Mysterious ‘shadow zone’ traps 2000-year-old water

A MYSTERIOUS abyss in the ocean known as the “shadow zone” traps ancient water dating back to 400AD. We now know why it’s there.

IT’S called the “shadow zone” and it lies around two kilometres below the surface in an ocean abyss where trapped water dates back to the fourth century.

This ancient water, which is between 1000 and 2000 years old, dates back to when the ancient Germanic tribe the Goths instigated the end of the Western Roman Empire and the rise of Medieval Europe.

Lying in a 6000km by 2000km patch of the North Pacific Ocean between 1km and 2.5km below the surface, the shadow zone’s reason for existence has remained a mystery until now.

Carbon dating identified its age and location some time ago, but scientists struggled to understand why the stagnant zone lay where it did.

An international team led by the University of NSW has now established that the zone is prevented from circulating up to make contact with the atmosphere by the shape of the sea floor.

The UNSW School of Mathematics and Statistics’ Dr Casimir de Lavergne is the lead author of a research paper about the zone, published in the latest edition of the scientific journal Nature.

The “shadow zone” lies around 2km below the surface of the North Pacific Ocean and is thousands of kilometres long.

Schematic illustration of water currents. Credit: Fabien Roquet and Casimir de Lavergne

“Abyssal ocean overturning shaped by sea floor distribution” explains that the zone has barely any vertical movement because factors prevent it from rising.

The zone lies between rising currents caused by the rough topography and geothermal heat sources below 2.5km and the shallower wind driven currents closer to the surface.

“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 timescales,” said the paper’s fellow author Dr Fabien Roquet of Stockholm University.

Swedish scientist Dr Fabien Roquet is part of the UNSW team which has researched the shadow zone.

The zone traps water dating back to the 4th century Goths (above).

Shadow zone team leader Dr Casimir de Lavergne.

The low oxygenation of the zone means marine life is restricted, but not completely absent.

“It’s not a zone of very flourishing life but that doesn’t mean it’s a dead zone,” Dr de Lavergne said.

The Atlantic and Indian oceans also have a shadow zone, but of younger origin, while waters in the Antarctic have a relatively high oxygen content.

The UNSW researchers hope their work will help other scientists understand the capacity of the oceans to absorb heat trapped by rising greenhouse gases.