Isotopic composition of the precipitations in the central Mediterranean: origin marks and orographic precipitation effects

Abstract

The isotopic composition of the rainfall in northwestern Sicily (Italy, central Mediterranean) was investigated in the period February 2002 to March 2003. A rain gauge network was installed and sampled monthly. The monthly values of the D and 18O ratios showed a wide range that reflected seasonal climatic variations. Mean weighted values were used to define an isotopic model of precipitation. Temporal variations in deuterium excess were also investigated. Using mean volume weighted values, the Local Meteoric Water Line (LMWL) can be represented by the equation: dD = 4.7d18O 8.2 (r2 = 0.96). Deuterium excess (d = dD 8d18O) was found to be strongly related to orography. The coastline samples were characterized by mean weighted deuterium excess values close to 12.5%; samples from inland areas showed values of 16%, while samples taken from the main reliefs showed values close to 19%. In inland areas, isotopic exchange between raindrops and moisture could shift the deuterium excess values slightly. On the higher reliefs, the interaction between falling raindrops and orographic clouds could shift the deuterium excess values significantly. The low slope of the LMWL could be referred to the high deuterium excess values of the higher sites and is related to orographic precipitation rather than to evaporation processes during the fall of the raindrops. The results obtained suggest that local orographic features may significantly change the isotopic composition of precipitation.