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Isotope Hydrology: 3 H/ 3 He dating. Peter Schlosser, February 19, 2008. Syllabus. Syllabus. Transient Tracers. Tracers: trace substances of natural or anthropogenic origin (stable and radioactive isotopes; chemical compounds. Sometimes toxic or otherwise harmful (contaminants)
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Isotope Hydrology: 3H/3He dating Peter Schlosser, February 19, 2008
Transient Tracers • Tracers: trace substances of natural or anthropogenic origin (stable and radioactive isotopes; chemical compounds. Sometimes toxic or otherwise harmful (contaminants) • Transient tracers: ‘Dyes’with known delivery rates to the environment (e.g.,3H, 3He, CFCs, 129I, SF6, 85Kr) • Radioactive clocks (e.g., 14C,39Ar, 3H/3He) • Special sources (e.g., 18O, Ba, nutrients, Ra isotopes, Rn isotopes) • Deliberately released tracers (e.g., SF6, 3He)
3He Radioactive daughter of 3H 1939: Luis Alvarez discovered an isotope of mass 3 and distinguished it from tritium. He noted that the isotope was the rare isotope of natural helium. Mass: 3.01603 amu (HD/H3+: 3.0219/3.0235 amu) Abundance in the atmosphere: 5.24 ppm * 1.384 *10-6 (3He/4He)atm = 1.384 * 10-6 Solubility of He in water: Ostwald coefficient: ca. 0.007 to 0.01 3He is less soluble than 4He: α ca. 0.983 for seawater at low temperatures Low abundance requires sophisticated analytical methods for precise measurement.
Tritium/3He method Determination of 3H/3He age requires measurement of tritium and helium isotopes at time t. No knowledge of tritium and 3He concentrations at time t0; i.e., at time of isolation of water parcel, is needed.
Tritiogenic 3He in TU R = [3He]/[4He]
Early 3H/3He literature Early theoretical work by Tolstikhin and Kamensky, Geochemical International, 6, 810 – 811, 1969
3H/3He profiles Adapted from Schlosser et al., 1988 (EPSL)
3H/3He profiles Adapted from Schlosser et al., 1989 (EPSL)
Separation of tritiogenic 3He From Schlosser et al., 1989 (EPSL)
Separation of tritiogenic 3He From Schlosser et al., 1989 (EPSL)
3H/3He profiles Adapted from Schlosser et al., 1989 (EPSL)
Future of 3H/3He method From Schlosser et al., 1989 (EPSL)
Future of 3H/3He method From Schlosser et al., 1989 (EPSL)
Future of 3H/3He method From Schlosser et al., 1989 (EPSL)
3He confinement From Schlosser et al., 1989 (EPSL)
3H/3He age: mixing The 3H/3He age is non – linear with respect to mixing. It favors the age of the water component with the higher tritium concentration. From Jenkins and Clarke, 1976 (DSR)
Summary • Combined tritium/3He can extend the utility of tritium as absolute time marker • They also can be used as a radioactive clock • Terrigenic helium can complicate the separation of tritiogenic 3He from the total measured helium. In many cases this problem can be solved by additional measurement of Ne • The 3H/3He age is an apparent age. It has to be interpreted with care to account for non – linearities introduced during mixing of two or more water masses • 3H/3He can be used for another several decades to come before tritium will have decayed to a large degree and dispersion will have homogenized the 3He signal