# Paleoclimate variations and impact on groundwater recharge in multi-layer aquifer systems using a multi-tracer approach (northern Aquitaine basin, France)

Marc Saltel, Romain Rebeix, Bertrand Thomas, Michel Franceschi, Bernard Lavielle, Pascal Bertran

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Marc Saltel, Romain Rebeix, Bertrand Thomas, Michel Franceschi, Bernard Lavielle, et al., Paleoclimate variations and impact on groundwater recharge in multi-layer aquifer systems using a multi-tracer approach (northern Aquitaine basin, France). Hydrogeology Journal, 2019, 27, pp.1439-1457. 10.1007/s10040-019-01944-x. hal-02154742

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# Paleoclimate variations and impact on groundwater recharge in multi-layer aquifer systems using a multi-tracer approach (northern Aquitaine basin, France)

Marc Saltel<fnref n="1" /> · Romain Rebeix<fnref n="2" />,<fnref n="3" /> · Bertrand Thomas<fnref n="2" />,<fnref n="3" /> · Michel Franceschi<fnref n="4" />,<fnref n="5" /> · Bernard Lavielle<fnref n="2" />,<fnref n="3" /> · Pascal Bertran<fnref n="6" />,<fnref n="7" />

## 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.

**Keywords** Stable isotopes · Groundwater age · Paleohydrology · Groundwater recharge · France

## Introduction

Multi-layer aquifer systems are an important strategic water resource. In order to manage properly those resources, a good

knowledge of the systems is needed. 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). Comparison of isotopic data sampled in north European aquifers suggested a “recharge gap” during part of the Last Glacial, which was attributed to the presence of permafrost or to low atmospheric moisture supply (Darling 2004). 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, therefore, led to strong reduction of aquifer recharge (Vidstrand 2003). Groundwater analysis may thus provide a useful tool for paleopermafrost reconstruction. In southwest France, the presence of late Pleistocene permafrost and its extension is a matter of debate. According to the available studies, permafrost was supposed to range

<figure><img src="image_2.png" /></figure>

<footnote>Marc Saltel
m.saltel@brgm.fr</footnote>

<footnote n="1">¹ BRGM Nouvelle Aquitaine, Parc Technologique Europarc, 24,
Avenue Léonard de Vinci, 33600 Pessac, France</footnote>

<footnote n="2">² University of Bordeaux, CENBG, 19 Chemin du Solarium - CS
10120, 33175 Gradignan Cedex, France</footnote>

<footnote n="3">³ CNRS (UMR 5797) CENBG, 19 Chemin du Solarium - CS 10120,
33175 Gradignan Cedex, France</footnote>

<footnote n="4">⁴ Bordeaux INP, G&E, EA 4592, 33600 Pessac, France</footnote>

<footnote n="5">⁵ Université Bordeaux Montaigne, G&E, EA 4592,
33600 Pessac, France</footnote>

<footnote n="6">⁶ Inrap, 140 Avenue du Maréchal Leclerc, 33130 Bègles, France</footnote>

<footnote n="7">⁷ PACEA, University of Bordeaux-CNRS, allée
Geoffroy-Saint-Hilaire, 33615 Pessac, France</footnote>
<figure><img src="image_3.png" /><figcaption>(a) Study area</figcaption></figure>

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /><figcaption>(b) Hydrogeological cross sections</figcaption></figure>

<caption>Fig. 1 a Location and b hydrogeological cross sections of the North Aquitaine Basin (southwest France)</caption>
from continuous to nearly absent during the coldest events of
the glacial period.

The purpose in the present study is to investigate this ques-
tion through analyses of groundwater sampled in the northern
part of the Aquitaine basin. Natural isotopic tracers have been
used to determine the groundwater residence time (¹⁴C), and
to highlight the paleoclimate fluctuations (¹⁸O, ²H). Noble gas
measurements (He, Ne, Ar, Kr, Xe) were used to determine
the mean annual temperature at the water table. Temperature
records and associated groundwater ages allow for
paleoclimate reconstruction in the study area, whereby a com-
parison with other regional proxies is then proposed.

Hydrogeological and paleoclimatic context

Hydrogeological context

The study region is located in the northern part of the
Aquitaine basin, southwest France (Fig. 1). The basin is filled
with Mesozoic and Cenozoic sedimentary deposits of con-
trasting lithologies and permeabilities, thus creating a multi-
layer aquifer system with alternating aquifers and aquitards.
This system is bounded by two major anticlines: Jonzac to the
north and Villagrains to the south (Gély and Sztrakos 2000).
The various aquifers of this system are of uneven extensions
and can be subdivided into two groups:

1. The Upper Cretaceous multi-layer system mainly com-
posed of fissured limestone, in which four aquifers can
be defined including (from the deeper to the shallower):
Cenomanian, Turonian, Coniacian and Campanian. In
some areas, the aquifers are not separated by a significant
aquitard and, thus, form a single reservoir. This is partic-
ularly the case underneath the city of Bordeaux where
Cenomanian, Turonian and Coniacian deposits are part
of a combined reservoir composed of cemented siliceous
sandstone overlain with limestone and dolomitic lime-
stone (Pédron et al. 2008). The range of transmissivity
reported in the literature is between 3·10⁻³ and 6·
10⁻³ m²/s (Gombert 1988). The upper aquitard consists
of compact chalky marl, ca. 200–300 m thick. The karstic
Campanian aquifer is formed by bioclastic limestone and
sandy limestone of low interstitial porosity balanced by a
large fracture porosity.

2. The Cenozoic complex is composed of six aquifers: lower Eocene, middle Eocene, upper Eocene, Oligocene, Miocene and Plio-Quaternary. Because of their accessibility, they are currently widely exploited. The middle Eocene aquifer is the main reservoir; it shows strong lateral lithofacies variations from sandy alluvial deposits to the east to shallow marine limestones to the west. Thus, two major ranges of transmissivity can be identified, the

first around 5·10⁻⁴ m²/s corresponding to sandy layers
and the second around 2·10⁻³ m²/s corresponding to lime-
stones (Larroque 2004). The aquifer is overlaid by a 50-
m-thick aquitard composed of marl and clay. The
Oligocene aquifer comprises mainly limestone. Most of
the measurements show transmissivity values between 1·
10⁻³ and 6·10⁻³ m²/s. Like the middle Eocene aquifer, this
Oligocene limestone aquifer is widely used for drinking
water. The Miocene aquifer is composed of sandstone and
is mainly used for irrigation, as is the Plio-Quaternary
aquifer, which corresponds to a shallow system within
alluvial or eolian gravel and sand formations.

On a large scale, groundwater in the north-Aquitaine multi-
layer system flows toward the Gironde estuary, which consti-
tutes a drainage area for the deeper aquifers. West of the estu-
ary, flow occurs from a high piezometric area westwards to the
ocean on the one hand and eastward to the Gironde estuary on
the other hand. The leakage exchange flows between the aqui-
fers through the aquitards are mainly downwards in the east-
ern part of the basin and are more complicated in the rest of the
basin, with fluxes both upwards and downwards according to
local piezometric conditions (Larroque et al. 2008).

Paleoclimatic context

Across Western Europe, major climate changes occurred dur-
ing the late Pleistocene and are well-documented by a large
array of data, both in continental (Fletcher et al. 2010; Genty
et al. 2010) and marine settings (Eynaud et al. 2012). These
changes involved significant variation in precipitation and
groundwater recharge (Jiráková et al. 2011). Available studies
show a contrast in the rate of groundwater recharge between
northern and southern Europe (Bath et al. 1979; Celle-Jeanton
et al. 2009; Deák et al. 1987; Dray et al. 1997; Geyer et al.
1993; Geyh et al. 1984; Huneau et al. 2001). A gap in re-
charge, which lasted some millennia to tens of millennia, typ-
ifies the northern catchments and is assumed to reflect perma-
frost development during the coldest part of the glacial. Data
on southwest France remain scarce so far, but suggest ground-
water recharge to have taken place continuously. In contrast,
the speleothem records from Villars Cave, northern Aquitaine,
point to a stop in stalagmite growth between ~30 to ~16 ka
(Genty et al. 2003, 2006, 2010). As indicated by isotopic data,
this gap corresponds to a strong cooling and, therefore, was
thought to reflect restricted water percolation through the host
limestone due to permafrost.

