Water Isotopes and Temperature Reconstruction
Ocean water is mostly, with small amounts of and, where D denotes Deuterium, i.e. Hydrogen with an extra neutron. In Vienna Standard Mean Ocean Water (VSMOW) the ratio of D to H is and O-18 to O-16 is . Fractionation occurs during changes between condensed and vapour phases: the vapour pressure of heavier isotopes is lower, so vapour contains relatively more of the lighter isotopes and when the vapour condenses the precipitation preferentially contains heavier isotopes. The difference from VSMOW is expressed as δ18O = 1000‰ ; and a similar formula for δD. δ values for precipitation are always negative. The major influence on δ is the difference between ocean temperatures where the moisture evaporated and the place where the final precipitation occurred; since ocean temperatures are relatively stable the δ value mostly reflects the temperature where precipitation occurs. Taking into account that the precipitation forms above the inversion layer, we are left with a linear relation:
- δ 18O = aT + b
which is empirically calibrated from measurements of temperature and δ as a = 0.67 ‰/oC for Greenland and 0.76 ‰/oC for East Antarctica. The calibration was initially done on the basis of spatial variations in temperature and it was assumed that this corresponded to temporal variations (Jouzel and Merlivat, 1984). More recently, borehole thermometry has shown that for glacial-interglacial variations, a = 0.33 ‰/oC (Cuffey et al., 1995), implying that glacial-interglacial temperature changes were twice as large as previously believed.
Read more about this topic: Proxy (climate)
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