Banned gases reveal the age of water

For more than fifty years CFC gases like freons and halons were used in everything from refrigerators and fire extinguishing systems to insulation foams and hairspray. Stable and of low toxicity, these gases were well-suited for so many applications that the CFC concentration in the atmosphere increased exponentially from the 1960s.

Then it was discovered that CFCs harmed the ozone layer. In 1987 46 nations signed the Montreal Protocol, agreeing to phase out the use of CFCs. By now all of the world's nations have signed the protocol, an example of a successful international environmental agreement.

Since around the millennium the concentration of CFCs in the atmosphere has declined, and the ozone layer has begun to recover. But, as CFCs break down slowly, a large share of the emitted substances still remains in nature.

Something good still comes out of this. Only trace amounts of CFCs in sea water are required to detect their presence. Such measurements may be used to follow the water's journey into the depths – crucial for monitoring changes in ocean currents. 

Sea water sinks to great depths

"Here we find the age of the water."

Emil Jeansson points to a sturdy steel frame secured to a bench with a cargo strap. The oceanographer from the Bjerknes Centre is the Research Director of Ocean Observations at NORCE. 

The bench with the steel framed device stands in a laboratory onboard the research vessel Kronprins Haakon, on a cruise in the Greenland Sea.

Warm water from the Gulf Stream flows into the Nordic Seas. There the surface water cools, gets denser and mixes downward. In the Greenland Sea such mixing causes the formation of dense deep water. 

Continuing southward in the straits between Greenland, Iceland, the Faroe Islands and Scotland, the dense deep water plunges like an underwater cascade to the bottom of the Atlantic Ocean.

The deep water formation in the Greenland Sea is crucial, not only locally, but for the whole system of ocean currents in the Atlantic.

"It is important to know how long time it takes the water to travel from point A to point B," says Emil Jeansson.

If you can find out how long ago the water left the surface, you will also know how fast it sinks.

Measures CFC in the deep ocean

The steel frame on the bench is about a meter long in each direction. Inside the frame, wires, tubes and rainbow-colored cables twist through glass flasks and a box with frost on the outside and seventy degrees Celsius below zero inside. 

"A jungle," says Emil Jeansson.

A jungle and a jumble, yet a systematic assembly line. Water samples are fed in, and after the water has passed through all the tubes and containers – as well as an instrument standing beside – the results pop up as numbers and curves on a computer screen:

The water contains this much CFC. That means it is so many years old.

In addition to CFC-12 – freon – the instruments registers SF₆, a compound used in industries such as aluminum production. Unlike CFCs, SF₆ emissions continue to rise.

"SF₆ og KFK-12 have a well-known history in the atmosphere," says Emil Jeansson.

Together these two substances provide a reliable reference to determine which year the water was last at the surface.

Age tells of deep water formation

Water samples are collected from the sea outside, some not many meters below the hull of Kronprins Haakon, others at several thousand meters' depth.

When the sampled water was at the surface, it took up CFC from the air – substantial amounts in years with a lot of CFC in the atmosphere, less in other years. When the water was mixed downward, CFC and other gases were brought along. As a result, you may compare the concentration of CFC in a water sample with the development in the atmosphere.

Then you know when water began to sink.

"It may have been ten, fifty or a hundred years ago," says Emil Jeansson.

The age of the water indicates how fast the water reaches great depths and how extensive the sinking and formation of deep water in the Greenland Sea is. Combined with other measurements, these gases may also be used to calculate how much anthropogenic CO₂ the deep water contains – a measure of how much CO₂ the ocean absorbs from the atmosphere.

No shortage of CFCs

Almost forty years have passed since nations started phasing out CFCs. The CFC concentration in the atmosphere is declining. Will there be a time when there is not sufficient CFC-12 to trace sinking sea water?

"It is possible that other tracers may be relevant in the future," says Emil Jeansson.

The oceanographer dismisses the idea that a lack of CFCs will pose any problems. For now, SF₆ concentration still rises, and CFC-12 is present at concentrations high enough to be easily detected.

"I will be able to measure CFCs for as long as I live," he says.