Features such as ice-wedge pseudomorphs and the rem-
nants of other ground ice bodies provide another line of evi-
dence for the existence of Pleistocene permafrost in France.
The studies on their distribution show that a large part of the
territory was affected by permafrost (Van Vliet-Lanoë 1989,
1996; Texier 1996; Vandenberghe et al. 2014; Andrieux et al.
<figure><img src="image_6.png" /><figcaption>**Fig. 2** Location of the sampling sites</figcaption></figure>

2016 and other references therein). Available radiometric ages suggest that ice wedges developed mostly between 32 and 25 ka (Antoine et al. 2014; Bertran et al. 2014), i.e. just before and during the Last Glacial Maximum (LGM, 27–23 ka as defined by Hughes and Gibbard 2015). However, these studies do not agree on the land surface affected by permafrost during the phase of maximal extension. In most of the Paris Basin and northern France permafrost was thought to be either continuous (Van Vliet-Lanoë 1989, 1996; Vandenberghe et al. 2014) or discontinuous (Andrieux et al. 2016). In southwest France, field observations suggest that permafrost developed during shorter periods. It was either continuous (Legigan 1979; Texier 1996), discontinuous (Van Vliet-Lanoë 1989; Vandenberghe et al. 2014) or only restricted to a few favorable contexts (sporadic permafrost), or even completely lacking in the southern part of the basin (Andrieux et al. 2016).

Paleotemperature reconstructions for the Last Glacial and the Lateglacial in Western Europe have been derived from pollen data (Guiot et al. 1993; Kageyama et al. 2006; Ramstein et al. 2007), and insects (Ponel 1995; Coope 2002), and may help tackle the issue of paleopermafrost. Temperature estimates have been also provided by simulations using global climate models (GCMs; Jost et al. 2005; Roche et al. 2007; Strandberg et al. 2011). All these approaches, however, suffer from their own biases and remain difficult to reconcile. Andrieux et al. (2016), for instance, have pointed out that mean annual temperature reconstructed by GCMs are significantly too warm in Western Europe to account for the known distribution of ice-wedge pseudomorphs.

A similar mismatch between simulated temperatures and those provided by vegetation is also underlined by Kageyama et al. (2006) and Ramstein et al. (2007).

# Materials and methods

## Sampling strategy

Although groundwater can be used as a paleoclimate recorder, several phenomena such as leakage between aquifers or molecular diffusion in stagnant groundwater bodies may smooth the record of climate variations. In addition, modeled ages are associated with a large uncertainty for the oldest groundwater (Fontes et al. 1993). As a consequence, only major events have a significant impact on the isotopic signal carried by water molecules, and any climatic fluctuation lasting less than a thousand years is assumed not to be detectable (Stute and Schlosser 1993). Establishing a sampling strategy appropriate to the local context is of prime importance to reduce as far as possible the uncertainties on climate reconstruction. Nevertheless, only a combination of methods is able to provide robust information on past climate changes and their potential impacts on groundwater.

A total of 36 new water samples were collected for $^{14}$C dating and measurement of stable isotopes (Fig. 2). The samples were taken in five aquifers to obtain a wide range of residence times. Indeed, according to the Tóth theoretical diagram (Tóth 1995), the residence time increases with depth.
These new data add to measurements made within the framework of previous studies (Hosteins 1982; Chery 1993; Chery et al. 1994; Chery and Gadalia 2001; Bonnery et al. 1994; Platel et al. 1999; Schnebelen et al. 2002; Franceschi 2005; Saltel 2008; Malcuit 2012). A total of 120 samples were used to obtain corrected ages to give an overview of the distribution of residence times in the multi-layered aquifer system. Furthermore, 138 measurements of $\delta^{18}$O and $\delta^{2}$H have been used to document the isotopic composition of groundwater. Among these samples, 17 samples collected in 14 wells were analyzed for noble gases. The aim of the borehole selection for sampling was to obtain a paleotemperature data set covering the whole age range of groundwater that can be found in northern Aquitaine. Thus, seven samples were retrieved from the Oligocene aquifer, four from the Eocene, one from the Aquitanian and two from the Campanian.

## Analytical methods

Physical parameters such as electrical conductivity (EC), temperature (T) and pH were measured directly in the field using a WTW 3310 multiparameter gauge.

### Water stable isotopes

Major element concentrations and water stable isotopes were analyzed at the laboratory EA 4592 “Géoressources et Environnement” at the Institut Polytechnique at Bordeaux (Bordeaux-INP). Groundwater samples were filtered through 0.45-µm membranes and stored in polyethylene bottles for major elements, and collected in amber glass bottles for stable isotopes. All samples were stored at 4 °C until analysis. The major ion concentrations were determined by high-performance liquid chromatography using a Dionex 320 chromatograph (IonPac CS16) for cations and a Dionex 1100 chromatograph (IonPac AS9-HC) for anions. The stable isotope analyses were performed using a liquid-water stable isotope analyzer DLT-100 Los Gatos Research according to the analytical scheme recommended by the IAEA (Aggarwal et al. 2009; Penna et al. 2010). The quality of the isotopic analysis was checked using a standard deviation condition up to 1 for $\delta$D and up to 0.1 for $\delta^{18}$O for six analyses of the same sample.

### Carbon isotopes

The $^{14}$C and $^{13}$C composition of the groundwater samples was measured at the University of Waterloo-Environmental Isotope Laboratory (Canada) and the Rafter Radiocarbon Laboratory (New Zealand) using the AMS usual protocols. The radiocarbon activities are expressed as percent modern C (pmc), and the isotopic $\delta^{13}$C values are reported in the conventional notation “per mil” as a deviation from the VSMOW (Vienna Standard Mean Ocean Water) for water

and PDB (American Belemnitella from the Pee Dee formation, North Carolina, USA) for carbonate and total dissolved inorganic carbon (TDIC).

### Noble gases

Noble gases are considered as efficient hydrological tracers since they are not sensitive to their close environment. Chemically inert, they are not involved in chemical reaction or biological activity. The dissolved noble gases in groundwater have mainly an atmospheric origin. Their concentration in solution results from physico-chemical equilibriums with their atmospheric component according to Henry's law, and depend on the physical conditions (temperature, salinity and pressure which is linked to altitude) prevailing in the recharge area during water infiltration. Once the saturated zone is reached, the atmospheric component of the dissolved noble gases is generally preserved and allows the determination of the ground temperature at the water table, which reflects the mean surface temperature at the time of recharge (Beyerle 1999; Aeschbach-Hertig et al. 1999; Aeschbach-Hertig and Solomon 2013; Stute and Schlosser 1993; Seltzer et al. 2015).

To these dissolved initial concentrations is added an atmospheric component due to the presence of small air bubbles trapped during water infiltration, referred to as “excess air” (Heaton and Vogel 1981; Beyerle 1999). A radiogenic component has also to be considered. Some radiogenic isotopes are produced within the minerals in the host rocks and are partially released in groundwater. The main radiogenic isotopes to be considered are $^{4}$He and $^{40}$Ar produced respectively by the radioactive decay of U, Th and $^{40}$K. Finally, in some deep aquifers, a diffusive component arising from the continental crust or the mantle can be observed (Beyerle 1999); whereby simultaneous analysis of five noble gases (He, Ne, Ar, Kr, Xe) in groundwater allows better determination of their origin.

