|The global water cycle:
what can isotopes tell us?
The radioactive isotope tritium (3H) and the stable isotopes deuterium (2H) and oxygen-18 (18O) are rare components of the water molecule H2O.They offer a broad range of possibilities for studying processes within the water cycle. Tritium was released to the atmosphere during the test phase for hydrogen bombs. The very low natural levels were overwhelmed by concentrations several orders of magnitude higher. Since then, tritium levels have been progressively decreasing due to washout processes and the admixture of moisture from the oceans. Due to the long residence time of ocean water as compared to the half-life of tritium (up to 2,000 years and 12.4 years respectively) the oceans act as a sink for tritium. Tritium in precipitation has been monitored since the beginning of the bomb tests in late 1952. These data enabled the establishment of timescales for the transport of water through different compartments of the hydrological cycle. For example, groundwater, which does not contain tritium, must have infiltrated as rain before 1952 because the natural concentrations have already decayed below the detection limit. If one measures 1,000 tritium units (TU) in glacier ice, this layer must originate from snowfalls in 1963. It was the time, when tritium injections to the atmosphere reached a maximum.
The stable isotopes label in a different way: since a water molecule
containing deuterium (2H) or oxygen-18 (18O) is heavier
than a normal 1H1H16O molecule, water
vapour forming precipitation will be depleted in heavy isotopes relative
to ocean water. Condensation forming raindrops from a cloud reverses this
process. The heavier molecule condenses first, i.e. the rain is isotopically
enriched, and the cloud moisture is subsequently depleted as the rain out
A global network of isotopic input data is necessary to decipher this ongoing tracer experiment in the laboratory of nature.