Abstract

The northern Aquitaine basin (southwest France) is a large multi-layer aquifer system that contains groundwater with strong residence-time variability ranging from years to tens of thousands of years. This system constitutes an archive of paleoclimate variations. A multi-parameter approach involving isotopic tracers (14C, 18O, 2H) was used to determine the residence time of groundwater and to document climate fluctuations, while dissolved noble gases were used to estimate mean annual temperatures (noble gas recharge temperatures, NGRT) at the water table. Near-surface ground temperature reconstruction from 40 ka cal BP to the present was made using data collected from five aquifers. The coldest temperatures are recorded for late Marine Isotopic Stage (MIS) 3 and MIS 2, i.e. between 36 and 18 ka cal BP. The mean NGRT for the period 27–18 ka cal BP is estimated at 5.9 ± 0.9 °C, and a strong increase towards modern values (11–13 °C) is observed after 15 ka cal BP. The temperature change between the Holocene and the Last Glacial ranges from 5 to 7 °C, in agreement with previous NGRT studies in Europe. Since mean near-surface ground temperatures during the glacial were well above 0 °C, long-term presence of permafrost in northern Aquitaine is unlikely. However, a possible warm bias in reconstructed temperatures during the coldest events lies in the fact that NGRTs in cold regions do not reflect annual means but rather the ground temperature during the thaw months.

Highlights

  • Multi-layer aquifer systems are an important strategic water resource

  • Since present groundwater is inherited from former aquifer recharge, which occurred under changing climate conditions (Schoeller 1971), this residence time variability gives the opportunity to use the aquifers as paleoclimate archives for this timespan (Fontes et al 1993)

  • Comparison of isotopic data sampled in north European aquifers suggested a Brecharge gap^ during part of the Last Glacial, which was attributed to the presence of permafrost or to low atmospheric moisture supply (Darling 2004)

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Summary

Introduction

Multi-layer aquifer systems are an important strategic water resource. In order to manage properly those resources, a good33600 Pessac, France 6 Inrap, 140 Avenue du Maréchal Leclerc, 33130 Bègles, France 7 PACEA, University of Bordeaux-CNRS, alléeGeoffroy-Saint-Hilaire, 33615 Pessac, France knowledge of the systems is needed. Multi-layer aquifer systems are an important strategic water resource. The Aquitaine basin in southwest France is a large multi-layer aquifer system where groundwater has residence times from a few years or decades to thousands of years (Chery et al 1994). Since present groundwater is inherited from former aquifer recharge, which occurred under changing climate conditions (Schoeller 1971), this residence time variability gives the opportunity to use the aquifers as paleoclimate archives for this timespan (Fontes et al 1993). Because permafrost acted as an impermeable layer, groundwater movement was restricted to few favourable areas such as unfrozen ground (taliks) below lakes or large rivers (Ford 1993; French 2007) and, led to strong reduction of aquifer recharge (Vidstrand 2003). According to the available studies, permafrost was supposed to range from continuous to nearly absent during the coldest events of the glacial period

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