Hydrogeology as a climate recorder: Sahara–Sahel (North Africa) and the Po Plain (Northern Italy)

Abstract 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.

[1]  K. Różański.  Deuterium and oxygen-18 in European groundwaters — Links to atmospheric circulation in the past , 1985 .

[2]  F. Gasse,et al.  Hydrologie et géochimie isotopique , 1998 .

[3]  F. Gasse.  Hydrological changes in the African tropics since the Last Glacial Maximum , 2000 .

[4]  J. Taupin,et al.  Rainfall characteristics (δ18O, δ2 H, ΔT and ΔH r ) in western Africa: Regional scale and influence of irrigated areas , 2000 .

[5]  R. Nativ,et al.  Stagnant groundwater stored in isolated aquifers: implications related to hydraulic calculations and isotopic dating—Reply , 1994 .

[6]  Dan Yakir,et al.  Production of CO2 in the capillary fringe of a deep phreatic aquifer , 1998 .

[7]  J. Fontes,et al.  Subsidence of the Venice Area during the Past 40,000 yr , 1973, Nature.

[8]  C. Weyhenmeyer,et al.  Cool glacial temperatures and changes in moisture source recorded in oman groundwaters , 2000, Science.

[9]  L. Carbognin,et al.  The Lagoon of Venice: natural environmental trend and man-induced modification / La Lagune de Venise: l'évolution naturelle et les modifications humaines , 1981 .

[10]  U. Beyerle,et al.  Interpretation of dissolved atmospheric noble gases in natural waters , 1999 .

[11]  J. W. Beck,et al.  INTCAL98 Radiocarbon Age Calibration, 24,000–0 cal BP , 1998, Radiocarbon.

[12]  J. Jouzel,et al.  Stable water isotopes in atmospheric general circulation models , 2000 .

[13]  J. Jouzel,et al.  Global Climatic Interpretation of the Deuterium-Oxygen 18 Relationship , 1979 .

[14]  C. Causse,et al.  Successions of sea-level changes during the Pleistocene in Mauritania and Senegal distinguished by sedimentary facies study and U/Th dating , 2000 .

[15]  N. Sturchio,et al.  Precipitation Source Inferred from Stable Isotopic Composition of Pleistocene Groundwater and Carbonate Deposits in the Western Desert of Egypt , 1997, Quaternary Research.

[16]  F. Gasse,et al.  PALHYDAF (Palaeohydrology in Africa) program: objectives, methods, major results , 1991 .

[17]  R. Fairbanks.  A 17,000-year glacio-eustatic sea level record: influence of glacial melting rates on the Younger Dryas event and deep-ocean circulation , 1989, Nature.

[18]  E. I. Hamilton,et al.  Isotope techniques in the study of past and current environmental changes in the hydrosphere and the atmosphere , 1996 .

[19]  A. Dodo,et al.  Étude des écoulements souterrains dans le bassin de Bilma-Djado à l'aide des isotopes de l'environnement , 1997 .

[20]  R. Gonfiantini.  On the isotopic composition of precipitation , 1998 .

[21]  W. Edmunds,et al.  Groundwater recharge estimation using chloride, stable isotopes and tritium profiles in the sands of northwestern Senegal , 1996 .

[22]  V. Markgraf,et al.  Land, sea and climate in the northern adriatic region during late pleistocene and holocene , 1977 .

[23]  W. Edmunds,et al.  Lakes, groundwater and palaeohydrology in the Sahel of NE Nigeria: evidence from hydrogeochemistry , 1999, Journal of the Geological Society.

[24]  B. Blavoux,et al.  Signal pluie et traçage par les isotopes stables en Méditerranée occidentale. Exemple de la région avignonnaise (Sud-Est de la France) , 2000 .

[25]  J. Fontes,et al.  Isotopes du milieu et circulations dans les aquiferes du sous-sol Vénitien , 1973 .

[26]  C. B. Gaye,et al.  La recherche du pic de tritium thermonucléaire en zone non saturée profonde sous climat semi-aride pour la mesure de la recharge des nappes : première application au Sahel , 1992 .

[27]  G. Marsily.  Quantitative Hydrogeology: Groundwater Hydrology for Engineers , 1986 .

[28]  F. J. Pearson,et al.  Sources of dissolved carbonate species in groundwater and their effects on carbon-14 dating , 1970 .

[29]  Giovanni Martinelli,et al.  Geochemistry of heavily exploited aquifers in the Emilia-Romagna region (Po Valley, northern Italy) , 1998 .

[30]  R. Koster,et al.  Water isotopes in precipitation:. data/model comparison for present-day and past climates , 2000 .

[31]  D. B. Smith.  Statistical treatment of data on environmental isotopes in precipitation: Technical report series no. 331, International Atomic Energy Agency (IAEA), Vienna, 1992, 781 pp., softback, 2100 Austrian Sch., ISBN 92-0-100892-9 , 1993 .

[32]  C. Ballentine,et al.  Determining paleotemperature and other variables by using an error-weighted, nonlinear inversion of noble gas concentrations in water , 1999 .

[33]  Martin Williams,et al.  The Sahara and the Nile. , 1980 .

[34]  E. P. Wright,et al.  Groundwater recharge and palaeoclimate in the Sirte and Kufra basins, Libya , 1979 .

[35]  C. Baroni,et al.  The Alpine “Iceman” and Holocene Climatic Change , 1996, Quaternary Research.

[36]  P. Shand,et al.  PALAEOCLIMATIC INFORMATION CONTAINED IN GROUNDWATERS OF THE GRAND ERG ORIENTAL, NORTH AFRICA , 1998 .

[37]  G. Martinelli,et al.  Geochemistry of the formation waters in the Po plain (Northern Italy): an overview. , 2000 .