Noble gas measurements were performed at the Centre d'Etudes Nucléaires de Bordeaux Gradignan (CENBG). To avoid air contamination or degassing, the water sample was transferred through a short plastic tube as soon as possible from the sampling tap to an airtight container. Around 28 cm$^3$ of water was sampled in a pure copper tube held on an aluminum rack. Stainless steel clamps placed at each end of the support crimp the tube to insure sample airtightness. A third clamp located in the middle of the rack allowed splitting the water sample into two sub-samples of 14 cm$^3$ each. Prior to its closure, the copper tube was rinsed thoroughly for few minutes to avoid air bubbles being trapped with the sample. At the laboratory, gas extraction of the water sample was performed by connecting the copper tube directly to a high vacuum extraction line. By opening a clamp of the sampler, water flowed into an expansion vessel. Gases were extracted by gentle agitation of the vessel for at least 1 h and then they were transferred to a purification line by means of a capillary
<table><caption>Table 1 Physical and chemical parameters; the aquifers are indicated (Aquit. Aquitanian, Camp. Campanian, Ceno. Cenomanian, Eoc. Eocene, Olig. Oligocene). Cond. conductance</caption><thead><tr><th>N°BSS</th><th>Aquifer</th><th>T (°C)</th><th>Cond. (µS/cm)</th><th>pH</th><th>Ca<sup>2+</sup> (mg/L)</th><th>Mg<sup>2+</sup> (mg/L)</th><th>Na<sup>+</sup> (mg/L)</th><th>K<sup>+</sup> (mg/L)</th><th>NH<sub>4</sub><sup>+</sup> (mg/L)</th><th>NO<sub>2</sub><sup>-</sup> (mg/L)</th><th>NO<sub>3</sub><sup>-</sup> (mg/L)</th><th>Cl<sup>-</sup> (mg/L)</th><th>F<sup>-</sup> (mg/L)</th><th>SO<sub>4</sub><sup>2-</sup> (mg/L)</th><th>HCO<sub>3</sub><sup>-</sup> (mg/L)</th></tr></thead><tbody><tr><td>08507×0013</td><td>Aquit.</td><td>16.80</td><td>323</td><td>7.63</td><td>45.00</td><td>3.40</td><td>16.00</td><td>2.60</td><td>0.05</td><td>0.03</td><td>1.00</td><td>23.00</td><td>0.10</td><td>1.00</td><td>160.00</td></tr><tr><td>08515×0006</td><td>Aquit.</td><td>14.50</td><td>513</td><td>7.35</td><td>84.00</td><td>4.70</td><td>18.00</td><td>2.30</td><td>0.15</td><td>0.03</td><td>1.00</td><td>29.00</td><td>0.10</td><td>25.00</td><td>251.00</td></tr><tr><td>08504×0004</td><td>Aquit.</td><td>14.90</td><td>273</td><td>7.85</td><td>42.00</td><td>2.80</td><td>9.90</td><td>2.40</td><td>0.06</td><td>0.03</td><td>1.00</td><td>17.00</td><td>0.10</td><td>5.00</td><td>137.00</td></tr><tr><td>07804×0003</td><td>Camp.</td><td>17.50</td><td>397</td><td>7.67</td><td>38.00</td><td>12.00</td><td>27.00</td><td>3.70</td><td>0.14</td><td>0.03</td><td>1.00</td><td>28.00</td><td>0.54</td><td>17.00</td><td>178.00</td></tr><tr><td>08033×0162</td><td>Camp.</td><td>27.40</td><td>663</td><td>7.48</td><td>30.10</td><td>11.70</td><td>90.00</td><td>10.20</td><td>0.00</td><td>0.00</td><td>0.00</td><td>51.00</td><td>2.40</td><td>49.00</td><td>250.00</td></tr><tr><td>08523×0095</td><td>Camp.</td><td>22.50</td><td>493</td><td>7.35</td><td>51.00</td><td>23.00</td><td>30.00</td><td>5.30</td><td>0.11</td><td>0.03</td><td>1.00</td><td>19.00</td><td>1.41</td><td>33.00</td><td>270.00</td></tr><tr><td>07294×0012</td><td>Ceno.</td><td>31.70</td><td>1.660</td><td>7.59</td><td>31.00</td><td>20.00</td><td>288.00</td><td>22.00</td><td>0.72</td><td>0.03</td><td>1.00</td><td>300.00</td><td>2.80</td><td>123.00</td><td>311.00</td></tr><tr><td>08036×0948</td><td>Ceno.</td><td>50.60</td><td>651.00</td><td>7.24</td><td>25.00</td><td>12.50</td><td>78.00</td><td>14.90</td><td>0.00</td><td>0.00</td><td>0.10</td><td>84.00</td><td>1.40</td><td>67.00</td><td>178.00</td></tr><tr><td>07305×0049</td><td>Eoc.</td><td>16.40</td><td>542</td><td>7.28</td><td>63.00</td><td>8.60</td><td>34.00</td><td>2.40</td><td>0.26</td><td>0.03</td><td>1.00</td><td>67.00</td><td>0.10</td><td>8.00</td><td>205.00</td></tr><tr><td>07552×0003</td><td>Eoc.</td><td>14.80</td><td>222</td><td>6.05</td><td>17.00</td><td>3.20</td><td>19.00</td><td>3.20</td><td>0.05</td><td>0.03</td><td>1.00</td><td>29.00</td><td>0.10</td><td>28.00</td><td>39.00</td></tr><tr><td>07778×0036</td><td>Eoc.</td><td>20.20</td><td>407</td><td>7.65</td><td>50.90</td><td>2.10</td><td>35.00</td><td>3.10</td><td>0.00</td><td>0.00</td><td>0.10</td><td>42.00</td><td>0.60</td><td>2.80</td><td>172.00</td></tr><tr><td>07783×0002</td><td>Eoc.</td><td>16.40</td><td>447</td><td>7.58</td><td>49.30</td><td>5.10</td><td>37.60</td><td>3.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>53.40</td><td>0.14</td><td>12.10</td><td>170.00</td></tr><tr><td>07793×0004</td><td>Eoc.</td><td>16.40</td><td>350</td><td>7.25</td><td>49.19</td><td>5.71</td><td>10.78</td><td>3.86</td><td>0.00</td><td>0.00</td><td>0.08</td><td>12.35</td><td>0.16</td><td>5.55</td><td>188.00</td></tr><tr><td>07795×0105</td><td>Eoc.</td><td>15.80</td><td>579</td><td>7.55</td><td>48.00</td><td>18.00</td><td>49.00</td><td>8.10</td><td>0.22</td><td>0.03</td><td>1.00</td><td>27.00</td><td>1.87</td><td>76.00</td><td>227.00</td></tr><tr><td>07806×0067</td><td>Eoc.</td><td>19.90</td><td>185</td><td>6.25</td><td>21.25</td><td>3.47</td><td>9.98</td><td>2.17</td><td>0.00</td><td>0.00</td><td>0.08</td><td>14.10</td><td>0.13</td><td>10.80</td><td>83.00</td></tr><tr><td>08033×0170</td><td>Eoc.</td><td>23.20</td><td>394</td><td>7.49</td><td>41.00</td><td>16.00</td><td>16.00</td><td>4.30</td><td>0.08</td><td>0.03</td><td>1.00</td><td>15.00</td><td>0.96</td><td>19.00</td><td>212.00</td></tr><tr><td>08036×0015</td><td>Eoc.</td><td>23.70</td><td>764</td><td>7.42</td><td>43.43</td><td>16.78</td><td>56.87</td><td>9.16</td><td>0.00</td><td>0.00</td><td>0.08</td><td>39.70</td><td>1.17</td><td>94.40</td><td>195.00</td></tr><tr><td>08037×0402</td><td>Eoc.</td><td>24.40</td><td>926</td><td>7.46</td><td>80.00</td><td>27.00</td><td>75.00</td><td>11.00</td><td>0.33</td><td>0.03</td><td>1.00</td><td>90.00</td><td>1.67</td><td>208.00</td><td>203.00</td></tr><tr><td>08042×0007</td><td>Eoc.</td><td>15.70</td><td>381</td><td>7.33</td><td>43.10</td><td>16.80</td><td>13.00</td><td>3.00</td><td>0.00</td><td>0.00</td><td>0.10</td><td>15.20</td><td>1.00</td><td>18.50</td><td>205.50</td></tr><tr><td>08261×0030</td><td>Eoc.</td><td>24.40</td><td>336</td><td>7.82</td><td>24.00</td><td>11.00</td><td>27.00</td><td>4.40</td><td>0.07</td><td>0.03</td><td>1.00</td><td>27.00</td><td>0.26</td><td>14.00</td><td>139.00</td></tr><tr><td>08277×0162</td><td>Eoc.</td><td>21.90</td><td>381</td><td>7.64</td><td>32.00</td><td>10.00</td><td>32.00</td><td>5.70</td><td>0.18</td><td>0.03</td><td>1.00</td><td>10.00</td><td>0.42</td><td>31.00</td><td>190.00</td></tr><tr><td>08284×0011</td><td>Eoc.</td><td>21.10</td><td>368</td><td>7.25</td><td>40.50</td><td>16.40</td><td>13.70</td><td>3.00</td><td>0.00</td><td>0.00</td><td>0.10</td><td>14.80</td><td>0.81</td><td>18.90</td><td>194.00</td></tr><tr><td>08301×0002</td><td>Eoc.</td><td>18.30</td><td>324</td><td>7.67</td><td>43.70</td><td>16.30</td><td>8.20</td><td>2.15</td><td>0.00</td><td>0.00</td><td>0.08</td><td>7.90</td><td>0.55</td><td>8.54</td><td>226.00</td></tr><tr><td>08764×0005</td><td>Eoc.</td><td>21.30</td><td>1.562</td><td>7.12</td><td>129.00</td><td>47.00</td><td>140.00</td><td>15.00</td><td>0.44</td><td>0.03</td><td>1.00</td><td>167.00</td><td>2.63</td><td>413.00</td><td>232.00</td></tr><tr><td>07545×0021</td><td>Eoc.</td><td>14.90</td><td>547</td><td>7.83</td><td>41.30</td><td>27.80</td><td>33.80</td><td>4.20</td><td>0.00</td><td>0.00</td><td>0.10</td><td>100.50</td><td>0.19</td><td>2.10</td><td>170.00</td></tr><tr><td>07538×0010</td><td>Olig.</td><td>17.40</td><td>721.00</td><td>7.56</td><td>55.00</td><td>9.60</td><td>78.00</td><td>3.30</td><td>0.69</td><td>0.03</td><td>1.00</td><td>142.00</td><td>0.10</td><td>4.00</td><td>183.00</td></tr><tr><td>07786×0057</td><td>Olig.</td><td>14.90</td><td>450.00</td><td>7.53</td><td>55.00</td><td>5.70</td><td>28.00</td><td>2.50</td><td>0.16</td><td>0.03</td><td>1.00</td><td>50.00</td><td>0.10</td><td>1.00</td><td>192.00</td></tr><tr><td>08018×0004</td><td>Olig.</td><td>21.20</td><td>327.00</td><td>7.94</td><td>22.57</td><td>14.08</td><td>20.94</td><td>3.11</td><td>0.00</td><td>0.00</td><td>0.08</td><td>42.90</td><td>0.24</td><td>2.77</td><td>128.00</td></tr><tr><td>08036×1858</td><td>Olig.</td><td>17.70</td><td>833.00</td><td>7.06</td><td>128.00</td><td>11.00</td><td>35.00</td><td>5.80</td><td>0.07</td><td>0.03</td><td>22.00</td><td>52.00</td><td>0.13</td><td>77.00</td><td>354.00</td></tr><tr><td>08257×0073</td><td>Olig.</td><td>21.90</td><td>271.00</td><td>7.10</td><td>15.10</td><td>8.10</td><td>31.00</td><td>4.00</td><td>0.00</td><td>0.00</td><td>0.10</td><td>32.60</td><td>0.31</td><td>7.05</td><td>131.00</td></tr><tr><td>08262×0096</td><td>Olig.</td><td>19.20</td><td>299.00</td><td>7.79</td><td>27.91</td><td>9.00</td><td>22.34</td><td>2.21</td><td>0.00</td><td>0.00</td><td>0.08</td><td>22.64</td><td>0.19</td><td>2.35</td><td>141.00</td></tr><tr><td>08264×0012</td><td>Olig.</td><td>14.00</td><td>372.00</td><td>7.32</td><td>55.30</td><td>5.24</td><td>26.80</td><td>2.35</td><td>0.00</td><td>0.00</td><td>0.10</td><td>35.60</td><td>0.13</td><td>3.20</td><td>207.00</td></tr><tr><td>08272×0058</td><td>Olig.</td><td>15.50</td><td>740.00</td><td>6.89</td><td>138.00</td><td>5.90</td><td>21.00</td><td>3.60</td><td>0.05</td><td>0.03</td><td>13.00</td><td>34.00</td><td>0.10</td><td>38.00</td><td>395.00</td></tr><tr><td>08277×0166</td><td>Olig.</td><td>15.10</td><td>526.00</td><td>7.23</td><td>87.00</td><td>4.30</td><td>18.00</td><td>3.00</td><td>0.46</td><td>0.09</td><td>5.00</td><td>29.00</td><td>0.10</td><td>16.00</td><td>261.00</td></tr><tr><td>08762×0025</td><td>Olig.</td><td>19.80</td><td>933.00</td><td>7.84</td><td>33.00</td><td>20.00</td><td>146.00</td><td>2.60</td><td>0.14</td><td>0.03</td><td>1.00</td><td>26.00</td><td>1.46</td><td>281.00</td><td>189.00</td></tr></tbody></table>
<table><caption>**Table 2** Physical and chemical parameters of water samples from older studies - sampling points for noble gases (Camp. Campanian, Eoc. Eocene, Olig. Oligocene). *Cond.* conductance</caption><thead><tr><th>N° BSS</th><th>Aquifer</th><th>T (°C)</th><th>Cond. (µS/cm)</th><th>pH</th><th>Ca<sup>2+</sup> (mg/L)</th><th>Mg<sup>2+</sup> (mg/L)</th><th>Na<sup>+</sup> (mg/L)</th><th>K<sup>+</sup> (mg/L)</th><th>Cl<sup>-</sup> (mg/L)</th><th>SO<sub>4</sub><sup>2-</sup> (mg/L)</th><th>F<sup>-</sup> (mg/L)</th><th>HCO<sub>3</sub><sup>-</sup> (mg/L)</th></tr></thead><tbody><tr><td>08763×0017</td><td>Olig.</td><td>17.3</td><td>711</td><td>7.53</td><td>53.41</td><td>32.13</td><td>55.14</td><td>5.02</td><td>59.29</td><td>16.92</td><td>0.35</td><td>349.00</td></tr><tr><td>08035×0360</td><td>Olig.</td><td>16.0</td><td>672</td><td>7.10</td><td>73.48</td><td>8.36</td><td>24.16</td><td>3.78</td><td>35.60</td><td>12.44</td><td>0.12</td><td>331.20</td></tr><tr><td>08276×0091</td><td>Olig.</td><td>14.8</td><td>366</td><td>7.75</td><td>41.97</td><td>4.56</td><td>18.00</td><td>2.14</td><td>20.84</td><td>3.97</td><td>0.14</td><td>174.10</td></tr><tr><td>08035×0465</td><td>Olig.</td><td>16.1</td><td>487</td><td>7.53</td><td>56.91</td><td>7.07</td><td>24.24</td><td>2.54</td><td>37.05</td><td>12.51</td><td>0.18</td><td>215.90</td></tr><tr><td>08525×0028</td><td>Olig.</td><td>17.6</td><td>567</td><td>8.16</td><td>22.75</td><td>12.02</td><td>82.98</td><td>4.39</td><td>19.49</td><td>98.00</td><td>1.31</td><td>185.00</td></tr><tr><td>08277×0215</td><td>Eoc.</td><td>24.7</td><td>348</td><td>7.70</td><td>35.50</td><td>10.50</td><td>30.00</td><td>5.60</td><td>9.80</td><td>34.45</td><td>0.35</td><td>189.10</td></tr><tr><td>08272×0136</td><td>Camp.</td><td>30.5</td><td>406</td><td>8.28</td><td>14.84</td><td>6.00</td><td>49.98</td><td>13.35</td><td>29.49</td><td>9.56</td><td>1.46</td><td>202.00</td></tr></tbody></table>

