Paleoclimate variations and impact on groundwater recharge in multi-layer aquifer systems using a multi-tracer approach (northern Aquitaine basin, France).
Résumé
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 years. This system was studied using a multi-parameter approach involving isotopic tracers (14C, 18O, 2H) to determine the groundwater residence time and to document climate fluctuations, while dissolved noble gases were used to estimate mean annual temperatures at the water table (NGRT, Noble Gas Recharge Temperature). Near-surface ground temperature reconstruction from 40 ka cal BP to the present was made using data collected from 5 aquifers. The coldest temperatures are recorded during late Marine Isotopic Stage (MIS) 3 and MIS 2, i.e. between 36 and 18 ka cal BP. The mean NGRT for the period 25-18 ka cal BP is estimated at 5.9 ± 0.9°C, and a strong increase towards modern values (11 to 13°C) is observed after 15 ka cal BP. The temperature drop 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.