Groundwater as an archive of climatic and environmental change - the PEP III transverse
by
W.M. Edmunds
British Geological Survey, Wallingford, UK
Coauthors: A. Dodo (University of Niamey, Niger), D. Djoret (University of N'Djamena, Chad), F. Gasse (CEREGE, Franc), C.B. Gaye (IAEA, Vienna), I.B. Goni (University of Maiduguri, Nigeria), Y. Travi (University of Avignon, France), K. Zouari (University of Sfax, Tunisia), G.M. Zuppi (University of Venice, Italy)

Groundwater is gradually increasing in significance as an archive of past climatic and hydrological change. Large groundwater bodies are of low resolution (typically +/- 1000 yr) in relation to other proxy indicators such as ice cores due to dispersion of the signal. Nevertheless, specific indications of palaeotemperature, air mass circulation and vegetation history may be retained. Dated groundwaters contain the direct evidence of prolonged wet episodes, although by corollary not necessarily the drought periods, except by the absence of dated waters over a specific interval. The unsaturated zone may under favourable circumstances contain records at decadal scale resolution of both wet and dry periods contained as variations in salinity and stable isotope enrichments. In this paper the evidence contained in selected groundwaters in semi-arid and arid areas of northern Africa is used to demonstrate the current issues in using groundwater archives and to compare this evidence with palaeowaters in Europe.

Inert tracers (stable isotopes (delta¹⁸O, delta²H), together with noble gases, Cl, Br/Cl), which retain some information on the input conditions, are the main indicators for reconstruction. Nitrate also may be used in arid regions since it remains inert under the prevalent oxidising conditions. Above all, radiocarbon activities, corrected where possible to give ages, remain the essential chronological tool.

Cl and oscillations in moisture from the unsaturated zone from Senegal, Nigeria, Niger, Cyprus and Tunisia provide a good correlation with the instrumental record for climatic changes over the past 100-130 years with validation provided by ³H. Extrapolation can be made to examine likely climatic changes over the past 1000+ yr where little other archive information is yet available from arid continental regions.

Selected non-carbonate sedimentary basins in Africa, containing reliable radiocarbon and stable isotope records, are examined, sometimes with noble gas information. Results from studies carried out over 30 years in Senegal, Niger, Mali, Nigeria, Chad, Cameroun, Sudan, Egypt, Libya, Tunisia and Algeria are compared on a regional basis. The groundwaters in the Sahel region retain a clear record of late Pleistocene recharge episodes and then evidence of desiccation in late Pleistocene prior to the onset of wet early-mid Holocene conditions and then the onset of the modern climate around 4500 BP. The late Pleistocene groundwaters record evidence of cooler climates towards the time of the LGM. Air mass circulation in the late Pleistocene was not significantly different from the present day although there is evidence notably over North Africa of a reinforcement and southward shift of the Atlantic westerly flow during the period; evidence also points to a northward extension of the African monsoon, notably during the early to mid-Holocene. The extent of cooling at the LGM recorded in the noble gas ratios from was up to 7°C. There is clear evidence in the isotopic signature of expansion of rivers such as the Niger as well as an extensive groundwater-fed fluvial system, remnants of which today are recorded in the phreatic aquifers. The isotopic evidence in different places records strong variations in humidity of the air masses supplying moisture across the continent at different times over the past 30000 yrs.

High nitrate concentrations are preserved in unconfined groundwaters across northern Africa, both in palaeowaters up to 35000 years old, as well as modern waters. The widespread distribution in space and time supports the idea that N-fixing vegetation such as Acacia species has persisted across much of north Africa in the Holocene and Pleistocene. Evidence from selected non-carbonate aquifers in Europe records strongly contrasting climatic conditions controlling recharge to aquifers. In Estonia, strongly depleted isotopic signatures record recharge under pressure directly beneath the ice cap. In UK and in Switzerland there is evidence of a recharge gap interpreted as frozen ground at the time of the LGM; however recharge in the late Pleistocene contains a depleted isotope signature compared to the present day indicating cooler climate, confirmed by the noble gas recharge temperatures (NGRT) mean value of 5-6°C. In southern France and in Portugal there is evidence from NGRT of cooling of 5-7°C, as well as continuous recharge, although in coastal Portugal, unlike in France, a slight enrichment in delta¹⁸O indicates the proximity to the ocean and the constancy of the SW Atlantic air circulation over the whole of the late Pleistocene and Holocene. Thus, the groundwater evidence records a gradation at the LGM from glacial recharge, through permafrost, to continuous rainfall recharge at Mediterranean latitudes and then desiccation in the modern Sahara region.

Date received: May 10, 2001

Copyright © 2001 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # cahi-87.