5 NATURAL ABUNDANCE OF THE STABLE ISOTOPES ... - Falw.vu

Stable Isotopes of Hydrogen, Carbon and Oxygenexchangeatmosph.CO 2 7.5‰precipitationoceanic HCO 3+ 1‰plants / humus 25‰soil H 2 Omarine CaCO 3+ 1‰humus CO 2 25‰erosion / sedimentationdissolved CO 2 25 ± 1‰soil CaCO 3+ 1 ± 1‰groundwaterHCO 3 12 ± 1‰groundwaterDIC11 to 16‰Fig. 7.7Schematic representation of the formation of dissolved inorganic carbon in groundwaterfrom soil carbonate and soil CO 2 . This is the main process responsible for the carbonatecontent of groundwater and the consecutive components of the water cycle. Generally,dissolved bicarbonate is by far the largest component. The ‰ values referring to therespective 13 have been kept simple for the sake of clarity. DIC is the dissolved inorganiccarbon content of the water, i.e. HCO 3 , CO 2 (aq) and CO 3 2 .7.2 STABLE OXYGEN ISOTOPES7.2.1 THE NATURAL ABUNDANCEThe chemical element oxygen has three stable isotopes, 16 O, 17 O and 18 O, with abundances of99.76, 0.035 and 0.2%, respectively (Nier, 1950). Observation of 17 O concentrations provideslittle information on the hydrological cycle in the strict sense above that which can be gainedfrom the more abundant and, consequently, more accurately measurable 18 O variations(Sect.3.7). We shall, therefore, focus our attention here on the 18 O/ 16 O ratio ( 0.0020).Values of 18 show natural variations within a range of almost 100‰ (Fig.7.9). 18 O is oftenenriched in (saline) lakes subjected to a high degree of evaporation, while high-altitude andcold-climate precipitation, especially in the Antarctic, is low in 18 O. Generally in thehydrological cycle in temperate climates we are confronted with a range of 18 not exceeding30‰.101