tube. The latter allowed recovery of almost all the extracted gases while limiting vapor transfer (Osenbrück et al. 1998; Beyerle et al. 2000). Helium and neon were cryogenically separated from the other noble gases by means of activated charcoal at the temperature of liquid nitrogen that traps the condensable gaseous phases (mainly water vapor and heavy noble gases: Ar, Kr and Xe). Getters composed of titanium sponge allowed purification of the phase of light noble gases (He, Ne). The phase of heavy noble gases was first exposed to

zeolites traps to eliminate water prior to exposure to the getters. After cryogenic separation from the heavy noble gases, He and Ne were introduced into a VG Micromass 1200 mass spectrometer for isotopic analysis. Ar, Kr, and Xe were also cryogenically separated and successively introduced into the mass spectrometer. Absolute concentrations were determined using an isotopic dilution method with two specially prepared noble gas spikes. The precision on measured concentrations were in the range of 4–6% (2σ; Lavielle et al. 2012).

<figure><img src="image_7.png" /><figcaption>**Fig. 3** Groundwater cation and anion composition plotted in a Piper diagram</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Fig. 4 Plot of δ¹⁸O vs. δ²H of groundwater collected in the North Aquitaine multi-layer aquifer system; annual weighted mean for the Cestas and the Dax Station (Millot et al. 2010)</figcaption></figure>

## Results

### Major elements

The geochemical water-type of the samples is mainly bicarbonate-calcic (Tables 1 and 2). Water sampled in the early and middle Eocene aquifers is more mineralized and has variable content in sulfates and chlorides (Fig. 3). Oligocene groundwater, showing limited mineralization, has a bicarbonate-calcic water-type. Upper Cretaceous (Santonian–Campanian) water is also little mineralized, while most of the Cretaceous samples have a bicarbonate calcic type.

The groundwater samples from the early and middle Eocene aquifers are the more scattered and the Piper diagram highlights a clear trend. The processes involved in the acquisition of mineralization are complex and seem to occur in two steps (Malcuit et al. 2014). Firstly, the processes of water-rock interaction in a carbonate system give water its mineralization, as shown by the bicarbonate-calcic water-type of the low mineralized samples, and secondly, water is increasingly enriched in dissolved elements, especially sulfates and chlorides mainly provided by evaporite dissolution (Malcuit et al. 2014). During evaporite dissolution, sulfate, fluoride and calcium are released and the water becomes over-saturated with respect to calcite which precipitates, leading to a decrease of the $HCO_3^-$ concentration and an increase in pH. Some water exceeds the standard of sulfates concentration for drinking

water (250 mg/L). The others aquifers sampled do not show any similar trend.

