Details

Autor(en) / Beteiligte

• Minissale, A.
• Vaselli, O.

Titel

Karst springs as “natural” pluviometers: Constraints on the isotopic composition of rainfall in the Apennines of central Italy

Ist Teil von

• Applied geochemistry, 2011-05, Vol.26 (5), p.838-852

Ort / Verlag

Elsevier Ltd

Erscheinungsjahr

2011

Quelle

Alma/SFX Local Collection

Beschreibungen/Notizen

• ► Isotopic compositions of karstic springs in central Italy have been reviewed. ► Isotopic gradients of rainfalls for elevations have been evaluated in an Alpine valley. ► Karstic drops have been calculated by using isotopic compositions of springs. ► Isotopic compositions of rainfalls in central Italy have been re-calculated using the isotopic compositions of karstic springs. This paper describes an indirect method to calculate the isotopic composition of rainfall by using the isotopic composition of karst springs fed by waters circulating in the most important regional aquifer of central Italy, i.e. the Mesozoic limestone sequence that forms the backbone of the Apennines. By using δ18O and δD data and the δ18O (and/or δD) average gradient for elevation, evaluated through the use of literature rainfall data and new measurements from a typical Alpine valley in northern Italy, the altitude of precipitation of their parent water has been re-calculated. Vertical descents of more than 2000m, from recharge to discharge, have been assessed in some high flow-rate cold springs in the morphologically steep Adriatic sector of central Italy. A clear correlation between the vertical descents and more negative isotopic compositions at their relative emergence elevations is highlighted. In contrast, in the Tyrrhenian sector lower karstic drops (generally lower than 500m) correlate with less negative isotopic composition of recharge areas. The δ18O iso-contour map of the “recalculated” parent rainfall in central Italy is more detailed than any possible isotopic map of rainfall made using pluviometers, unless large number of rainfall collectors were deployed on mountaintops. The data also show that the isotopic composition of rainfall depends on the source of the storm water. In particular, precipitation is isotopically heavier when originating in the Mediterranean Sea, and lighter when formed in the Atlantic Ocean. Consequently, the collision between air masses with such a different isotopic signature results in a relatively small latitudinal fractionation effect. The peninsular part of central Italy is very narrow, with several mountains and massifs more that 2000m high, and any latitudinal variation in the isotopic composition between rainfall sourced in the Atlantic Ocean and in the Mediterranean Sea is much lower than that due to the isotopic fractionation due to elevation.