Deep in the middle of the deepest oceans of the world lies a shadow zone, where ancient sea water hangs, trapped in vast volume, stagnant. The water last floated to the surface some 1000 years ago.
Research by a led by mathematician Casimir de Lavergne of the University of New South Wales in Australia has revealed how this water gets trapped, midway between the abyssal depths and the sunlit surface. The combination of temperature gradients and the shape of the ocean floor sets strong constraints on the possible circulation of water positioned in the middle depths – effectively holding it hostage for centuries.
To make their findings the team studied the waters of the North Pacific Ocean. The movement in the area can be broadly described as a northward flow of very dense water along the ocean floor that gradually turns into a general southward flow of much less-dense waters in the upper levels – a cycle known as abyssal overturning circulation.
Lavergne and his team, however, discovered that north of latitude 32 degrees South the shape of the ocean floor compels the dense north-flowing water to turn and head south again without rising to higher than 2.5 kilometres. Without strong surface currents flowing from the north, this leaves a body of water between 2.5 and one kilometres down just sitting there, unable to circulate.
The researchers established the age of the trapped pocket of water by carbon dating it, and established that it had not been to the top or bottom of the ocean since the Goths sacked the western Roman empire.
In a paper published in the journal Nature, the scientists write that the ocean floor structure that gives rise to the Northern Pacific shadow zone is also to be found in the Indian and Atlantic ocean basins – and therefore place “a major external constraint” on the way the water circulates.
The findings are likely to modify assumptions regarding the ability of the ocean to influence global warming.
“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,” says co-author Fabien Roquet, from Stockholm University in Sweden.