Groundwater as an archive of climatic and environmental change: Europe to Africa

Abstract

Droughts and wet periods in recent earth history have a clear link with human habitation and migration notably in the Middle East and in Africa. Water availability in the form of perennial rivers, lakes and springs dictated the first settlement patterns. Reference is found in the first written records to dramatic climatic changes affecting water availability (Lambert and Millard 1969) and a fascinating challenge exists to reconstruct and to relate the hydrological records to the early historical and archaeological evidence (Issar 1990; Hassan 1997) as well as the rapidly growing body of data on palaeoclimate. Groundwater is emerging as an archive at both low and mid-latitudes of past climatic and hydrological change, which may be used alongside other proxy data. Indirect evidence of the palaeohydrology in the late Pleistocene and Holocene, has been deduced from various sources especially lake sediments (Fontes and Gasse 1991; Gasse 2000; Hoelzmann et al. 2000; this volume) and speleothems (Bar-Matthews et al. 1997). In contrast to other archives such as ice cores or tree rings, which contain high-resolution information, data available in large groundwater bodies are of low resolution (typically ±1000 yr). This is due to the advection or dispersion of any climatic input signal in the water body. Many groundwater data are obtained from pumped samples where sample intervals may extend over tens of metres. Nevertheless, specific indications of palaeo-temperature, air mass circulation and vegetation history may be retained in a range of different chemical and isotopic signals (Fontes et al. 1993a; Stute and Schlosser 1993), notably in confined aquifers where sequential changes may be recorded along flow lines, or in the stratification of phreatic aquifers. Dated groundwaters are important since these contain the direct evidence of prolonged wet episodes. Even the absence of dated waters over a specific time interval may indicate periods of drought (Sonntag et al. 1978). The correlation between groundwater records and aeolian deposition in semi-arid/arid regions (Stokes et al. 1998; Swezy 2001) can also provide complimentary evidence of wet and dry intervals. Moisture in the unsaturated zone may under favourable circumstances also contain records of past environments and climate at decadal to millennial scale resolution, contained mainly as variations in salinity and in stable isotope enrichments (Edmunds and Tyler 2002). Such records are found in porous media in areas of low moisture flux, notably beneath modern arid or semi-arid areas. The resolution of unsaturated (vadose) zone records will depend on the dispersion of the signal (Cook et al. 1992) but decadal scale records may be retained, as in West Africa, over several hundred years (Edmunds and Gaye 1997), or at the millennial scale over the late Pleistocene (Tyler et al. 1996). Some of the classical studies of hydrogeological systems that contain palaeo-waters have been conducted in Northern Africa and in Western Europe over the past three decades. In this paper the evidence contained in unsaturated zone profiles and especially in phreatic and confined aquifers in semi-arid and arid areas of Northern Africa is used to demonstrate the current possibilities in using groundwater archives. The results from Northern Africa are then compared with the evidence found in palaeowaters in Europe and elsewhere along the PEP III transect.