Deep below the surface of the North Pacific Ocean, there’s a strange patch of water that lies practically stagnant and trapped beneath the surface. In fact, the last time this water came into contact with Earth’s atmosphere the world was home to Vikings, Mayans, and Romans.
A new study in the scientific journal Nature has looked into the so-called “shadow zone” of the North Pacific to see why it has remained trapped for over 1,000 years.
"Carbon-14 dating had already told us the most ancient water lied in the deep North Pacific. But until now we had struggled to understand why the very oldest waters huddle around the depth of 2 kilometers [1.2 miles]," lead author Dr Casimir de Lavergne, from the University of New South Wales, Australia, said in a statement.
"What we have found is that at around 2 kilometers [1.2 miles] below the surface of the Indian and Pacific Oceans there is a 'shadow zone' with barely any vertical movement that suspends ocean water in an area for centuries.”
Armed with previous research on this zone, the scientists on this project began to look where others previously hadn’t – the seabed. They found that the depth and shape of the ocean floor in this area plays a huge part in trapping the water in this one spot.
Water currents in the Pacific usually started with salty Antarctic ice melting and heading to the bottom of the ocean. As water heads northwards into the Pacific, it’s drawn upwards by surface currents of warmer upper ocean water from the tropical seas. Currents in the deep sea also help "push" dense waters upwards and northwards. Together, these forces help to ensure the waters flow with a vertical circulation and the layers begin to mingle more.
However, in this “shadow zone”, the water remains too far from these surface currents. Crucially, it's also not reached by deeper sea currents either because of the seabed's rough topography and distance from any geothermal heat sources. Since the bottom water cannot rise above 2.5 kilometers (1.5 miles) below the surface, the water above just sticks around in a motionless limbo.
"When this isolated shadow zone traps millennia old ocean water it also traps nutrients and carbon which have a direct impact on the capacity of the ocean to modify climate over centennial time scales," said Dr Fabien Roquet, a study author from Stockholm University in Sweden.