How the “shadow zone” traps the world’s oldest ocean water

New research from the ARC Centre of Excellence for Climate System Science at the University of New South Wales (UNSW) has revealed why the oldest water in the ocean in the North Pacific has remained trapped in a shadow zone 2km below the sea surface for over 1000 years.

The last time this water encountered the atmosphere the Goths had just invaded the Western Roman Empire.

The research published in Nature suggests the time the ancient water spent below the surface is a direct consequence of the shape of the ocean floor and its impact on vertical circulation.

“Carbon-14 dating had already told us the most ancient water lies in the deep North Pacific. But until now we had struggled to understand why the very oldest waters huddle around the depth of 2km in this ocean,” said lead author from the UNSW School of Mathematics and Statistics, Dr Casimir de Lavergne.

“What we have found is that at around 2km below the surface of the Indian and Pacific Oceans there is a ‘shadow zone’ with barely any vertical movement that suspends ocean water in the area for centuries.

The shadow zone is a volume of almost stagnant water between the rising currents caused by the rough topography and geothermal heat sources in the deep ocean below 2.5km and the shallower wind driven currents closer to the surface.

Importantly, unlike the Indian and Pacific Oceans, the Atlantic Ocean does not have such a shadow zone as it receives a second influx of cold water from the north that prevents this forming.

Before this research, models of deep ocean circulation did not accurately account for the constraint of the seafloor geometry on upwelling. Once the UNSW researchers factored it in they found the upwelling could not lift the bottom water any higher than 2.5km below the surface.

The still, isolated area that resulted explains why parts of the ocean are so old in the Pacific and Indian Oceans.

While the researchers have unlocked one part of the puzzle their results also have the potential to tell us much more.

“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 influence climate over time,” said fellow author from the ARC Centre of Excellence for Climate System Science Dr Ryan Holmes.

“So, while the research may have answered one question about deep ocean water, it has also opened doors to answer more questions that relate to the future impacts of climate change and the ecology of our major oceans.”



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