Karst springs as ‘‘natural’’ pluviometers: Constraints on the isotopic composition of rainfall in the Apennines of central Italy A. Minissale a,⇑ , O. Vaselli a,b a CNR – Italian Council for Research, Institute of Geosciences and Earth Resources (Section of Florence) – Via La Pira 4, 50121 Firenze, Italy b Department of Earth Sciences, University of Florence – Via La Pira 4, 50121 Firenze, Italy article info Article history: Received 18 May 2010 Accepted 10 February 2011 Available online 13 February 2011 Editorial handling by W.M. Edmunds abstract This paper describes an indirect method to calculate the isotopic composition of rainfall by using the iso- topic composition of karst springs fed by waters circulating in the most important regional aquifer of cen- tral Italy, i.e. the Mesozoic limestone sequence that forms the backbone of the Apennines. By using d 18 O and dD data and the d 18 O (and/or dD) average gradient for elevation, evaluated through the use of liter- ature 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 2000 m, 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 500 m) correlate with less negative isotopic composition of recharge areas. The d 18 O 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 Med- iterranean 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 2000 m 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. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Gat and Carmi (Gat and Carmi, 1970) showed how the isotopic composition of precipitation in the Mediterranean Sea, at least in its eastern sector, is affected by what they called ‘‘deuterium ex- cess’’, with respect to the Global Meteoric Water Line (GMWL; Craig, 1961). For the Mediterranean, they suggested the relation- ship: dD =8d 18 O + 15 between 18 O and D, instead of the GMWL equation: dD =8d 18 O + 10. They explained the deuterium excess as being caused by the interaction between northern continental air masses with those of the Mediterranean Sea. The Mediterra- nean Sea, particularly in its eastern sector, and especially in sum- mer, undergoes strong evaporation; its isotopic composition is relatively heavy (i.e.: dD = +9.8‰; d 18 O = +1.33‰; Shinohara and Matsuo, 1986) with respect to Standard Mean Ocean Water (SMOW), conventionally established as 0 delta ‰ . Central Italy (and the entire Italian Peninsula as a whole), being in the middle of the Mediterranean Sea, is affected by atmospheric disturbances prevalently deriving from either the north, especially from the Atlantic Ocean (‘‘cold’’ rain, more frequent in winter) or Africa (‘‘warm’’ rain, more frequent in summer). For this reason the isotopic composition of precipitation in Italy does not have a clear latitudinal gradient, and their monthly mean temperatures do not correlate with the isotopic composition of precipitation (Longinelli and Selmo, 2003). The present study focuses on the d 18 O and dD values measured in cold springs in central Italy, in the large regional-wide karstic reservoir represented by a thick (2–4 km) Mesozoic limestone se- quence (Fig. 1). The idea is that such springs, especially those with a very high flow-rate (such as the Peschiera spring, supplying more than 20 t/s of drinkable water to the city of Rome), can be used as natural pluviometers. These springs are indeed able to isotopically integrate and buffer the relative contributions of precipitation coming from either north Europe and the Atlantic Ocean or Africa and the Mediterranean Sea. 0883-2927/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2011.02.005 ⇑ Corresponding author. Tel.: +39 0552757521; fax: +39 055284571. E-mail address: minissa@igg.cnr.it (A. Minissale). Applied Geochemistry 26 (2011) 838–852 Contents lists available at ScienceDirect Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem