How old is the ocean?

But if essentially none of the water on earth is "fresh," just how old is it, and how did it get here? The Earth and other planets in our solar system probably formed about 4.54 billion years ago from the condensation and accretion of dust and rocks into growing lumps that eventually became planets, due to gravitational attraction. But there was originally probably no liquid water on the earth (if this theory of planet formation is correct, it would have been far too hot for liquid water at first).

There are two main scenarios that scientists think could have formed the oceans. Either the oceans originated from off-gassing of water vapor that cooled, condensed and rained out as liquid water from the Earth's molten interior (this continues today with volcanic activity) - basically steam escaping out of Earth's rocks; and/or delivery from space via collisions of icy comets or meteors with the Earth. Which of these scenarios is responsible for the majority of water in the oceans is still unclear, but we know that most of the water in the oceans (and on the rest of the planet) is very ancient - on the order of 4 billion years old.

However, when scientists refer to the "age" of ocean waters, they often mean something completely different - how long the water has been out of contact with the atmosphere at the surface of the ocean. The oceans aren't stagnant - there are currents across the surface moving warm tropical waters towards the poles on the western sides of ocean basins, and cold Arctic and Antarctic waters moving back towards the equator on the eastern sides (see this page for some amazing visualizations).

But the oceans also overturn vertically - cold and salty (and therefore dense) surface waters sink to the bottom in a few places around the world, then move through the basins down in the cold, dark depths before resurfacing elsewhere. This is called Thermohaline Circulation because it's driven by differences in water density caused by temperature - "thermo", and salinity - "haline". It's also called the Ocean Conveyor Belt. One way to get very dense water is when the ocean freezes: salt is rejected out of the ice and the surrounding liquid ocean water becomes super salty.

So, how long does it take for the average drop of seawater to make one full trip around the ocean basins, from the surface in the north Atlantic to the deep north Pacific? About 1000 years.

If you think that this means that old water is totally free from any of the gross things humans dump into the ocean, and therefore that you should drink it, indeed you can, for a premium price (note: I think this entire idea is nuts).

How do we know that deep Pacific water is about 1000 years "old"? It doesn't have a freshness date stamped on it, or useful dating features like the rings in a tree. One neat tool for aging the sea is radiocarbon, or ¹⁴C dating. This radioactive element is created naturally in the atmosphere by solar radiation, and makes up a small percentage of all of the carbon on Earth (the rest is composed of stable forms of the element: ¹²C and ¹³C). Carbon-containing compounds like atmospheric CO₂ that contain "fresh" radiocarbon can dissolve in ocean waters in contact with the air. Those surface waters would then have the same amount of radiocarbon as the atmosphere - but when they sink, the radioactive carbon begins to decay at a set rate.

By measuring the amount of remaining radiocarbon in ocean waters at different depths and different places around the world, we know that the deep Pacific holds the ocean's oldest waters, which have been out of contact with the atmosphere for about 1000 years before they mix to the surface again.

So, what do you think? Why is ocean mixing important? What might happen if circulation slows down due to global climate change? (Hint: have you seen the fantastical movie The Day After Tomorrow?)

References

Elkins-Tanton, L. T. Formation of early water oceans on rocky planets. Astrophysics and Space Science 332, 359-364 (2011).

Hartogh, P., et al. Ocean-like water in the Jupiter-family comet 103P/Hartley 2. Nature 478, 218-220 (2011).

Bryden, H. L., Longworth, H. R., Cunningham, S. A. Slowing of the Atlantic meridional overturning circulation at 25° N Nature 438, 655-657 (2005).