New Hampshire
ROVER is an unusual creature. It explores, spends much of its time digging
and sniffing mud, but always returns home when its owners whistle. But this is
no dog. Rover is an underwater tractor, a small treaded tank, designed to spend
months wandering in the murky ocean depths, scouring the ocean floor for signs
of life.
Last year, Rover set out on its maiden journey. About 75 kilometres off Point
Conception on the Californian coast, the mini tractor was lowered over the side
of a research vessel into the water. After a 4000-metre journey to the seabed,
Rover began to measure the rate at which microscopic life in the seafloor鈥檚 mud
takes up and releases carbon.
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Rover鈥檚 mission is hugely important. The metabolism of seabed organisms is
part of the carbon cycle, the continuous use and replenishment of carbon in the
Earth鈥檚 ecosystems. Since carbon is the fundamental building block of life, it
is absorbed by organisms as they eat, respire and grow, and released as carbon
dioxide when they die and decay. This system was delicately balanced until about
1700 when humans began to burn fossil fuel and the amount of carbon dioxide in
the atmosphere suddenly began to rise from 280 parts per million to 355 ppm
today.
But strangely, fossil fuels should have produced a much bigger increase than
this. Many researchers believe that the oceans have absorbed much of this the
extra carbon (鈥淲here has all the carbon gone?鈥�, 快猫短视频, 8
January 1994, p 30). If they have, what effect does this have on life on the
seafloor?
The truth is that scientists do not yet understand the carbon cycle well
enough answer this question. 鈥淲e know almost nothing about the role of the
seabed in that process, so we鈥檝e created the Rover to help us find out,鈥� says
Kenneth Smith, a marine biologist at Scripps Institution of Oceanography in La
Jolla, California, who thought up Rover and led the engineering team which
created it.
鈥淭he organisms on the seafloor are like you and me,鈥� explains Smith. 鈥淭hey
take in oxygen and expel carbon dioxide, and the rate at which they do that is
related to the rate at which they process carbon.鈥� By measuring changes in the
amount of oxygen dissolved in a water sample, Smith and his team can compute how
fast carbon is processed. If the number of organisms is increasing or if each
one is growing, then the population will, as a whole, be absorbing carbon. If,
on the other hand, the population is decreasing, carbon is released. The key is
to measure the way this process changes over time. 鈥淚t鈥檚 a fundamental baseline
measurement of the biological processes of the ocean,鈥� he adds. But seafloor
measurements over time have been impossible until now.
快猫短视频s have been able to take the same basic measurements that Rover
collects for some time. Thousands of times during the past three decades,
instrument pods have been dropped from ships to settle on the ocean floor on
wide-based tripods. Each two-metre long pod pushes a small, open-bottomed
cylinder a few centimetres into the seabed, calculates the proportion of oxygen
dissolved in the water, then casts off its steel anchor and floats back to the
surface.
鈥淭he problem is that these pods go down in one place, make one measurement,
and come back up,鈥� Smith explains. 鈥淭o understand the role of the sea bottom in
the ocean鈥檚 carbon cycle, we need to make a series of measurements not just
across space, but also across time.鈥�
Rover is ideal for this. Unlike vehicles carrying people, it can stay at the
bottom for six months or more at a time and unlike one-shot instrument pods, can
move from place to place, making a series of measurements. 鈥淭he Rover is the
first instrument that makes it practical to take continuous measurements at the
seabed over time,鈥� says Smith.
Smith and his team began building Rover in 1992 with an $800 000
development grant from the National Science Foundation, based in Washington
DC. They faced two hefty challenges. As well as ensuring that their
vehicle would tread lightly on the seafloor, they had to give Rover enough power
to carry out its work. So they built their vehicle using plastics, fibreglass
and lightweight titanium and made it buoyant with 17 football-sized glass
globes. This means Rover weighs 1500 kilograms on land but only 50 kilograms on
the sea-floor. 鈥淚t was critically important to be able to make these
readings without damaging the sediment we wanted to study,鈥� points out
Smith.
Yet at the same time, the vehicle had to be strong enough to withstand the
600 atmospheres of pressure common at depths of 6000 metres. 鈥淎t that depth, we
can reach 95 per cent of the ocean floor, adds Smith. To make the Rover strong
enough to reach the rest would have been vastly more expensive.鈥�
To power the tractor, the group needed a low-cost, self-contained energy
source that could work in the dark. They settled on a pair of electric motors,
one for each tread, run by batteries. The one-eighth horsepower motors were
picked from a catalogue, but the choice of batteries was more complex: the
lead-acid variety gave off gases and mercury cells showed unpredictable sudden
power losses after long exposure to the near-freezing temperatures that they
would experience in the deep. Lithium batteries worked well, but were too
expensive. Ultimately, the engineers equipped Rover with 300 alkaline D-cell
batteries, the everyday variety that power large torches.
