快猫短视频

FOR sheer strangeness, it鈥檚 hard to beat the bizarre world of deep-sea vents
or 鈥渂lack smokers鈥�. Dotted along the mighty chains of undersea volcanoes that
form the backbone of the Earth, these vents spew out hot seawater loaded with
chemicals and mineral particles, which build chimneys on the ocean floor and
nourish communities of exotic sea creatures.

But oceanographers diving in deep-sea submersibles to explore this alien
world seldom realise that they could be passing through something even stranger.
Several hundred metres above the ocean floor, giant whirlpools swirl in the
darkness, fed by the fluids erupting from the vents below. Over the past few
years, scientists have discovered that these spinning masses of water can break
free and wheel across the underwater landscape like flying saucers, carrying
heat, chemicals and even vent animals heading for new pastures. While most
oceanographers concentrate on the vents themselves, a handful of physicists,
chemists and biologists are studying the spinning plumes that they spawn. These
eddies, they say, have probably been scudding through the depths since the
oceans were first formed and may once have caused tropical storms that make
modern hurricanes look like minor squalls.

Since the vents were discovered 20 years ago, oceanographers have known that
they spew out hot seawater. The fluid rushes upwards as a billowing column of a
blue-black mineral 鈥渟moke鈥�, mixing with colder seawater as it goes. When the
fluid first emerges it encounters seawater that is much denser, giving it
buoyancy, but as the plume rises it gradually reaches water with the same
density, and then it spreads out sideways. Knowing this, researchers assumed
that the plumes would reach this height and then simply carry on spreading out,
becoming fainter as they diffused farther from their source.

However, this simple picture overlooked a crucial fact: we live on a rotating
planet. 鈥淣one of the early studies had rotation in them. A plume would just
spread to infinity or fill up a basin,鈥� says Kevin Speer, a physical
oceanographer at the French Research Institute for the Exploitation of the Sea
in Brest, France. Physical oceanographers knew that the rotation of the Earth
could parcel spreading bodies of water into spinning vortices, but no one had
thought of applying that to deep-sea vents.

In 1988, while doing his PhD at Woods Hole Oceanographic Institution in
Massachusetts, Speer developed a model of the plume behaviour that incorporated
the Earth鈥檚 rotation. Even if a body of water looks stationary to someone
perched on the Earth鈥檚 surface, it is actually moving to match the planet鈥檚
spin. If it spreads out, it will begin to spin less quickly, just as spinning
ice-skaters slow down when they extend their arms. Now it will be rotating more
slowly than the planet beneath, and will seem to an observer to be spinning
slowly the other way. Incorporating this effect, Speer鈥檚 model predicted that a
spreading plume would become a rotating 鈥渓ens鈥� of water, roughly 200 metres
thick and 2 kilometres across, around 300 metres above the seafloor.

The notion that rotation could package a plume into a spinning vortex was
particularly significant, because anything swept into the vortex would be kept
there. Rotating plumes would become coherent bodies, distinct from the water
around them. What鈥檚 more, they would retain their unusual chemical signature for
much longer than if they simply diffused outwards. Intrigued by Speer鈥檚
calculations, his colleagues Karl Helfrich and Thomas Battisti decided to test
them using a large fish tank on a turntable. Using coloured dye to simulate the
vent fluid, they confirmed that plumes of water could form distinct rotating
vortices.

They also discovered something potentially much more important. As the
spinning lenses of dye reached a certain size, eddies would break off and wander
across the tank. The implication was clear. If the spinning lenses were formed
at vents in the ocean, they too might be able to break off and roam. Dissolved
chemicals, mineral particles and living creatures could be carried away across
the ocean.

