Helium ocean

As can be seen in Fig. 2-1 (abundance of elements), hydrogen and oxygen (along with carbon, magnesium, silicon, sulfur, and iron) are particularly abundant in the solar system, probably because the common isotopic forms of the latter six elements have nuclear masses that are multiples of the helium (He) nucleus. Oxygen is present in the Earth s crust in an abundance that exceeds the amount required to form oxides of silicon, sulfur, and iron in the crust the excess oxygen occurs mostly as the volatiles CO2 and H2O. The CO2 now resides primarily in carbonate rocks whereas the H2O is almost all in the oceans. 

Hilton DR, Barling J, Wheiler GE (1990) The effect of shallow-level contamination on the helium isotope systematics of ocean island lavas. Nature 348 59-62... 

Soo [96] determined picogram amounts of bismuth in seawater by flameless atomic absorption spectrometry with hydride generation. The bismuth is reduced in solution by sodium borohydride to bismuthine, stripped with helium gas, and collected in situ in a modified carbon rod atomiser. The collected bismuth is subsequently atomised by increasing the atomiser temperature and detected by an atomic absorption spectrophotometer. The absolute detection limit is 3pg of bismuth. The precision of the method is 2.2% for 150 pg and 6.7% for 25 pg of bismuth. Concentrations of bismuth found in the Pacific Ocean ranged from < 0.003-0.085 (dissolved) and 0.13-0.2 ng/1 (total). 

Poreda R, Schilling J-G, Craig H (1986) Helium and hydrogen isotopes in ocean-ridge basalts north and south of Iceland. Earth Planet Sci Lett 78 1-17... 

Kurz M.D. and Jenkins W.J. (1981) The distribution of helium in oceanic basalt glasses. Earth Planet. Sci. Lett. 53, 41-54. 

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. 

The first, second and third periods are short periods. There are two elements in the first period and eight elements in both the second and the third periods. The elements in the short periods constitute almost 97% of the earth s crust, oceans and atmosphere. Among these elements, helium (He) and neon (Ne) gases occur only in trace amounts in the atmosphere but argon (Ar) makes up about 1 % of the atmsophere. The other elements are abundant. 

Esser and Turekian (1988) estimated an accretion rate of extraterrestrial particles in ocean bottom and in varved glacial lake deposit on the basis of osmium isotope systematics and concluded a maximum accretion rate of between 4.9 x 104 and 5.6 x 104 tons/a. The discrepancy between this estimate and those derived from helium can easily be attributed to the difference in the size of the cosmic dust particles under consideration. Cosmic dusts of greater than a few ten micrometers may not be important in the helium inventory of sediments because the larger grains are likely to lose helium due to atmospheric impact heating (e.g., Brownlee, 1985). Stuart et al. (1999) concluded from studies on Antarctic micrometeorites that 50- to 1 OO-qm micrometeorites may contribute about 5% of the total flux of extraterrestrial 3He to terrestrial sediments. Therefore, the helium-based estimate deals only with these smaller particles. 

Craig, H., Lupton, I. (1976) Primordial neon, helium and hydrogen in oceanic basalts. Earth Planet. Sci. Lett., 31, 369-85. 

Lupton, J. E., Craig, H. (1975) Excess 3He in oceanic basalts Evidence for terrestrial primordial helium. Earth Planet. Set Lett., 26, 133-9. 

Nier, A. O., Schlutter, D. J., Brownlee, D. E. (1990) Helium and neon isotopes in deep Pacific Ocean sediments. Geochim. Cosmochim. Acta, 54, 173-82. 

Helium core forms. On Earth, the oceans boil away.)... 

Hanan B. and Graham D. (1996) Lead and helium isotope evidence from oceanic basalts for a common deep source of mantle plumes. Science 111, 991-995. 

The dawn of the modern era of terrestrial noble gas studies can be traced to a suggestion by Suess and Wanke (1965) that the ocean floor should be characterized by a steady-state loss of helium equal to its production in the mantle. Although they measured a 6% excess helium saturation anomaly in Pacific deep water, doubts were... 

Figure 3 Compilation of helium isotope data from MORBs, continental hotspots, ocean island basalts, and HIMU sources (reproduced by permission of Mineralogical Society of America from Rev. Mineral. Geochem., 2002,...

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