The water that last saw the sky around the year 600

It starts in winter, in the far north

In the Labrador Sea and in the seas between Greenland and Iceland, winter winds strip heat from the surface. The water cools until it is heavier than the water beneath it, and it sinks two or three kilometres towards the ocean floor. The drop goes down with it.

Something similar happens, even deeper, around Antarctica, in the Weddell and Ross seas, where cold and the salt left behind by freezing sea ice make the water sink. Most deep water starts in a few cold places like these.

How do you date water?

While it is at the surface, seawater takes carbon from the air, and among those atoms is a little radiocarbon, made by cosmic rays high in the atmosphere. Once the water sinks, nothing replaces it. Radiocarbon decays at a fixed rate: half of it every 5,730 years. The less is left, the longer the water has been away from the air.

The colours on the globe are that measurement, from ocean surveys of the 1990s at 33 depths: pale where the water keeps a lot of radiocarbon, violet and then glowing ember where the least is left.

The clock has a catch: it does not start at zero. Surface water mixes with older water from below and swaps carbon with the air slowly, so by this clock the sea surface already looks about 400 to 1,000 years old over most of the ocean, oldest near Antarctica. That is why the ages in this story come from a study that corrects for it (Gebbie and Huybers, 2012).

The descent

As you scroll, the globe goes down with the drop. Each layer is a real depth in the map. Grey is the seafloor: the ridges down the middle of the oceans rise above this depth, and the basins shrink as you go down.

The new deep water heads south, keeping close to the American side of the ocean.

Three or four hundred years at 30° north

On the Atlantic floor at 30° north, the water around the drop left the surface 300 to 400 years ago. It sank at about the time Dimitrie Cantemir, the scholar prince of Moldavia, was writing his description of the country for the Berlin Academy.

At the average pace of the water here, your drop would arrive around 2330–2430.

Almost eight centuries in the South Atlantic

Further south, the water mixes with older water. At 40° south, the oldest water in the South Atlantic left the surface almost 800 years ago, around the years when the Mongols rode into the lands that would become Moldavia and Wallachia, in 1241.

Round Antarctica

Here it meets the densest water in the ocean, which sinks off Antarctica in the Weddell and Ross seas and spreads along the bottom. The current that circles the continent carries the mixture on, into the Indian Ocean and the Pacific.

Over a thousand years, north of the equator in the Pacific and Indian oceans

In the deep Pacific and Indian oceans, north of the equator, the water at 2,500 metres is on average more than a thousand years old. Nothing new arrives here quickly: the water creeps north and grows old.

The oldest water in the sea

At 2,500 metres under the north-east Pacific lies the oldest water yet dated in the ocean. It last touched the air 1,200 to 1,500 years ago; the best estimate is 1,427. It sank between the years 526 and 826: from the time of Justinian, who finished Hagia Sophia in 537, to the time of Charlemagne, crowned in Rome on Christmas Day 800.

The point with the least radiocarbon on the whole map lies to the north-west, under the Bering Sea, at the same depth. The raw clock reads 2,310 years there, but like every raw reading it runs ahead of the real age: the water set out with a surface clock already hundreds of years old.

If the drop takes this road at the average pace, it gets here around 3200–3500.

The deep still remembers the Little Ice Age

Water this old carries the climate of the time it sank. Between the 1870s, when the British ship HMS Challenger lowered thermometers on hemp ropes across the world’s oceans, and the surveys of the 1990s, the Atlantic warmed down to 2,800 metres. The deep Pacific, between 1,600 and 2,800 metres, cooled by 0.02–0.08 °C.

Gebbie and Huybers (2019) explain it: in the deep Pacific, water that sank in the warm Middle Ages is still being replaced by water that sank in the colder centuries of the Little Ice Age. The deep ocean is still answering a cooling that began several hundred years ago.

The way back

The road back to the surface runs through the Southern Ocean, where the winds draw deep water up (Marshall and Speer, 2012). There the heat and carbon stored in the deep ocean meet the air again, and the radiocarbon clock starts over.

The drop sinks this winter. If it takes the long way round, through the North Pacific, it would not see the sky again, on average, before the 3200s.

Send your drop (image)