Meteorological and geographical control on stable isotopic signature of precipitation in a western Mediterranean area (Tuscany, Italy): Disentangling a complex signal

The stable isotopes of oxygen and hydrogen in precipitation are widely used to track processes occurring within the hydrological cycle and understand regional atmospheric patterns that influence a specific area. Moreover, the oxygen isotopic composition of continental carbonates is extensively used for palaeohydrological reconstruction. Nevertheless, few comprehensive investigations have been performed in the Western Mediterranean to analyze the statistical relationships between oxygen and hydrogen stable isotopes in precipitation and meteorological variables. For understanding the factors that regulate the rainfall stable isotopic signature at present day in this area of Mediterranean region we selected the Tuscany region (central Italy), due to its considerable climatic seasonality, complex morphology and orography. Tuscany is affected by both Atlantic atmospheric disturbances from NW and moist air masses originated in the Mediterranean, and also by the secondary cyclogenesis center of the Gulf of Genoa, the most active in the entire Mediterranean. The ideal position of the Tuscany region thus offers the opportunity to investigate the complex influence of moisture sources on the rainfall isotopic composition. Moreover, in this region many hydrogeological and palaeohydrological studies were performed and require a more precise and quantitative interpretation. In this work, 644 isotope monthly data (delta18O, delta2 H, and deuterium excess) of precipitation collected in 12 sites through Tuscany from 1971 to 2018 were gathered in a database. Then, only sites whose monthly data covered almost one year were considered for processing, resulting in 553 precipitation samples archived along with monthly mean temperature and rainfall amount. In this framework, the LMWL for Tuscany was determined by applying different regression techniques and statistical analyses were performed to define the influence of meteorological and geographical variables on the rainfall isotopic composition. The outcomes point out the variations of rainfall isotopic signature as result of several climate and geographical variables than a univocal proxy for both local temperature and precipitation amount. The Atlantic and Mediterranean moisture sources differently contribute for generating the observed isotopic variability, and their influence may seasonally change. Overall, the temperature and amount effect allow to explain the 50% of the isotopic variability, but temperature appears to be more important. These local effects are more pronounced during the periods with low rainfall (spring and summer), whereas the large-scale processes (moisture origin, trajectories, and rainout) are prevailing in winter and autumn. The altitude effect is the main driver of isotopic spatial variability. The deuterium excess parameter is revealed to be a very complex but useful signal for disentangling the different contribution of Atlantic and Mediterranean sources in Tuscan precipitation, even if it is sometime misused.