Diving for knowledge under Antarctic sea ice

From ships to satellites, and now submersibles. Marine researchers have found a new means of studying the world below the surface. From Gothenburg, Sebastiaan Swart will coordinate a fleet of underwater vessels collecting unique data for oceanographers, ecologists and climate researchers.

Sebastiaan Swart

PhD in Physical Oceanography

Wallenberg Academy Fellow 2015

Institution:
University of Gothenburg

Research field:
Studies of ocean processes, mainly physical, with the help of underwater autonomous vehicles

The oceans of the world play an absolutely key role in global climate. Without these huge bodies of water to even out temperatures across the world, the poles would have been even colder and the equator even hotter. Photosynthesis by ocean phytoplankton enables the ocean to absorb carbon dioxide roughly the equivalent of all the world’s forests. Therefore, the interaction between the ocean and the atmosphere is fundamental to climate change and our understanding thereof. Yet scientists still know relatively little about ocean processes below the surface, particularly those taking place under sea ice-covered regions at the poles.

“Antarctica is fringed by a wide belt of sea ice. It’s called the marginal ice zone, and it melts and forms depending on the seasons. The ice covers many millions of square kilometers, it’s larger than the entire Antarctic continent. Although this area is vital to the earth’s climate and ecosystems, we basically have no observations of it. It has been technically impossible to find out what’s happening under the ice – until now,” says Swart.

“As a Wallenberg Academy Fellow I will be able to conduct curiosity-driven research which is very risky – but that has huge potential. Oceanography is such a small field; it’s great that the Foundation is supporting it. Also, Sweden has had a limited representation in this field so far, and this project can help change that.”

Customized submersibles

He is an oceanographer, and Wallenberg Academy Fellow at the University of Gothenburg. Together with colleagues across the world – particularly in South Africa, where he lived and worked until recently – Swart has begun to study the oceans of the world in a completely new way. He is using AUVs, autonomous underwater vehicles, controlled via satellite, to gather data on or below the sea surface.

Currently he is customizing AUVs to enable them to collect data below ice as well. To do this, the vehicles must be able to determine whether there is open water above them. Normally the vehicle would dive, gather data, and return to the surface to transmit the information via satellite and receive new instructions. The ice-adapted vehicles will be fitted with a type of sonar, an echo sound device making it possible for them to check if it is likely that they can reach the surface. A strong echo means ice – a weak echo open water. If it seems the surface is ice covered, the vehicle will continue to dive until it finds open water. While it is under water the researchers have no direct control over it.

“It’s a risky experiment, but we have to believe that sooner or later they will find open water. We can program them not to dive more than, say, fifty times. After that they will be programmed to head north to find open water. There is a high risk we will lose some of the vehicles. But if we succeed, we will obtain extremely valuable data,” Swart says.

More data for less money

The AUVs are about the same size as a person, and weigh around 60 kilograms. There are wave-powered models, but the ones exploring below the ice are driven by lithium batteries lasting up to ten months. To save power, the vessel uses low energy mechanisms to propel itself and its scientific sensors. It submerges and surfaces by pumping oil between a pressure chamber on the inside, and a rubber bladder on the outside, which alters its buoyancy characteristics. When the oil is inside the chamber, the vessel sinks; when it fills the bladder, it acquires a lower density than water, acting like a balloon and pulling the AUV back to the surface.

Until twenty years ago oceanographers acquired most of their data from ships. But few ships go to the Southern Ocean, and those that do only go in summer. This resulted in sparse and sporadic data. For the last couple of decades, it has been possible to collect data by satellite, but they can only detect what happens on the surface. AUVs offer radically new opportunities, and are also much less costly than ships. An ice-breaking ship’s fuel alone costs about SEK 200,000 a day.

“Collecting data from the Southern Ocean covered by sea ice is something unique to this project. No-one has ever been able to do that before and achieve the level of detail that we will get. Among other things, this project can make an invaluable contribution to improving our climate models,” Swart adds.

Antarctic fascination

The submersibles will be fitted with equipment to measure temperature, salinity, oxygen content, light, fluorescence and currents. Each added scientific instrument makes the vehicle heavier. On the other hand, new lightweight sensors are being developed all the time, as well as technology to make these vehicles more robust, and reduce biofouling; growth of aquatic organisms on the hull, which can compromise the AUV.

“I didn’t think growth on the AUV would be a problem. These craft dive to a depth of a thousand meters, and what would want to grow on something traveling between such different pressure levels? But we nearly lost one vehicle due to the extra weight it was carrying in the form of growing barnacles,” Swart recalls, showing a photograph of the AUV when it was taken out of the water, covered with a thick layer of tiny crustaceans.

Sebastiaan Swart never thought he would be an oceanographer. He studied environmental science and archaeology, but was given the opportunity to go on a research voyage to the Southern Ocean, where he caught the science bug.

“The Antarctic was like visiting another planet. It was so beautiful. Now I find it incredibly fascinating to explore it from a distance. We throw in a robot no bigger than a human being, and leave it to its fate under ridiculously harsh conditions – waves ten meters high, temperatures below freezing point…and it sends data to me in my comfortable office. I never know what it’s going to find. It’s really exciting.”

Text Lisa Kirsebom
Translation Maxwell Arding
Photo Magnus Bergström