### Stable isotopes

Water δ¹⁸O and δ²H values range from -7.88 to -4.64‰ and from -51.65 to -30.28‰ respectively. These results are in good agreement with previous analyses, which yielded δ¹⁸O values from -8.1 to -3.7‰ and δ²H values from -51.9 to -21.6‰ (Hosteins 1982; Chery 1993; Chery et al. 1994; Chery and Gadalia 2001; Bonnery et al. 1994; Platel et al. 1999; Schnebelen et al. 2002; Franceschi 2005; Saltel 2008; Malcuit 2012).

All samples almost fit the Global Meteoric Water Line (Fig. 4). Two International Atomic Energy Agency (IAEA) stations are located near the study area: the Cestas station and the Dax station (IAEA 2018). The Cestas station covers the most recent and complete period (2007–2016). A local meteoric line has been established from these measurements. The best fit is obtained between this local line and groundwater measurements. The depletion in ²H and ¹⁸O relative to other samples is associated with low ¹⁴C content, i.e. with old ages and, therefore, points to the cooler climate of the Last Glacial. The most depleted water samples come from the middle Eocene aquifer and show a strong deuterium excess, while water from the other aquifers is less depleted.
<table><caption>**Table 3** Isotopic composition of water, chemical speciation for total inorganic carbon (CD/T concentration in moles per liter) and radiocarbon ages for the new sampling points</caption><thead><tr><th>N°BSS</th><th>δ<sup>18</sup>O<br>(‰ VSMOW)</th><th>δ<sup>2</sup>H<br>(‰ VSMOW)</th><th>δ<sup>13</sup>C<br>(‰ VSMOW)</th><th>δ<sup>13</sup>C Eq<br>(‰ VSMOW)</th><th>CDIT<br>(moles/L)</th><th>H<sub>2</sub>CO<sub>3</sub><br>(moles/L)</th><th>HCO<sub>3</sub><sup>-</sup><br>(moles/L)</th><th>CO<sub>3</sub><sup>2-</sup><br>(moles/L)</th><th><sup>14</sup>C activity<br>(pcm)</th><th>Mean age<br>(years)</th><th>±</th><th>Mean age<br>(years)</th><th>±</th></tr></thead><tbody><tr><td>07305×0049</td><td>-5.54</td><td>-36.07</td><td>-12.10</td><td>-13.1</td><td>3.76 ×10<sup>-3</sup></td><td>4.05 ×10<sup>-4</sup></td><td>3.30 ×10<sup>-3</sup></td><td>5.73 ×10<sup>-5</sup></td><td>32.78</td><td>3,400</td><td>1,200</td><td>3,770</td><td>1,440</td></tr><tr><td>07538×0010</td><td>-4.77</td><td>-30.29</td><td>-12.97</td><td>-14.0</td><td>3.17 ×10<sup>-3</sup></td><td>1.85 ×10<sup>-4</sup></td><td>2.93 ×10<sup>-3</sup></td><td>5.54 ×10<sup>-5</sup></td><td>9.65</td><td>13,300</td><td>1,140</td><td>15,870</td><td>1,540</td></tr><tr><td>07545×0021</td><td>-6.26</td><td>-41.48</td><td>-6.37</td><td>-7.5</td><td>2.86 ×10<sup>-3</sup></td><td>9.60 ×10<sup>-5</sup></td><td>2.70 ×10<sup>-3</sup></td><td>6.51 ×10<sup>-5</sup></td><td>0.70</td><td>29,400</td><td>1,200</td><td>33,690</td><td>1,060</td></tr><tr><td>07552×0003</td><td>-5.96</td><td>-37.08</td><td>-18.06</td><td>-17.7</td><td>2.05 ×10<sup>-3</sup></td><td>1.41 ×10<sup>-3</sup></td><td>6.37 ×10<sup>-4</sup></td><td>3.29 ×10<sup>-6</sup></td><td>73.98</td><td>1,200</td><td>130</td><td>1,120</td><td>130</td></tr><tr><td>07778×0036</td><td>-4.91</td><td>-30.50</td><td>-12.43</td><td>-13.5</td><td>2.94 ×10<sup>-3</sup></td><td>1.37 ×10<sup>-4</sup></td><td>2.76 ×10<sup>-3</sup></td><td>4.71 ×10<sup>-5</sup></td><td>0.58</td><td>36,600</td><td>1,450</td><td>40,940</td><td>1,490</td></tr><tr><td>07783×0002</td><td>—</td><td>—</td><td>-11.48</td><td>-12.5</td><td>2.95 ×10<sup>-3</sup></td><td>1.70 ×10<sup>-4</sup></td><td>2.73 ×10<sup>-3</sup></td><td>4.29 ×10<sup>-5</sup></td><td>1.42</td><td>28,900</td><td>850</td><td>33,250</td><td>770</td></tr><tr><td>07786×0057</td><td>-4.64</td><td>-31.11</td><td>-14.91</td><td>-16.0</td><td>3.36 ×10<sup>-3</sup></td><td>2.21 ×10<sup>-4</sup></td><td>3.09 ×10<sup>-3</sup></td><td>4.97 ×10<sup>-5</sup></td><td>37.27</td><td>3,500</td><td>870</td><td>3,870</td><td>1,080</td></tr><tr><td>07793×0004</td><td>-5.88</td><td>-37.01</td><td>-13.92</td><td>-14.8</td><td>3.48 ×10<sup>-3</sup></td><td>4.06 ×10<sup>-4</sup></td><td>3.04 ×10<sup>-3</sup></td><td>4.15 ×10<sup>-5</sup></td><td>28.09</td><td>5,700</td><td>530</td><td>6,520</td><td>580</td></tr><tr><td>07795×0105</td><td>-7.36</td><td>-49.64</td><td>-4.91</td><td>-6.0</td><td>3.95 ×10<sup>-3</sup></td><td>2.42 ×10<sup>-4</sup></td><td>3.64 ×10<sup>-3</sup></td><td>6.63 ×10<sup>-5</sup></td><td>0.32</td><td>37,400</td><td>2,100</td><td>41,520</td><td>2,060</td></tr><tr><td>07804×0003</td><td>-7.01</td><td>-47.16</td><td>-11.01</td><td>-12.01</td><td>3.05 ×10<sup>-3</sup></td><td>1.42 ×10<sup>-4</sup></td><td>2.86 ×10<sup>-3</sup></td><td>4.74 ×10<sup>-5</sup></td><td>0.15</td><td>47,600</td><td>2,700</td><td>52,070</td><td>3,950</td></tr><tr><td>07806×0067</td><td>-5.92</td><td>-37.64</td><td>-20.93</td><td>-20.8</td><td>3.10 ×10<sup>-3</sup></td><td>1.74 ×10<sup>-3</sup></td><td>1.35 ×10<sup>-3</sup></td><td>8.90 ×10<sup>-6</sup></td><td>69.08</td><td>1,480</td><td>410</td><td>1,450</td><td>430</td></tr><tr><td>08018×0004</td><td>-6.39</td><td>-41.90</td><td>-3.20</td><td>-4.3</td><td>2.13 ×10<sup>-3</sup></td><td>5.17 ×10<sup>-5</sup></td><td>2.04 ×10<sup>-3</sup></td><td>3.81 ×10<sup>-5</sup></td><td>0.50</td><td>25,600</td><td>1,160</td><td>30,270</td><td>1,160</td></tr><tr><td>08033×0162</td><td>—</td><td>—</td><td>-2.63</td><td>-3.6</td><td>4.35 ×10<sup>-3</sup></td><td>2.65 ×10<sup>-4</sup></td><td>4.02 ×10<sup>-3</sup></td><td>6.19 ×10<sup>-5</sup></td><td>0.19</td><td>30,500</td><td>2,800</td><td>35,840</td><td>2,960</td></tr><tr><td>08033×0170</td><td>-7.54</td><td>-49.56</td><td>-3.35</td><td>-4.4</td><td>3.70 ×10<sup>-3</sup></td><td>2.34 ×10<sup>-4</sup></td><td>3.40 ×10<sup>-3</sup></td><td>6.20 ×10<sup>-5</sup></td><td>0.27</td><td>30,500</td><td>1,600</td><td>35,190</td><td>1,800</td></tr><tr><td>08036×0015</td><td>-7.68</td><td>-49.61</td><td>-7.90</td><td>-8.9</td><td>3.43 ×10<sup>-3</sup></td><td>2.48 ×10<sup>-4</sup></td><td>3.13 ×10<sup>-3</sup></td><td>5.59 ×10<sup>-5</sup></td><td>0.14</td><td>44,300</td><td>2,800</td><td>48,400</td><td>3,190</td></tr><tr><td>08036×0948</td><td>-6.97</td><td>-44.96</td><td>-2.77</td><td>-3.6</td><td>3.20 ×10<sup>-3</sup></td><td>2.89 ×10<sup>-4</sup></td><td>2.86 ×10<sup>-3</sup></td><td>4.49 ×10<sup>-5</sup></td><td>1.16</td><td>16,800</td><td>1,860</td><td>20,230</td><td>2,140</td></tr><tr><td>08036×1858</td><td>-5.56</td><td>-36.55</td><td>-11.87</td><td>-12.9</td><td>3.80 ×10<sup>-3</sup></td><td>2.52 ×10<sup>-4</sup></td><td>3.45 ×10<sup>-3</sup></td><td>6.02 ×10<sup>-5</sup></td><td>44.75</td><td>470</td><td>30</td><td>510</td><td>30</td></tr><tr><td>08037×0402</td><td>-7.73</td><td>-51.30</td><td>-8.66</td><td>-9.7</td><td>3.54 ×10<sup>-3</sup></td><td>2.26 ×10<sup>-4</sup></td><td>3.23 ×10<sup>-3</sup></td><td>8.69 ×10<sup>-5</sup></td><td>0.17</td><td>43,600</td><td>2,370</td><td>47,480</td><td>2,610</td></tr><tr><td>08042×0007</td><td>-7.80</td><td>-49.44</td><td>-3.96</td><td>-4.9</td><td>3.73 ×10<sup>-3</sup></td><td>3.70 ×10<sup>-4</sup></td><td>3.31 ×10<sup>-3</sup></td><td>5.29 ×10<sup>-5</sup></td><td>0.43</td><td>28,600</td><td>1,200</td><td>33,670</td><td>980</td></tr><tr><td>08257×0073</td><td>-6.11</td><td>-40.05</td><td>-2.05</td><td>-3.1</td><td>3.81 ×10<sup>-3</sup></td><td>3.76 ×10<sup>-4</sup></td><td>3.35 ×10<sup>-3</sup></td><td>5.4 ×10<sup>-5</sup></td><td>1.02</td><td>16,300</td><td>900</td><td>19,070</td><td>1,020</td></tr><tr><td>08261×0030</td><td>-6.66</td><td>-43.82</td><td>-3.75</td><td>-4.8</td><td>2.33 ×10<sup>-3</sup></td><td>7.11 ×10<sup>-5</sup></td><td>2.23 ×10<sup>-3</sup></td><td>3.71 ×10<sup>-5</sup></td><td>0.33</td><td>30,400</td><td>1,200</td><td>34,780</td><td>1,240</td></tr><tr><td>08262×0096</td><td>-4.81</td><td>-31.46</td><td>-6.20</td><td>-7.3</td><td>2.38 ×10<sup>-3</sup></td><td>8.37 ×10<sup>-5</sup></td><td>2.26 ×10<sup>-3</sup></td><td>3.45 ×10<sup>-5</sup></td><td>0.50</td><td>31,800</td><td>1,300</td><td>36,500</td><td>1,600</td></tr><tr><td>08264×0012</td><td>-4.94</td><td>-31.15</td><td>-10.59</td><td>-11.5</td><td>3.78 ×10<sup>-3</sup></td><td>3.95 ×10<sup>-4</sup></td><td>3.34 ×10<sup>-3</sup></td><td>4.81 ×10<sup>-5</sup></td><td>0.44</td><td>38,600</td><td>1,800</td><td>42,930</td><td>1,290</td></tr><tr><td>08277×0162</td><td>-7.89</td><td>-50.62</td><td>-12.15</td><td>-13.2</td><td>3.25 ×10<sup>-3</sup></td><td>1.52 ×10<sup>-4</sup></td><td>3.05 ×10<sup>-3</sup></td><td>4.64 ×10<sup>-5</sup></td><td>0.25</td><td>43,600</td><td>3,300</td><td>47,960</td><td>3,540</td></tr><tr><td>08284×0011</td><td>-7.56</td><td>-49.48</td><td>-3.66</td><td>-4.6</td><td>3.56 ×10<sup>-3</sup></td><td>3.85 ×10<sup>-4</sup></td><td>3.12 ×10<sup>-3</sup></td><td>5.08 ×10<sup>-5</sup></td><td>0.58</td><td>24,900</td><td>1,790</td><td>29,380</td><td>1,840</td></tr><tr><td>08301×0002</td><td>-7.61</td><td>-49.83</td><td>-4.39</td><td>-5.5</td><td>3.86 ×10<sup>-3</sup></td><td>1.77 ×10<sup>-4</sup></td><td>3.62 ×10<sup>-3</sup></td><td>7.01 ×10<sup>-5</sup></td><td>0.73</td><td>25,500</td><td>1,100</td><td>30,200</td><td>1,140</td></tr><tr><td>08504×0004</td><td>-5.68</td><td>-36.07</td><td>-10.44</td><td>-11.6</td><td>2.31 ×10<sup>-3</sup></td><td>7.63 ×10<sup>-5</sup></td><td>2.20 ×10<sup>-3</sup></td><td>3.44 ×10<sup>-5</sup></td><td>11.71</td><td>11,100</td><td>1,200</td><td>13,080</td><td>1,720</td></tr><tr><td>08507×0013</td><td>-4.97</td><td>-33.64</td><td>-13.37</td><td>-14.4</td><td>2.75 ×10<sup>-3</sup></td><td>1.43 ×10<sup>-4</sup></td><td>2.57 ×10<sup>-3</sup></td><td>3.79 ×10<sup>-5</sup></td><td>15.34</td><td>10,100</td><td>600</td><td>11,660</td><td>830</td></tr><tr><td>08515×0006</td><td>-5.45</td><td>-35.25</td><td>-13.52</td><td>-14.5</td><td>4.54 ×10<sup>-3</sup></td><td>4.36 ×10<sup>-4</sup></td><td>4.03 ×10<sup>-3</sup></td><td>7.71 ×10<sup>-5</sup></td><td>22.60</td><td>6,800</td><td>600</td><td>7,640</td><td>620</td></tr><tr><td>08523×0095</td><td>-7.24</td><td>-48.41</td><td>-1.91</td><td>-2.9</td><td>4.82 ×10<sup>-3</sup></td><td>4.09 ×10<sup>-4</sup></td><td>4.32 ×10<sup>-3</sup></td><td>9.10 ×10<sup>-5</sup></td><td>1.12</td><td>10,700</td><td>960</td><td>12,370</td><td>1,270</td></tr><tr><td>08762×0025</td><td>-6.85</td><td>-45.09</td><td>-5.06</td><td>-6.2</td><td>3.17 ×10<sup>-3</sup></td><td>9.42 ×10<sup>-5</sup></td><td>3.02 ×10<sup>-3</sup></td><td>5.95 ×10<sup>-5</sup></td><td>0.22</td><td>36,800</td><td>1,300</td><td>41,180</td><td>1,340</td></tr></tbody></table>
<table><caption>**Table 4** Isotopic composition of water and radiocarbon ages for older studies (sampling points for noble gases)</caption><thead><tr><th>N°BSS</th><th>δ<sup>18</sup>O<br>(‰ VSMOW)</th><th>δ<sup>2</sup>H<br>(‰ VSMOW)</th><th>δ<sup>13</sup>C<br>(‰ VSMOW)</th><th><sup>14</sup>C activity<br>(pcm)</th><th>Mean age<br>(years)</th><th>±</th><th>Mean age (years)<br>mod. BP</th><th>±</th></tr></thead><tbody><tr><td>08763×0017</td><td>-6.95</td><td>-43.38</td><td>-12.48</td><td>2.77</td><td>22,800</td><td>2,000</td><td>27,160</td><td>2,300</td></tr><tr><td>08035×0360</td><td>-5.59</td><td>-34.51</td><td>-12.25</td><td>22.33</td><td>6,300</td><td>350</td><td>7,130</td><td>360</td></tr><tr><td>08276×0091</td><td>-5.20</td><td>-31.37</td><td>-12.09</td><td>3.01</td><td>21,600</td><td>2,400</td><td>25,900</td><td>2,790</td></tr><tr><td>08035×0465</td><td>-5.38</td><td>-33.65</td><td>-10.45</td><td>16.37</td><td>7,500</td><td>300</td><td>8,370</td><td>325</td></tr><tr><td>08525×0028</td><td>-6.90</td><td>-43.40</td><td>-7.27</td><td>3.31</td><td>17,700</td><td>370</td><td>21,230</td><td>590</td></tr><tr><td>08277×0215</td><td>-7.92</td><td>-50.58</td><td>-7.30</td><td>0.90</td><td>28,300</td><td>550</td><td>32,840</td><td>580</td></tr><tr><td>08272×0136</td><td>-6.49</td><td>-41.19</td><td>-6.33</td><td>3.06</td><td>20,000</td><td>650</td><td>23,380</td><td>790</td></tr></tbody></table>

Carbon isotopes

The δ¹³C value of the TDIC (Tables 3 and 4) range from -20.93 to -1.91‰ vs PDB (-12.48 to -6.33‰ for older analyses). ¹⁴C values range from 0.14 to 74 pmc.