This possibility caught the imagination of Lauren Mullineaux, a biologist
also working at Woods Hole. Mullineaux was puzzled by how vent creatures could
travel from one vent site to another when the distance between them can be
several hundred kilometres. It鈥檚 a long-standing conundrum in oceanography
(鈥淢onster journeys鈥�, 快猫短视频 supplement, 2 November 1996, p 14).
The ability to emigrate to another vent is important for the animals, because
vents don鈥檛 last forever: earthquakes or volcanic eruptions can disrupt their
plumbing, or they can become clogged with mineral deposits.

When a vent fails, the animals around it die. However, their progeny could
live on if only there were a way to transport them to a new site. There is
plenty of evidence that this happens, but nobody knows exactly how. Mullineaux
began to wonder if the plumes might be the answer. Many of the animals around
the vents don鈥檛 move much as adults, but they do have tiny offspring that could
be carried away by currents. Perhaps spinning eddies could transport these
larvae in concentrated patches from one vent site to another.

Looming hazard

In 1990, she tested this idea by trawling nets through spreading plumes above
vents to see if larvae were more concentrated inside them compared to the water
outside. The results were suggestive, but not conclusive. The problem Mullineaux
faces is that towing a net through a plume is a tricky business. 鈥淚t鈥檚 a little
like flying a kite near high-tension wires on a very long string from a moving
car,鈥� she says. The string in this case is a cable up to 5 kilometres long towed
behind a moving ship, and the hazards are the underwater cliffs and mountains
that loom around volcanic ridges, threatening to snag the net.

So Mullineaux tried another tack. She teamed up with Helfrich to work out how
likely it is that the tiny offspring of vent animals were being sucked up by
plumes. During a series of submersible dives in 1991, Mullineaux and her student
Stacey Kim squirted fluorescent dye into the water around a vent. By measuring
the amount of dye that was swept into the rising plume, they estimated how many
larvae could be captured by the plume from a single black smoker. They
discovered that plenty of larvae would be sucked onboard the plume鈥攁bout a
hundred per hour. Mullineaux likens these larvae to 鈥渦nwitting passengers in a
hot air balloon鈥�.

Hints like this are all very well, but what was really needed was a way to
catch the plumes in the act of roaming. Following the behaviour of plumes in the
ocean is much harder than watching patterns of dye in a tank. For one thing,
it鈥檚 not easy to take a clear snapshot of exactly where they are and what they
may be doing. A surface ship takes several hours to survey a plume thoroughly,
but in the meantime ocean tides slosh it back and forth on a 12-hour cycle. It鈥檚
like trying to photograph someone who won鈥檛 sit still. However, while looking
for new vents on the seafloor in the early 1990s, Chris German and Bramley
Murton from the Southampton Oceanography Centre in Britain found tantalising
hints that the plumes might be spinning off on their own. Sometimes, for
instance, they saw two plumes where there was only one set of vents below. But
still they couldn鈥檛 be sure if the plumes were rotating, or detaching.

Enter John Lupton, a chemical oceanographer at the Pacific Marine
Environmental Laboratory in Newport, Oregon, who decided to nail the question
once and for all. He settled on a different approach: rather than surveying a
plume from a surface ship, he released a buoy into a plume to watch its antics.
The buoy was designed to sink to the level of the spreading plume and then drift
with it. With just enough ballast to reach a depth of 2.2 kilometres, it settled
inside the envelope of the plume. As it drifted along, the buoy recorded 鈥減ings鈥�
from acoustic transponders that had already been scattered across the seafloor.
Twice a day it used these signals to fix its position by comparing their arrival
times. At the end of its 60-day mission in the summer of 1996, the buoy ditched
its weights and floated back to the surface, transmitting its cargo of data to
an expectant Lupton via a satellite.

During its travels, the buoy had covered 127 kilometres as it spiralled
around inside the plume鈥攖he first hard evidence that plumes do form the
swirls found in the experiments and models. What鈥檚 more, when Lupton went to
collect the buoy, he found that the plume was still beneath it, 8 kilometres
from where it had started. As the edges of this plume were several kilometres
from the nearest volcanic ridge, it had clearly detached and moved off from its
source.

