Tritium ocean

Fig. 10-10 Tritium section of the western Atlantic from 80 N to the equator versus depth (m). Vertical exaggeration is 2000 1. Horizontal scale is proportional to cruise track. (Reproduced with permission from H. G. Ostland and R. A. Fine (1979). Oceanic distribution and transport of tritium. In Behaviour of Tritium in the Environment" (Proceedings of a Symposium, San Francisco, 16-20 October 1978, IAEA-SM-232/67, pp. 303-314. International Atomic Energy Agency, Vienna.)...

Ostlund, G. G. and Fine, R. A. (1979). Oceanic distribution and transport of tritium. lAEA-SM-232162, pp. 303-314. Inti. Atom. Energy Agency, Vienna. 

Using the above laboratory relations we compute the values of 6D and 618 expected for rain made by a single distillation from the ocean and for rain made by a subsequent distillation for sea rain which fell on the land. These values show that when rain and snow exhibit very large depletions of the heavy isotopes, e.g., 6D of -200 and -300 ppt, two or three distillations have occurred. It is known from measurements of tritium in rain [76] that two or three distillations occur in the U.S. between evaporation from the Pacific Ocean and precipitation in the eastern U.S. The agreement between the number of distillations deduced from stable isotopes and deduced from tritium is gratifying. 

Ostlund, H.G. and C.G.H. Rooth. 1990. The North Atlantic Tritium and Radiocarbon Transients 1972-1983. Journal of Geophysical Research—Oceans 95 20,147-20,165. 

Radioactive or stable isotopes of noble gases are also used to determine vertical turbulent diffusion in natural water bodies. For instance, the decay of tritium (3H)— either produced by cosmic rays in the atmosphere or introduced into the hydrosphere by anthropogenic sources—causes the natural stable isotope ratio of helium, 3He/ 4He, to increase. Only if water contacts the atmosphere can the helium ratio be set back to its atmospheric equilibrium value. Thus the combined measurement of the 3H-concentration and the 3He/4He ratio yields information on the so-called water age, that is, the time since the analyzed water was last exposed to the atmosphere (Aeschbach-Hertig et al., 1996). The vertical distribution of water age in lakes and oceans allows us to quantify vertical mixing. 

Helium-3 is a decay product of radioactive tritium (3H, half-life = 12.44 years) that has been produced by nuclear bombs as well as naturally by cosmic rays in the upper atmosphere. Because virtually all 3He atoms escape from the surface ocean to the atmosphere, the 3He/tritium ratio in subsurface seawater samples indicates the time since the water s last exposure to the atmosphere. Both 3He and tritium are measured by gas mass spectrometry. Alternatively, tritium may be measured by gas counting with a detection limit of 0.05 to 0.08 tritium unit, where 1 tritium unit represents a 3H/H ratio of lxl0 18. A degassed water sample is sealed and stored for several months to allow the decay product 3He to accumulate in the container. The amount of 3He is then measured by mass spectrometry, yielding a detection limit of 0.001 to 0.003 tritium unit when 400-gram water samples are used. With this technique, the time since a water mass left the surface can be determined within a range from several months to 30 years. 

Begemann Libby (1957) estimated that 1.1 kg of T was released to atmosphere for each megatonne (MT) thermonuclear explosion. The tests between 1954 and 1963 had a fusion yield of 320 MT. Allowing for radioactive decay, the global inventory in 1963, including tritium in the atmosphere, groundwater and oceans, was about 330 kg. French and Chinese thermonuclear tests between 1968 and 1977 may have added another 20-30 kg. In 1972, by which time most of the pre-1963 tritium had returned to the earth s surface, a world-wide survey of oceanic waters gave a total of 164 kg (Ostlund Fine, 1979). Corrected for radioactive decay, this is equivalent to an inventory of 270 kg in 1963. 

When atmospheric HTO remains at a more or less constant level for months, water in soil and surface run-off approaches equilibrium with the atmosphere. In 1963, tritium in US surface water averaged 3000 pCi l-1 (110 Bq l-1) (NCRP, 1979), equivalent to 920 TU and rather greater than the level in oceanic rains (Fig. 4.2). 

Begemann, F. Libby, W.F. (1957) Continental water balance, ground water inventory and storage times, surface ocean mixing rates and worldwide water circulation patterns from cosmic ray and bomb tritium. Geochemica Cosmochemica Acta, 12, 277-96. 

Weiss, W., Roether, W. Dreisigacker, E. (1979) Tritium in the North Atlantic Ocean Inventory input and transfer into deep water. In Behaviour of Tritium in the Environment, pp. 315-35. Vienna IAEA. 

Craig, H., Clarke, W. B. (1970) Oceanic 3He Contribution from cosmogenic tritium. Earth Planet. Sci. Lett., 9, 289-96. 

The natural inventory of tritium produced by this reaction is about 4 x lO Bq (11 kg), of which 90% is contained in the ocean as HTO. The tritium produced by man s activities (largely thermonuclear weapons testing) has increased to approximately 2 x 10 ° Bq. Although most of this tritium has been transferred to the oceans, environmental tritium is still half an order of magnitude higher than was present before nuclear testing. 

Tritium in water vapor has also been utilized in the study of meteorology. Kigoshi and Yoneda observed daily variations in the tritium content of atmospheric moisture (water vapor) collected in Tokyo. High tritimn contents indicate the arrival of continental air masses from the north, and the low contents in tropical low-pressure air masses are nearly equal to that of surface ocean water. 

In order to make use of this transient, the contributions of the natural and bomb radiocarbon had to be separated. Three sets of observations went into this separation. First, use was made of measurements on surface waters collected very early in the nuclear testing era (Broecker etal., 1960) and also with results of measurements on prenuclear growth ring-dated corals and mollusks (Drulfel and Linick, 1978 Drulfel, 1981, 1989). Second, use was made of the tritium released to the atmosphere during nuclear tests. As this bomb-test tritium swamped the natural tritium present in the ocean, the vertical distribution of tritium in the sea could be used to establish the limit of penetration of bomb radiocarbon. Finally, based on the radiocarbon analyses made on thermocline waters free of bomb tritium, it was shown that there was a close correlation between the natural ratio and the dissolved silica... 

The average global production of tritium is —2,500 atom m s (Solomon and Cook, 2000). The deposition rate of the tritium varies with latitude, but it is also mixed with the bulk of precipitation originating from the ocean (which has a very low tritium content), and thus the average tritium content of precipitation tends to vary inversely with annual precipitation. Natural tritium in precipitation varies from —1 TU in oceanic high-precipitation regions to as high as 10 TU in arid inland areas. 

Jenkins W. J. and Rhines P. B. (1980) Tritium in the deep North Atlantic Ocean. Nature 286, 877-880. 

Kelley D. and Van Scoy K. A. (1999) A basin-wide estimate of vertical mixing in the upper pycnocline spreading of bomb tritium in the North Pacific Ocean. J. Phys. Oceanogr. 19, 1759-1771. 

Weiss W. M. and Roether W. (1980) The rates of tritium input to the world oceans. Earth Planet. Sci. Lett. 49, 435-446. 

The concentration of tritium in each lagoon was found to be higher than in the open ocean, as the result of leakages from a number of the cavity-chimneys created by underground nuclear tests. 

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