The Lost City hydrothermal vent field discovered just two years
ago this week is giving scientists reason:

— To believe that similar systems may be or have been present
on water-bearing, tectonically active planets. Researchers
also believe that systems like Lost City may be common on
Earth, according to Deborah Kelley, University of Washington
associate professor of oceanography and one the three people
on the first manned dive to the field the day after it was
discovered.

— To speculate that life on this planet may have started in
the relatively warm, alkaline waters rich with methane and
hydrogen that result when seawater reacts with mantle rock.
Today the mantle in most places is capped by oceanic crust.
But early in Earth’s history mantle rocks may have been
much more exposed to seawater, providing ample opportunity
for conditions to support microbial life at the seafloor.

Different from previously studied vent systems

Kelley says Lost City is distinctive because it sits on
mantle rock that’s about 1.5 million years old and its
hydrothermal venting — in which water circulates into the
seafloor, gaining heat and chemicals until there is enough
heat for the fluids to rise buoyantly and vent back into
the ocean — is much different from previously explored
vent fields.

It doesn’t appear that volcanic activity drives hydrothermal
venting at Lost City; fluids from there give no chemical
evidence of having been in contact with magma chambers. This
is unlike any system found at the Earthís spreading centers
where magma chambers are present. At speading centers very
young seafloor is created — often dramatically during
volcanic eruptions — and vented water can be as hot as 700
degrees F.

Instead, venting at Lost City occurs because of heat
generated by chemical changes in the rocks: seawater
permeates deeply into the fractured surface of the mantle
rocks where it transforms the mineral olivine into a new
mineral, serpentine, in a process called serpentinization.

The heat generated during serpentinization is not as great as
that at volcanically active sites but it is enough to power
hydrothermal circulation and produce vent fluids of 105 to
170 degrees F, Kelley says. The result is a field of dramatic
vents not made of sulfide but of carbonate minerals, or
limestone. The most massive, at 18 stories, is the tallest
vent structure ever seen anywhere. The vents support a
community of microorganisms able to live off the fluids
rich with methane and hydrogen, both byproducts of
serpentinization.

Lost and found

The Lost City hydrothermal vent field was discovered Dec.
4, 2000, in the mid-Atlantic, at about 30 degrees north
latitude, during a National Science Foundation-funded
expedition that had not set out to seek a hydrothermal
vent field. It is nine miles from the nearest volcanically
active spreading center.

It is on the summit of the mountain known as Atlantis Massif,
one reason for the name Lost City.

Mantle rock is usually many kilometers beneath the seafloor
but at the Atlantis Massif, the Earth’s forces thrust mantle
rock up exposing it directly at the seafloor. Spreading and
faulting stripped much of the mountain down to the underlying
mantle rocks.

The extent of the hydrothermal field is unknown. In the
limited time researchers were there they saw about 30 active
and inactive carbonate chimneys. Tallest is the 180-foot
vent scientists have named Poseidon. Previously studied
vents mostly reach 80 feet or less with the tallest being a
135-foot vent on the seafloor off the coast of Washington
(which toppled in recent years).

The new vents are nearly 100 percent carbonate, the same
material as limestone in caves, and range in color from a
beautiful clean white to cream or gray, in contrast to
black smoker vents that are a darkly mottled mix of sulfide
minerals.

It’s easy to imagine there could be many more such systems,
Kelley says. Within a mere 60-mile radius of the Atlantis
Massif are three similar mountains subject to the same
fracturing, the same intrusion of seawater and perhaps
the same reactions with mantle material. And those four
represent a tiny fraction of the potential sites along the
6,200 mile Mid-Atlantic Ridge, Indian ridges and the Arctic
Ridge, also considered slow- and ultra-slow-spreading
centers.

Kelley will lead the second scientific expedition to Lost
City from April 9 to May 9, 2003.

Kelley will be at AGU, Dec. 5-10; messages can be left for
her at Parc 55, 415-403-26602. Her contact information in
Seattle at 206-543-9279.

IMAGE CAPTION:
[http://www.washington.edu/newsroom/news/images/flange-hr.jpg (147KB)]
A ledge or flange made of carbonate juts out from the side
of a 160-foot chimney in the Lost City hydrothermal vent
field. The chimney and flange are made of carbonate minerals
and silica dissolved in 160 F fluids that flow out of the
seafloor and then precipitate when the fluids hit the icy
cold seawater. The flange is 1 meter across. Photo credit:
University of Washington

ADDITIONAL IMAGES AVAILABLE:
http://www.washington.edu/newsroom/news/images/lostcity

NOTE: This information about Lost City pertains to Friday’s
news conference, "Life in extreme environments on Earth —
and where else in the solar system?" at the annual American
Geophysical Union meeting; to Deborah Kelley’s presentation
B62A-03 Saturday afternoon; and Matt Schrenk’s presentation
B71B-0742 Sunday morning.