Temporal Variations of delta 2 H, delta 18 O and Deuterium Excess in Atmospheric Moisture and Improvements in the CO 2 -H 2 O Equilibration Technique for 18 O/ 16 O Isotope Ratio Analysis
A continuous 18-year record (1981-2000) of the stable isotope ratios D/H and 18O/16O in atmospheric water vapour collected at the Central European site Heidelberg (49° 25’ N and 8° 41’ E) is analysed. Its basic features, such as seasonal variations of D/H and 18O/16O, are discussed and compared with the isotope composition of precipitation. The correlation between local air temperature and isotopic composition of water vapour and precipitation is nearly identical for annual, but not for monthly means. This study focuses on the deuterium excess in water vapour, d(vap), such as its difference to the mostly analysed deuterium excess in precipitation, d(prec). Both d’s are assumed to be controlled primarily by the moisture deficit above the ocean, the major source of atmospheric moisture. In order to understand the seasonal and short-term variations of d(vap), the isotopic evolution of the moisture along 5-day backward trajectories is analysed. Although the subtropical Atlantic is an important moisture source for Europe, variations of deuterium excess are shown to be controlled by regional evaporation. This study also presents details of an optimized CO2-H2O equilibration device for 18O/16O measurements in water. A system designed for high throughput has been modified to yield results of excellent precision while keeping the operator time at a minimum. Results of an 18-months study of three laboratory standard waters and an international interlaboratory comparison organized by the IAEA shows the high analytical precision of 18O/16O determinations in natural waters now available.
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