In a carbonate aquifer, the δ¹³C value of the TDIC results from the following:

1. Isotopic equilibrium with biogenic CO₂ (open system with respect to soil CO₂) during mineralization, or

2. Partial isotopic mixing between biogenic CO₂ and the limestone matrix (closed system for soil CO₂). In this study, the δ¹³C content of the biogenic CO₂ is considered to be about -21‰, which is characteristic of the natural C-3 vegetation under temperate climate (Deines et al. 1974).

In order to assess the system in which C evolves, a calculation of δ¹³C<sub>gas_eq</sub>, i.e. δ¹³C of the gaseous phase in equilibrium with the measured TDIC, was carried out using the following equation (Le Gal La Salle et al. 1996):

$$ \delta^{13}C_{\text{gas\_eq}} = \delta^{13}C - \frac{|\text{CO}_2|\epsilon_{\text{CO}_2\text{-g}} + |\text{HCO}_3^-|\epsilon_{\text{HCO}_3\text{-g}} + |\text{CO}_3^-|\epsilon_{\text{CO}_3\text{-g}}}{C_{\text{tot}}} \quad (1) $$

where ε is enrichment factor between carbon species and the gas phase g; and C<sub>tot</sub> is total of carbon species.

The distribution of the carbon species is determined using the geochemical software PHREEQC 3.0 (Parkhurst and Appelo 2013). The calculated δ¹³C<sub>gas_eq</sub> varies from -2.9 to -20.8‰ (Tables 3 and 4). Therefore,

<table><caption>**Table 5** Concentrations of dissolved noble gases in groundwater (expressed in 10⁻⁷ cm³ STP/g water). The aquifers are indicated (Olig. Oligocene, Eoc. Eocene, Camp. Campanian, Aquit. Aquitanian)</caption><thead><tr><th>Site</th><th>Aquifer</th><th>He<br>(10<sup>-7</sup> cm<sup>3</sup><br>STP/g water)</th><th>Ne<br>(10<sup>-7</sup> cm<sup>3</sup><br>STP/g water)</th><th>Ar<br>(10<sup>-7</sup> cm<sup>3</sup><br>STP/g water)</th><th>Kr<br>(10<sup>-7</sup> cm<sup>3</sup><br>STP/g water)</th><th>Xe<br>(10<sup>-7</sup> cm<sup>3</sup><br>STP/g water)</th></tr></thead><tbody><tr><td>08257×0073</td><td>Olig.</td><td>2.57±0.32</td><td>2.59±0.11</td><td>4,546±257</td><td>1.00±0.05</td><td>0.151±0.007</td></tr><tr><td>08763×0017</td><td>Olig.</td><td>10.6±0.75</td><td>2.42±0.10</td><td>4,594±277</td><td>1.03±0.05</td><td>0.148±0.006</td></tr><tr><td>08262×0096/F</td><td>Olig.</td><td>0.86±0.07</td><td>2.42±0.17</td><td>4,535±281</td><td>0.95±0.06</td><td>0.138±0.006</td></tr><tr><td>07538×0010</td><td>Olig.</td><td>0.99±0.07</td><td>2.37±0.10</td><td>4,470±280</td><td>1.02±0.05</td><td>0.136±0.006</td></tr><tr><td>08276×0091</td><td>Olig.</td><td>0.92±0.21</td><td>2.61±0.11</td><td>4,370±280</td><td>0.97±0.06</td><td>0.129±0.006</td></tr><tr><td rowspan="2">08503×0010</td><td rowspan="2">Olig.</td><td>0.45±0.29</td><td>2.60±0.13</td><td>4,298±244</td><td>0.93±0.05</td><td>0.122±0.005</td></tr><tr><td>0.82±0.19</td><td>2.55±0.13</td><td>4,232±238</td><td>0.92±0.05</td><td>0.119±0.005</td></tr><tr><td rowspan="2">08525×0028</td><td rowspan="2">Olig.</td><td>15.2±1.15</td><td>2.66±0.16</td><td>4,472±276</td><td>1.07±0.06</td><td>0.150±0.006</td></tr><tr><td>16.0±1.13</td><td>2.67±0.14</td><td>4,685±296</td><td>1.07±0.07</td><td>0.150±0.007</td></tr><tr><td>08277×0215</td><td>Eoc.</td><td>18.8±1.33</td><td>2.32±0.10</td><td>4,339±272</td><td>0.98±0.05</td><td>0.138±0.006</td></tr><tr><td>08277×0152</td><td>Eoc.</td><td>17.7±1.25</td><td>2.33±0.10</td><td>4,390±258</td><td>0.99±0.06</td><td>0.138±0.006</td></tr><tr><td>08035×0393</td><td>Eoc.</td><td>90.1±7.28</td><td>2.41±0.30</td><td>4,740±266</td><td>1.06±0.06</td><td>0.151±0.007</td></tr><tr><td>07792×0006</td><td>Eoc.</td><td>0.83±0.14</td><td>2.92±0.20</td><td>4,607±336</td><td>1.02±0.06</td><td>0.135±0.005</td></tr><tr><td>08514×0047</td><td>Camp.</td><td>0.61±0.08</td><td>2.54±0.28</td><td>4,207±262</td><td>0.93±0.05</td><td>0.128±0.06</td></tr><tr><td rowspan="2">08272×0136</td><td rowspan="2">Camp.</td><td>5.67±0.40</td><td>2.53±0.23</td><td>4,926±274</td><td>1.13±0.06</td><td>0.157±0.007</td></tr><tr><td>5.38±0.38</td><td>2.42±0.12</td><td>4,757±292</td><td>1.08±0.06</td><td>0.154±0.006</td></tr><tr><td>08507×0013</td><td>Aquit.</td><td>1.19±0.23</td><td>2.69±0.12</td><td>4,451±273</td><td>0.94±0.06</td><td>0.135±0.007</td></tr></tbody></table>
<table><caption>**Table 6** Salinity measurements and atmospheric pressure at the sampling points, and outputs of the Closed System Equilibration Model (Noble90 software)</caption><thead><tr><th>Site</th><th>Aquifer</th><th>Salinity (‰)</th><th>Pressure (atm)</th><th>X<sup>2</sup></th><th>Probability (%)</th><th>NGRT (°C)</th><th>A (cm<sup>3</sup>STP.kg<sup>-1</sup>)</th><th>ΔNe (%)</th></tr></thead><tbody><tr><td>08257×0073</td><td>Olig.</td><td>0.25</td><td>0.999</td><td>0.54</td><td>76.4</td><td>6.8±1.1</td><td>2.8±0.6</td><td>24.6</td></tr><tr><td>08763×0017</td><td>Olig.</td><td>0.55</td><td>0.995</td><td>0.91</td><td>63.5</td><td>6.6±1.0</td><td>2.0±0.6</td><td>17.1</td></tr><tr><td>08262×0096/F</td><td>Olig.</td><td>0.23</td><td>0.994</td><td>1.71</td><td>42.4</td><td>8.7±0.9</td><td>2.4±0.1</td><td>21.2</td></tr><tr><td>07538×0010</td><td>Olig.</td><td>0.64</td><td>0.999</td><td>2.49</td><td>28.8</td><td>8.2±0.9</td><td>1.9±0.6</td><td>17.1</td></tr><tr><td>8276×0091</td><td>Olig.</td><td>0.27</td><td>0.994</td><td>1.44</td><td>48.7</td><td>10.4±1.2</td><td>3.4±0.6</td><td>30.9</td></tr><tr><td rowspan="2">08503×0010</td><td rowspan="2">Olig.</td><td>0.30</td><td>0.997</td><td>2.13</td><td>34.4</td><td>11.9±1.1</td><td>3.6±0.8</td><td>33.2</td></tr><tr><td>0.30</td><td>0.997</td><td>2.80</td><td>24.7</td><td>12.5±1.1</td><td>3.4±0.7</td><td>31.8</td></tr><tr><td rowspan="2">08525×0028</td><td rowspan="2">Olig.</td><td>0.45</td><td>0.991</td><td>0.24</td><td>88.9</td><td>6.5±1.1</td><td>3.2±0.8</td><td>28.3</td></tr><tr><td>0.45</td><td>0.991</td><td>0.57</td><td>75.4</td><td>6.2±1.1</td><td>3.3±0.8</td><td>28.6</td></tr><tr><td>08277×0215</td><td>Eoc.</td><td>0.30</td><td>0.999</td><td>0.65</td><td>72.4</td><td>8.4±1.1</td><td>1.6±0.5</td><td>13.8</td></tr><tr><td>08277×0152</td><td>Eoc.</td><td>0.31</td><td>0.996</td><td>1.14</td><td>56.6</td><td>8.2±1.2</td><td>1.6±0.6</td><td>14.4</td></tr><tr><td>08035×0393</td><td>Eoc.</td><td>0.76</td><td>0.998</td><td>1.41</td><td>49.5</td><td>5.9±1.0</td><td>2.6±1.7</td><td>22.3</td></tr><tr><td>07792×0006</td><td>Eoc.</td><td>0.16</td><td>0.997</td><td>1.36</td><td>50.5</td><td>9.7±1.0</td><td>5.2±1.0</td><td>47.2</td></tr><tr><td>08514×0047</td><td>Camp.</td><td>0.31</td><td>0.995</td><td>0.53</td><td>76.8</td><td>11.1±1.1</td><td>3.4±1.5</td><td>31.2</td></tr><tr><td rowspan="2">08272×0136</td><td rowspan="2">Camp.</td><td>0.30</td><td>0.998</td><td>2.26</td><td>32.3</td><td>4.7±1.0</td><td>2.8±1.2</td><td>23.8</td></tr><tr><td>0.30</td><td>0.998</td><td>1.25</td><td>53.5</td><td>5.3±1.0</td><td>1.8±0.6</td><td>15.6</td></tr><tr><td>08507×0013</td><td>Aquit.</td><td>0.26</td><td>0.995</td><td>0.88</td><td>64.5</td><td>9.9±1.2</td><td>3.8±0.7</td><td>34.1</td></tr></tbody></table>

