Why pour 200L of fluorescent dye into the ocean? For science

A barrel of fluorescent dye has allowed scientists to solve a mystery of ocean circulation.

Researchers released the non-toxic dye at the bottom of an ocean trench, allowing them to measure the movement of cold, deep water to the ocean surface.

They’ve found that the upwelling process is 10,000 times faster than the expected global rate.

The findings, published in Nature, also provide a possible answer to a question that has been vexing oceanographers for 25 years.

Water circles the globe in immensely complicated patterns. These patterns drive weather, biodiversity, and a host of other global systems, but they are very difficult to track.

One of the biggest influences on ocean circulation is cooling water at the poles, which sinks to the ocean floor, then travels to warmer regions and rises to the surface: this process is called upwelling.

Until now, this upwelling process has never been directly measured. In the 1960s, oceanographer Walter Munk proposed that undersea turbulence caused it, and calculated a rate at which ocean water rose around the globe: roughly 1 centimetre per day.

Boat on open ocean
A special high-speed winch that the researchers used to swiftly raise and lower instruments to track the dye’s movements underwater. Credit: San Nguyen

But in the last 25 years, measurements have suggested that turbulence is highest close to the seafloor. This should prevent upwelling from happening.

In this study, an international team of researchers lowered a drum of fluorescent dye – just over 200 litres, or 55 gallons, in volume – into an ocean trench some 370 kilometres northwest of Ireland. The trench was about 2,000 metres deep.

“We selected this canyon out of the roughly 9,500 we know of in the oceans because this spot is pretty unremarkable as deep sea canyons go,” says senior author Matthew Alford, a professor of oceanography at the University of California – San Diego, US.

“The idea was for it to be as typical as possible to make our results more generalisable.”

Person standing next to black barrel on ship
This barrel is filled with non-toxic fluorescent dye, which researchers released just above the sea floor to answer a longstanding question in oceanography. Credit: San Nguyen

Once the barrel was 10m away from the ocean floor, it was opened with a remote mechanism and fluorescein – a non-toxic dye used widely in medicine and environmental studies – flowed out.

Because the dye is fluorescent, the researchers were able to track its movement with devices called fluorometers. This allowed them to map how the water moved from a ship, floating over the deep sea trench for several days.

They found that the dye moved up much faster than expected: a rate of about 100m per day.

“We’ve observed upwelling that’s never been directly measured before,” says lead author Bethan Wynne-Cattanach, a postdoctoral fellow at the University of California – San Diego.

“The rate of that upwelling is also really fast, which, along with measurements of downwelling elsewhere in the oceans, suggests there are hotspots of upwelling.”

The researchers also saw some dye moving in directions they weren’t expecting, meaning that the water is moving in more complicated ways than the researchers expected.

Alford says this is “a call to arms for the physical oceanography community to understand ocean turbulence a lot better”.

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