Tritium and Helium Isotopes in the Oceans

Tritium ³H is a radioactive isotope of hydrogen. It has a half-life of 12.43 years and decays by beta emission to helium-3 (³He). Tritium is produced naturally by the interaction of cosmic rays with nitrogen and oxygen mainly in the upper atmosphere. Tritium exists in the atmosphere in the form of tritiated water molecules (HTO) and is transferred from the atmosphere to the sea-surface through vapor exchange and rainfall. This exchange occurs on short time scales, which makes the tritium distribution asymmetric between hemispheres, with higher concentrations in the northern hemisphere. The involvement of tritium in the hydrological cycle makes it an excellent transient tracer for tracing water movement in natural water systems. By the late 1950's and early 1960's large quantities of anthropogenic tritium were delivered to the atmosphere by the thermonuclear weapons tests. This anthropogenic signal completely masked the small quantities of naturally produced tritium. The tritium transient in the atmosphere is dominated by a sharp bomb signal, which peaks in 1963. The anthropogenic tritium is used as a tool for studying movement of water masses in oceans. In using tritium as an oceanographic tracer anthropogenic tritium has some added advantages:

1. its input function is better known than that of natural tritium
2. and its use is not limited to the upper waters.

The utility of tritium as a tracer is enhanced considerably by additional measurement of ³He. The combined measurement of tritium and ³He allows estimates of the time elapsed since a water parcel has been isolated from the atmosphere when the "tritium-helium clock" was set to zero. This method known as tritium-helium dating has been used extensively in oceanography, limnology and hydrology. The accuracy of the ³He measurements allow the detection of tritium/³He ages within a few months resolution. In using tritium-helium dating one has to be aware that tritium helium age does not respond in a simple manner to mixing. Mixing produces an average age which is biased towards the younger watermass.

Large excess of ³He relative to solubility equilibrium has been found in all the deep basins of the world ocean. This excess helium is attributed to the flux of mantle helium emanating from tectonically active spots such as mid-ocean ridges, convergent margins, and hot-spots zones. The mantle helium released in the deep ocean forms plumes that extend across ocean basins. The high signal to noise ratio and the conservative nature of mantle helium makes it an excellent tracer for mid-depth circulation.

Projects

• WOCE Pacific Ocean Helium/Tritium measurements (Abstract)
• Mantle ³He Distribution and Deep Circulation in the Indian Ocean (Abstract)
• Helium, Neon and Tritium measurements along five lines in the Indian and Pacific Oceans during WOCE (Sections and map of station locations and cruise tracks)