SAMS news room

The world’s first investigation of climate-related processes taking place in the deepest part of the global ocean, at the bottom of the Mariana Trench, has just been carried out by a research team led by Professor Ronnie Glud of SAMS and University of Southern Denmark, in an international collaboration with colleges from The Max Planck Institute for Marine Microbiology in Bremen (Germany), The Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and University of Copenhagen (Denmark).

To better understand the global carbon cycle, including the removal of carbon dioxide from the atmosphere, it is critical to know what role the oceans play in carbon sequestration. Pushing the frontiers of science, a sophisticated deep-diving autonomous lander has carried out a series of descents to the floor of the Challenger Deep, a canyon 10.9 km beneath the ocean surface. Here it performed detailed investigations of the microbial processes occurring there.  Such detailed science has never been carried out at these extreme depths, where ambient pressure is more than 1000 times that at the surface.

Deep ocean trenches make up only 2% of the seafloor but may be disproportionately important as a trap for carbon. The aim of this research was two-fold: 1) to measure the rate by which organic carbon is degraded at these extreme depths and 2) to estimate how much organic carbon is retained in the trenches (by examining the collected sediment samples).

Prof Glud said: "The fraction of carbon retained in the seabed is key to our understanding of carbon cycling and hence the climate of our planet."

Preliminary data from the instruments reveals that the turnover of carbon is much greater at the bottom of the Trench than on the Abyssal plain (6000 metres down). This demonstrates that the seabed in the trenches acts as a trap for organic material and may therefore have high rates of carbon retention. The research team will carry out analyses on recovered samples to reveal the rate at which sediment is accumulating at the bottom of the trench.

The pressure at these great depths is extreme, so to investigate microbial processes in samples from such depths can be very difficult – bringing the organisms to the surface can radically affect them. Therefore instruments capable of performing the measurements directly on the seafloor had to be developed. Specially constructed sensors probed the sediment in small grids and mapped out the distribution of oxygen in the seabed, providing key insight into the rate at which organic carbon is degraded.

To get the 'robot' to operate at 10.9km depth during the expedition with the Japanese research vessel Yokosuka (Cruise YK 10-16), commissioned by JAMSTEC, was a great challenge. Equipment designed by the team was specially engineered for the mission to function at pressures in excess of 1000 atmospheres. The deployed instrument consisted of parts developed at JAMSTEC (release and recovery system) and the central measuring module that was developed by Ronnie Glud and his team – and which was assembled and tested in Scotland and Germany.

Professor Glud continued: "We succeeded in performing detailed mapping of microbial activity using highly sophisticated, movable instrumentation and microsensor arrays."

Having completed its mission on the seafloor, the lander began its ascent to the sea surface and scientists onboard ship waited three hours for it to return from the bottom of the Trench with its unique data. In total, the sampling was successfully carried out four times.

Ronnie Glud added: "We are delighted this expedition went so well. The extreme pressure at the bottom of the Mariana Trench made it a huge challenge to design equipment to function at these depths. There is a great sense of achievement to study and bring back data from the deepest part of the ocean. We expect this information to help us answer some very important questions regarding carbon mineralisation and sequestration in the ocean trenches."

The research involved collaboration between Prof Glud and Dr Henrik Stahl of SAMS, German, Danish and Japanese scientists – both at the scientific and technological level.

"This is a great example of international scientific collaboration," said Prof Glud.

Other questions that will be addressed within the next six months include an examination of whether bacterial and viral communities are different from those of the abyssal plain.