Hydrogeology as a Climate recorder: Sahara-Sahel (North Africa) and the Po Plain (Northern Italy).

Information on palaeoclimate is encoded in groundwater in a variety of measurable “tracers”. A great variety of geochemical and isotopic methods has been developed and tested with the aim of defining the different moments and aspects of the water cycle, providing a fundamental information on the recharge and groundwater flow conditions. Furthermore, environmental tracers allow us to reconstruct past environmental changes. Particular emphasis is put on Western Africa and on the Po valley. In fact, two features are common to these regions: the movement of air masses crossing them and the geological characteristics. From the global perspective, Sahel, North Africa, Mediterranean Basin and Central Europe reveal a relative simple pattern of spatial and temporal composition of precipitation. The displacement of the Intertropical Convergence Zone (ITCZ) and the Azores anticyclone control the climatological regime and, consequently, the isotopic composition of rainfall. Moreover, both regions are characterised by big aquifers in the form of multilayer systems several hundred metres thick in large sedimentary basins. The aquifers are generally formed by sandstones (Sahara) or sands (Po valley) and are interbedded with argillaceous layers and outcrop only in fairly limited areas, constituting the recharge zones. The groundwater movement in confined aquifers is always lower than 1 m year-1, respecting the conditions of steady-state flow regime and hydraulic continuity, necessary for the “hydro-archives”. The comparison between the Sahara–Sahel and southern Alps allows to define the principal periods of aquifer replenishment associated with the displacement of the Intertropical Convergence Zone (ITCZ) and the Azores anticyclone. Continental isotope archives provide a means of extending the knowledge on the magnitude and the causes of environmental and climate changes. Nevertheless, for a quantitative interpretation of the isotope records preserved in continental archives, it is necessary to know the response of the isotopic composition of precipitation to long-term fluctuations in key climatic parameters over a given area. Further, the transfer functions relating the climate-induced changes of the isotopic composition of precipitation to the isotope record preserved in the given archive should be established. It appears, thus, that stable isotopes can, presently only be used as an indirect proxy dating method for palaeowaters.