The oldest water in the ocean didn't reach its advanced years by accident.
Deep in the North Pacific, a vast stretch of submerged ocean is trapped in a kind of stasis between powerful currents and the sea floor, and for the ancient waters caught in this airless 'shadow zone', it's almost like time stands still.
"What we have found is that at around 2 kilometres (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," says oceanographer Casimir de Lavergne from the University of New South Wales in Australia.
"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 kilometres."
The reason, according to a new analysis of deep water movements called abyssal overturning circulation, is due to the geometry of the seafloor, which for long periods prevents very deep, dense ocean waters from circulating to the surface.
"[N]orth of 32° S, the depth distribution of the seafloor compels dense southern-origin waters to flow northward below a depth of about 4 kilometres and to return southward predominantly at depths greater than 2.5 kilometres [1.5 miles]," the team explains in its paper.
What this means is that for an isolated section of the North Pacific running approximately 6,000 kilometres (3,728 miles) west to east by about 2,000 kilometres (1,243 miles) north to south, deep waters run in a loop, hardly ever getting a chance to reach the surface.
"Our main advance is actually to understand how and why this water is so old," one of the team, oceanographer Ryan Holmes from the ARC Centre of Excellence for Climate System Science, told The Sydney Morning Herald.
"We've developed a theory that explains all our observations – it's simply dependent on the shape of the sea floor."
Due to the seabed topography and the way the trapped water almost never reaches the ocean surface, oxygen concentrations in the shadow zone would be very low compared to waters sitting higher in the sea – but this almost stagnant water could still support life, the team thinks.
"It's not a zone of very flourishing life but that doesn't mean it's a dead zone," de Lavergne said.
The Atlantic Ocean and the Southern Ocean do not feature the same sorts of circulation traps, but the researchers say the Indian Ocean has a similar shadow zone – although its proximity to fresh waters sourced from Antarctica mean its content is less stagnant than the deep reaches of the North Pacific.
Now that we know about this submerged holding pattern, the researchers want to investigate what it means for oceans systems as a whole.
"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," Holmes explained in a press release.
"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."
The findings are reported in Nature.