Processes behind the isotopic water line: water cycle and climate

István Fórizs

Institute for Geochemical Research, Hungarian Academy of Sciences, H-1112 Budapest, Budaörsi út 45, Hungary.

The stable isotope compositions of precipitations on the δD-δ18O diagram plot along a line called meteoric water line. This shows seasonal and geographical variations as well. Its parameters (slope, deuterium excess = δD – 8*δ18O, intervals of δD and δ18O) bears a lot of information about the provenance of air vapour and about the history of vapour movement. During the evaporation of (sea) water the relative humidity has the main role, beside temperature and wind speed, in determining the isotopic composition of air vapour. This is called primary evaporation effect. The isotopic composition of the sea water is rather homogeneous dislike continental surface water bodies, where the isotopic composition of water plays a role as well. The process of falling out of liquid (rain) or solid (snow) precipitation during the movement of vapour containing air masses can be described by the Rayleight fractionation (sequence of events with equilibrium isotopic fractionation between the cloud and precipitation, and no feedback from the precipitation to the cloud (vapour)). The isotopic compositions of precipitations from such a sequence of events plot along a line with a slope 8 (global average). If vapour of terrestrial origin contributes to clouds of maritime origin the isotopic composition of precipitation from this mixed cloud goes off the mentioned water line, characteristically its deuterium excess (d-excess) increases.
On arid and semiarid areas in summertime, when the air temperature is high and the air humidity is low, rain droplets are affected by evaporation on their way in the air (secondary evaporation effect). As a result, their d-excess decreases in this part of a year, while the d-excess of winter precipitation remains unaffected, and so the slope of local meteoric water line calculated for the whole year is smaller than 8. For example the lowland areas of the Carpathian Basin are characterized by such meteoric water lines: Abádszalók (Hungary) δD = 7.2*δ18O + 0.1‰ (Deák 1995); Oradea (Romania) δD = 6.7*δ18O - 3.1‰ (Ţenu 1981). In both cases the slopes of the water lines (7.2 and 6.7) are significantly lower than 8.
The d-excess of the meteoric water lines depends on the elevation as well, because d-excess increases with increasing elevation. In the Bihor Mountains (Romania), very close to the above mentioned lowland areas in the Carpathian Basin, at 1100 meters a.s.l. elevation the water line of the ice block in the Focul Viu Ice Cave is δD = 7.5*δ18O + 11.5‰. Keeping in mind that the isotopic composition of this ice is affected by sublimation (evaporation), which decreased its d-excess value, the above ice-water line indicates a source precipitation with significantly higher d-excess than the global value of 10‰. At higher elevations this effect is more visible: e.g. the local meteoric water line at Villacher Alpe (altitude 2135 m) is δD = 8.1*δ18O + 12.5‰ (IAEA/WMO (2004).

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