On the level
Rover鈥檚 routine is a simple one. The two-metre-square tractor floats down
onto a flat part of the seabed. Once there, the vehicle trundles forward a few
metres until its on-board inclinometer tells the central computer that it is
level.
Once settled Rover lowers a shelf holding a package of instruments. It then
pushes two open-bottomed cylinders about six centimetres into the sediment.
Inside each cylinder, a stirring paddle spun at a lazy 9 rpm simulates the
motion of current and replicates the rate at which gases and nutrients are
naturally exchanged between the water and sediment. Every 15 to 30 minutes,
Rover sucks water samples from each cylinder, measures their oxygen content and
returns the samples to the cylinders, carefully filing away the results in its
memory.
After up to a week in one location, Rover is ready to move on. Pulling up its
cylinders, Rover checks the direction of current. Then, to prevent the clouds of
sediment its treads churn up from disturbing the next sample site, it moves
slowly against the current towards the next site a few metres away. Finding
level ground, the tractor plants its cylinders and begins again.
鈥淲e need a number of measurements from the same area because biological
activity on the seafloor can vary from one spot to another,鈥� Smith explains.
鈥淪omething as simple as a mound or burrow made by an animal has different kinds
of organisms associated with it. By taking several measurements close together,
we鈥檒l have enough data to draw general conclusions about an area.鈥�
The level of biological activity on the seafloor also depends on the amount
of carbon available for food. This reaches the seafloor as a 鈥渞ain鈥� of algae,
protozoa and other tiny organic particles which drift down from above. So Rover
takes samples with sediment traps, inverted cones which rest near the tractor.
In each trap, a series of collecting cups rotate over time, recording the rate
at which food arrives from waters closer to the surface.
Underwater whistles
After as long as six months and 30 sites later, Rover is ready to come home.
Scripps engineers sail back to the spot and broadcast an acoustic signal for
Rover to return to the surface. The message prompts Rover to release the clasps
holding a 110-kilogram lump of steel to its belly. As it begins to rise to the
surface, Rover emits an audible chirp to tell the ship where it is. Two hours
later, it reaches the surface and is hauled aboard to deliver its findings.
Rover鈥檚 first mission has thrown up some interesting results. Between 1 and 5
per cent of the organic material originating at shallow depths drifts to the
seabed in what had been assumed to be a steady stream. So until recently
researchers believed that biological processes there were constant and slow,
like their supply of food.
But Rover and its sediment traps have helped to confirm that this idea is
wrong. The rate at which manna from above arrives at the seafloor varies and the
tempo of life matches these changes. 鈥淭here are episodic fluxes which seem to
drive the entire biological system,鈥� Smith says. 鈥淢ore food is suddenly produced
on the surface and can鈥檛 be consumed fast enough, so more of it falls to the
产辞迟迟辞尘.鈥�
What Smith would really like is to work out exactly how life on the surface
and life on the seafloor are linked and how changes above effect life in the
depths. This is a vital link in the carbon cycle that has still to be explored.
鈥淲e鈥檙e trying to develop a scenario of how these cycles are connected over time
from the top to the bottom of the water column. The Rover will let us gather the
data that we can use to begin to make conjectures,鈥� he explains.
Return journey
Smith is hunting for partners to send Rover back to the seabed. Although
funding for new voyages has not yet materialised, academic geochemists and the
US Navy have shown an interest in Rover for their own research. Other groups
should begin to pay attention as Smith鈥檚 group equips the vehicle to take core
samples from the ocean floor, record the time and location that they were taken
and store them. Each core would be up to 20 centimetres long which would give
researchers an insight into the processes that laid down the sediments up to 10
000 years ago. 鈥淭he Rover is ideally suited for coring,鈥� says Smith. 鈥淭o
accomplish the same thing, other vehicles would have to surface many more
times鈥攔equiring the presence of a research ship, technicians, and much
more expense and trouble.鈥�
Eventually, Smith hopes that Rover鈥檚 descendants will become the
oceanographers鈥� all-purpose ocean floor research vehicles. 鈥淎nything you can do
on the seabed with a submersible vehicle, you can do with the Rover,鈥� he points
out. 鈥淎nd you can do it for a whole lot longer.鈥�
