By Rebecca Morelle Science reporter, BBC News

http://www.bbc.co.uk/news/science-environment-12183244

The climate secrets of the deepest part of the ocean, the Marianas
Trench in the western Pacific Ocean, have been probed by scientists.

The international team used a submersible, designed to withstand
immense pressures, to study the bottom of the 10.9km-deep underwater
canyon.

Their early results reveal that ocean trenches are acting as carbon
sinks.

This suggests that they play a larger role in regulating the Earth's
chemistry and climate than was thought.

Although two explorers, Jacques Piccard and Don Walsh, reached the
deepest part of the Marianas Trench - a point called the Challenger
Deep - in 1960, no humans have been back since.

And the handful of scientific missions, including this recent visit to
this deepest spot, have been carried out using unmanned underwater
vehicles.

Lead researcher Professor Ronnie Glud, from the University of Southern
Denmark and the Scottish Association for Marine Science (Sams), said
that working at more than 1,000 atmospheres of pressure was
challenging, but advances in technology had made it possible.

He told BBC News: "This is the first time we have been able to set
down sophisticated instruments at these depths to measure how much
carbon is buried there."

Under pressure

Professor Glud, working with scientists from the Japan Agency for
Marine Earth Science and Technology (Jamstec) and from the UK and
Germany, used a lander equipped with special sensors packed in a
titanium cylinder that was able to resist the remarkable pressures.
Don Walsh (left) and Jacques Piccard (right) in the bathyscaphe
Trieste (Noaa Ship Collection) Don Walsh (l) and Jacques Piccard's (r)
deep-sea record still stands

The lander was launched from a ship and took three hours to free-fall
to the sea bottom, where it carried out pre-programmed experiments
before releasing its ballast and returning to the surface.

The tests helped the scientists to assess the abundance of carbon at
those murky depths.

Professor Glud said: "Basically, we are interested in understanding
how much organic material - that is all the material produced by algae
or fish in the water above - settles at the sea bed, and is either
eaten by bacteria and degraded or is buried.

"The ratio that is either degraded or buried is the ultimate process
determining what are the oxygen and carbon dioxide concentrations of
the oceans and the atmosphere, and this gives us an overall picture of
how efficiently the sea can capture and sequester carbon in the global
carbon cycle."

While this has been studied in other parts of the ocean, such as the
abyssal plain - the large flat area of the ocean that lies between
4.6km and 5.5km of depth - the role deep sea trenches play in the
carbon cycle has until now remained largely unknown.
Continue reading the main story
“Start Quote

To see an experiment such as this carried out at these extreme
depths is a great leap forward in deep-sea science”

End Quote Dr Alan Jamieson Oceanlab

Professor Glud said: "Although these trenches cover just 2% of the
ocean, we thought they might be disproportionately important, because
it was likely that they would accumulate much more carbon because they
would act as a trap, with more organic matter drifting to the bottom
of them than in other parts of the ocean."

He explained that preliminary data from his experiments suggested that
this was the case.

He said: "Our results very strongly suggest that the trenches do act
as sediment traps. And they also had high activity, meaning that more
carbon is turned over by bacteria in the trenches than is turned over
at 6,000m of depth in the abyssal plain.

"What it means is that we have carbon storage going on in these
trenches that is higher than we thought before, and this really means
that we have a carbon dioxide sink in the deep ocean that wasn't
recognised before."

The next stage for the team is to quantify their results and work out
exactly how much more carbon is stored in deep sea trenches compared
with other parts of the sea, and how much carbon turnover by bacteria
is being carried out.

This, the researchers said, should help them to better establish the
role of the ocean trenches in regulating climate.

Surprising finds

This is not the first time deep sea trenches have surprised
scientists.
Continue reading the main story
Notoliparis kermadecensis

These fish were filmed at a depth of 7.7km

* Beyond the abyss

Recent studies by University of Aberdeen's Oceanlab team have revealed
that marine life is much more abundant in this hostile habitat than
was previously thought.

In 2008, they filmed the deepest living fish ever to be caught on
camera - a 17-strong shoal found at depths of 7.7km in the Japan
Trench, and the revealed other animals such as amphipods were present
in large numbers even deeper.

Dr Alan Jamieson, from Oceanlab, said the new study was helping
researchers to build up a better idea of what happens in the deepest
of the deep.

He said: "The trenches continue to amaze us.

"And to see an experiment such as this carried out at these extreme
depths is a great leap forward in deep-sea science.

"These studies will greatly enhance our understanding of how the deep
trenches contribute to carbon cycling in the world's oceans."