Hydrothermal vent structures serendipitously
discovered last December in the mid-Atlantic
Ocean, including a massive 18-story vent
taller than any seen before, are formed
in a very different way than ocean-floor
vents studied since the 1970s. The findings
are published by National Science Foundation
(NSF)-funded researchers in the July 12
edition of Nature. This new class
of hydrothermal vents apparently forms
where circulating seawater reacts directly
with mantle rocks, as opposed to where
seawater interacts with basaltic rocks
from magma chambers beneath the seafloor.
"This discovery is a tremendous example
of the results of targeted exploration
of the ocean," says Margaret Leinen, NSF
assistant director for geosciences. "We
knew that the areas of great faults on
the seafloor were interesting, but did
not realize that hydrothermal activity
of this sort could be taking place on
seafloor generated millions of years ago."
No one had previously seen a field quite
like this, but Deborah Kelley, a University
of Washington (UW) oceanographer and lead
author of the Nature paper, says this
kind of vent may be common on the seafloor.
If so, scientists may have underestimated
the extent of hydrothermal venting, the
amount of heat and chemicals pouring into
the world's oceans and the abundance of
life that thrives in such conditions.
"Rarely does something like this come along
that drives home how much we still have
to learn about our own planet," Kelley
says. "We need to shed our biases in some
sense about what we think we already know."
The Lost City Field, named partly because
it sits on the seafloor mountain Atlantis
Massif, was discovered December 4, 2000,
during an NSF-funded expedition led by
Scripps Institution of Oceanography's
Donna Blackman, UW's Kelley and Duke University's
Jeffrey Karson. Blackman and Karson are
among the paper's co-authors.
Lost City is like other hydrothermal vent
systems where seawater circulates beneath
the seafloor and gains heat and chemicals
until there is enough heat for the fluids
to rise buoyantly and vent back into the
ocean. As the warm fluids mix with cold
seawater the chemicals separate from the
vent fluids and solidify, sometimes piling
up into impressive mounds, spires and
chimneys of minerals.
It was immediately clear, however, that
the Lost City Field is unlike other hydrothermal
vent systems in a number of ways. First,
there is the height attained by some of
the structures - the mighty 180-foot vent
scientists named Poseidon compares to
previously studied vents that reach 80
feet or less. The new vents are nearly
100 percent carbonate, the same material
as limestone in caves, and range in color
from a clean white to cream or gray, in
contrast to black smoker vents that are
a darkly mottled mix of sulfide minerals.
And perhaps the Lost City's most distinctive
feature is that it is sitting on 1.5 million-year-old
crust formed from mantle material.
In the area of the Lost City, spreading
and faulting during the last 1 to 1.5
million years have stripped the mountain
down to the underlying mantle rocks. Hydrothermal
circulation appears to be driven by seawater
that permeates into the deeply fractured
surface and transforms olivine in the
mantle rocks into a new mineral, serpentine,
in a process called serpentinization.
Kelley says it's easy to imagine there
could be many more such systems. Within
a mere 50-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
only a tiny fraction of the potential
sites along the 6,200 mile Mid Atlantic
Ridge, as well as the Indian ridges and
the Arctic Ridge, also considered slow-
and ultraslow-spreading centers.
![Photo of carbonate growth on flange](https://webarchive.library.unt.edu/eot2008/20081106233717im_/http://nsf.gov/od/lpa/news/press/images/lc_flange.jpg)
A 5-foot-wide flange, or ledge, on the
side of a chimney in the Lost City Field
is topped with dendritic carbonate growths
that form when mineral-rich vent fluids
seep through the flange and come into
contact with the cold seawater.
Photo credit: University
of Washington/Woods Hole Oceanographic
Institution
A
larger version (53kb) is here.
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![](https://webarchive.library.unt.edu/eot2008/20081106233717im_/http://nsf.gov/od/lpa/images/spacer.gif) |
![Photo of flange on 18-story chimney](https://webarchive.library.unt.edu/eot2008/20081106233717im_/http://nsf.gov/od/lpa/news/press/images/lc_flange2.jpg)
Collecting samples from a 6-foot-wide
flange, or ledge, growing from the side
of the 18-story carbonate chimney in the
Lost City Field reveals an opening into
the hollow white interior of the flange
from which warm vent fluids escape in
a shimmering curtain.
Photo credit: University
of Washington/Woods Hole Oceanographic
Institution
A
larger version (64kb) is here.
![Graphic of the Atlantis Massif](https://webarchive.library.unt.edu/eot2008/20081106233717im_/http://nsf.gov/od/lpa/news/press/images/lc_atlantismap.jpg)
The Atlantis Massif is west of the Mid-Atlantic
Ridge and north of the Atlantis Fracture
Zone. The Lost City Field is on a terrace
on the side of the mountain, between 2,300
and 2,600 feet below the sea surface.
Graphic credit: Scripps
Institution of Oceanography
A
larger version (89kb) is here.
![Graphic of Mid-Atlantic map](https://webarchive.library.unt.edu/eot2008/20081106233717im_/http://nsf.gov/od/lpa/news/press/images/lc_midatlanticmap.jpg)
The Mid-Atlantic Ridge is one of the earth's
largest undersea mountain ranges at a
length of nearly 6,200 miles. The dots
represent active hydrothermal sites.
Graphic credit: University
of Washington
A larger
version (45kb) is here.
![Photo of underwater chimney](https://webarchive.library.unt.edu/eot2008/20081106233717im_/http://nsf.gov/od/lpa/news/press/images/lc_chimney.jpg)
The carbonate structures at the Lost City
Field include this chimney more than 30
feet in height. The white, sinuous spine
is freshly deposited carbonate material.
The top shows evidence of collapse and
re-growth, as indicated by the small newly
developed cone on its top.
Photo credit: University
of Washington/Woods Hole Oceanographic
Institution (Mitch Elend, University of
Washington, generated this composite mosaic
from digital images.)
A
larger version (146kb) is here.
![Photo of researcher and sample](https://webarchive.library.unt.edu/eot2008/20081106233717im_/http://nsf.gov/od/lpa/news/press/images/lc_kelley.jpg)
University of Washington oceanographer
Deborah Kelley contrasts the white porous
(almost wasp-nest-like) texture of a sample
from the Lost City's carbonate chimneys
with a sample from the sulfide chimneys
studied since the 1970s.
Photo credit: University
of Washington
A
larger version (115kb) is here.
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