Obstacle course

Knowing that real plumes rotate and can break free, the next hurdle will be
to find out how long they last. Is it long enough to carry larvae between vents?
鈥淭here are several theories about how a plume might spin down,鈥� says Speer.
Friction in the circulation should eventually act as a brake, but as water is
slippery stuff, this would probably take a year or two to stop the spinning.
However, the eddies would probably crash into something before they had time to
spin down. The volcanic ridges where vents are found are dotted with an
underwater obstacle course of tall peaks known as seamounts.

To determine the ultimate fate of a plume, Lupton plans to follow one for a
much longer period of time. 鈥淲e have three buoys ready to go, and we鈥檙e building
more,鈥� he says. The question is, how to find one that will be big and distinct
enough to track for a long period. Lupton鈥檚 strategy is to wait for a particular
kind of plume known as an event plume. Plumes come in two classes. There are
those formed by the continuous activity of a set of vents which fill up a lens
of water to a critical size before eddies break off and the whole process
repeats. But there are also larger ones that appear after eruptions on the ocean
floor. These are event plumes, believed to be created by new lava flows on the
seafloor heating a body of seawater very rapidly. The last plume that Lupton had
luck with was an event plume, so he鈥檚 waiting for another.

His colleagues in Oregon are using the US Navy鈥檚 SOSUS array of hydrophones
to listen out for eruptions on the nearby Juan de Fuca Ridge. In the meantime,
Lupton has a ship on standby. 鈥淲hen an event is detected, we鈥檒l look for plumes.
We hope to follow a plume for longer, up to a year or more,鈥� he says. Event
plumes are bigger than other plumes鈥攗p to 2 kilometres across and 800
metres thick鈥攁nd rise to around 1 kilometre above the seafloor, which
should make them easier to find.

Finding out exactly where plumes roam and what they get up to may throw up a
few surprises. In the early 1990s, Dan Walker at the University of Hawaii,
Honolulu, proposed that hydrothermal plumes might trigger El Ni帽o events
by rising high enough to affect the temperature of surface waters, and alter
ocean currents. Despite a flurry of press coverage, the idea was rapidly
dismissed. To rise all the way to the surface, a plume would have to be very,
very buoyant. This would require a colossal heat source, at least a thousand
times greater than that driving the largest event plume ever seen. 鈥淎s one
prominent El Ni帽o researcher put it, pissing in the ocean would have more
of an effect than hydrothermal plumes,鈥� says Speer.

However, a hundred million years ago during the Cretaceous, plumes might have
been able to reach the surface, according to Speer. The layering of the ocean
was less pronounced, and there was less of a difference in the density of
seawater between the surface and the bottom, making it easier for a plume to
rise. There were also occasional huge volcanic outbursts that would have
deposited massive amounts of hot lava on the seafloor. Speer thinks that these
heat sources might have been big enough to drive a plume all the way up through
the water column.

Kerry Emanuel, a climatologist at the Massachusetts Institute of Technology,
has suggested that if a patch of surface water were heated to 50 掳C, this
could trigger a massive tropical storm which he dubbed a 鈥渉ypercane鈥� (New
快猫短视频, Science, 4 February 1995, p 16). With wind speeds approaching
the speed of sound, these storms would whip huge amounts of water vapour and
dust up into the stratosphere, potentially affecting the climate of the whole
planet. Emanuel first thought of these storms as a possible consequence of a
large asteroid hitting the ocean, but Speer points out they could also be set
off by a monster plume rising from the ocean floor.

Nobody can know for sure whether the volcanic eruptions in the Cretaceous
happened rapidly enough to generate the intense heat needed to spawn such a
plume, but it is still an intriguing notion. If sufficient energy was available,
creatures standing on the shores of a Cretaceous ocean would have been made all
too aware of the whirling masses of water that now spin quietly in the
depths.