vents. “It will be interesting to see how they make their living.”
There are no large animals--crabs, clams or shrimp--in the vicinity of these new vents. That is probably because the physiology of larger animals requires trace amounts of metals. While metals are abundant in the sea floor rocks, they are not available to the animals because the water temperatures at the new vents are not hot enough to melt the metals and release them. Scientists are most excited because the new type of vent--and its intimate interaction between warm, salty water and rocks fresh from the Earth’s crust--may prove to be an analog for an early Earth devoid of an oxygen-rich environment and crawling with life very different from what we see on land today.
“Some people have suggested that the total number of creatures living within the [Earth’s crust] is larger than the total amount living on top of the crust,” Fisher said. “It may tell us something about the conditions under which life on Earth started.”
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Sea Floor Vent Evolution
Historically, hydrothermal vents form on the sea floor where volcanic activity is present, such as along the underwater Mid-Atlantic Ridge, where tectonic plates spread apart. A new kind of hydrothermal vent has been found 9 miles west of the ridge at the Lost City site, where volcanic eruptions are rare. This new type of vent differs from the predominant “black smoker” vents in that it is made of carbonate minerals, not iron sulfides; is on mantle rocks, not basalt; and is on 1.5-million-year-old crust, not volcanically active areas.
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How vents form
Cold, dense seawater seeps through ocean crust, sometimes miles away from vents.
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Seawater continues to pass through the uneven cracks in the mantle rock.
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Mantle rock begins to react with seawater, extracting chemicals from the rocks. This reaction gives off heat, which drives the circulation of fluids.
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The warm, buoyant fluids rise through cracks in the mantle. These 1-foot-wide cracks fill in over time to form veins, which block vent growth.
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Fluids continue through the porous limestone chimney.
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Fluids, 104--167, exit through pinnacles and mix with seawater to deposit carbonate. Gases in the fluids provide nutrients for microbial communities that live in the absence of sunlight.
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Sources: Dr. Deborah S. Kelley, University of Washington
RICHARD R. SANCHEZ/Los Angeles Times
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University of Washington oceanographer Deborah S. Kelley compares the white porous texture of a sample (at left) from the Lost City’s carbonate chimneys with a sample from the sulfide chimneys studied since the 1970s.
University of Washington
