Tritium profiles in the oceans determine the movement of water to depth and help to distinguish areas of up-welling.


Tritium, a natural hydrogen isotope (3H), enters the oceans as a part of the water molecule, either as water vapour exchange between the atmosphere and oceans, or by direct precipitation. This tritium is decaying to a stable, inert isotope called helium-3 (3He).

At the oceans surface this helium-3 will escape to the atmosphere, and not accumulate in the water. However, once the water leaves the surface and moves to depth, the tritium continues to decay but, since the 3He can no longer escape, it begins to accumulate. By measuring excess (accumulated) 3He and the amount of tritium remaining in the water, it is possible to determine the time that has elapsed since the water last was at the surface.

These times, ranging from months to a decades, can be used to study ocean mixing.