all the samples have δ¹³C<sub>gas eq</sub> enriched with respect to soil CO₂ showing a closed system or a rapidly closing system with respect to soil CO₂.

Low mineralized water ([SO₄²⁻]<60 mg/L) shows a rapid evolution of δ¹³C with mineralization, from -12.7 to -3.4‰ when sulfate concentration reaches 60 mg/L. Such an evolution is a good argument for a closed system. For mineralized water ([SO₄²⁻]>60 mg/L), δ¹³C<sub>gas eq</sub> is lower than -9.6‰, showing that mineralization was also gained in a closed system after infiltration. It may be noted that water enriched in δ¹³C with respect to soil CO₂, testifying to a closed system or a gradually closing system, was found in most calcareous aquifers. These samples also have low ¹⁴C content.

## ¹⁴C models

To assess the initial ¹⁴C activity A₀, correction models are necessary (Fontes 1992) for water with a ¹³C signature resulting from both limestone matrix and CO₂ dissolution. The term A₀ refers to the initial ¹⁴C content of the inorganic carbon dissolved in groundwater that occurs following recharge and water isolation from the CO₂ reservoir of the unsaturated zone, while A refers to the ¹⁴C activity measured in water. The correction models are based on different mixing processes, i.e. pure chemical and isotopic mixing (Tamers 1975; Ingerson and Pearson 1964) and chemical mixing with isotopic exchange (Evans et al. 1979; Fontes and Garnier 1979; Mook 1980; Salem et al. 1980; Eichinger 1983). The

C end-member introduced in these calculations has the following isotopic contents:

$$^{13}\text{C} = -21\text{‰} \text{ and } A^{14}\text{C} = 100\% \text{ for biogenic CO}_2 \quad (2)$$

$$^{13}\text{C} = 0\text{‰} \text{ and } A^{14}\text{C} = 0 \text{pmc for solid carbonate} \quad (3)$$

The calculated A₀ falls within a wide range of values (Tables 3 and 4). All the models taking into account a chemical and isotopic mixing with isotopic exchange in a dissolved C-soil CO₂-CaCO₃ solid system give similar values. Higher values are provided by Tamers' model (Tamers 1975). These can be rejected for the samples for which the ¹³C contents varies as a function of ¹⁴C activity, suggesting carbon isotopic exchange between TDIC and the limestone matrix to have taken place through time, since such an exchange process is not taken into account in Tamers' approach (Tamers 1975).

Ages from Fontes-Garnier and Ingerson-Pearson models are close to each other, so the two models have been used to estimate the mean age value and the measurement uncertainty. For water with δ¹³C greater than -6.2‰, the Eichinger model is also in good agreement with Fontes-Garnier and Ingerson-Pearson models, so, for these waters, the three models have been used to calculate a mean age value.

The modeled ¹⁴C ages were then converted into calendar ages using the Intcal13 calibration curve (Reimer et al. 2013) and the CalPal software (Weninger and Joris 2004). After calibration, the modeled ages are referred here to as age mod. BP (Before Present, i.e. 1950), to differentiate them from normal calibrated radiocarbon ages (age cal BP).
<figure><img src="image_9.png" /><figcaption>Fig. 5 Hydrogeological cross-section in the North Aquitaine multi-layered aquifer system and residence time distribution</figcaption></figure>

## Noble gases and paleotemperatures estimation

To estimate the reproducibility of noble gas measurements, four water samples were duplicated. For all noble gases, the reproducibility is generally better than 4%, except for one He sample (08503×0010), probably due to a diffusive loss. The concentrations of dissolved noble gases are reported in Table 5.

Noble gas recharge temperatures (NGRT) were estimated by using the Noble90 program (Aeschbach-Hertig et al. 1999; Peeters et al. 2002). This program allows for assessing various models that differ on their estimation of excess air. The input data in these models are the salinity of groundwater, the atmospheric pressure at the recharge area, and the measured concentrations of dissolved noble gases with the exception of He, which has a large non-atmospheric component. Salinities were estimated from the electrical conductivity at 20 °C (κ₂₀) according to the method proposed by Wüest et al. (1996) and the atmospheric pressure is related to the altitude of the recharge area (Aeschbach-Hertig et al. 1999). Electrical conductivities and altitudes of surface water were obtained from the BRGM (French Geological Survey) database (BRGM 2018). However, salinity and pressure have here a minor effect on NGRT results as their values are low in the studied area. The salinity of meteoritic groundwater is very low and estimated recharge heights range from 6 to 75 m.

The closed system equilibration (CE) model leans on the assumption that solubility equilibration is reached in a closed

system of initially air-saturated water and a finite volume of air under hydrostatic pressure, corresponding to air bubbles trapped in the nonsaturated ground (Aeschbach-Hertig et al. 2000). This model, with the hypothesis of a pure air excess, best fits with the dataset. Table 6 presents the output parameters modeled by the Noble90 software such as the NGRT, the amount of entrapped air $A_e$ (cm³ STP of pure air per kg of water), and the excess air expressed by neon oversaturation percentage (ΔNe). The mathematical method is based on the inverse theory: a combination of target parameters (NGRT and air excess for example) is calculated that would produce noble gas concentrations as close as possible to the measured concentrations. The minimum chi-squared value ($\chi^2$) corresponding to the fit reliability between modeled and measured noble gas concentrations, and the probability, are reported in Table 6.

## Discussion

### Residence time distribution

A total of 120 values of residence time from groundwater of five aquifers were calculated from all the data collected in northern Aquitaine (Hosteins 1982; Chery 1993; Chery et al. 1994; Chery and Gadalia 2001; Bonnery et al. 1994; Platel et al. 1999; Schnebelen et al. 2002; Franceschi 2005; Saltel 2008; Malcuit
<figure><img src="image_10.png" /><figcaption>**Fig. 6** δ¹⁸O isotopic signatures of European groundwater during the last 40 ka. a Locations; b Data from previous studies: 1 in East Midlands, UK (Edmunds and Milne 2001); 2 in Hungary (Stute and Deák 1989); 3 in Normandy, France (Barbecot et al. 2000); 4 in Lorraine, France (Celle-</figcaption></figure>

Jeanton et al. 2009); 5 in Valréas, France (Huneau et al. 2001) (1–4 show the recharge gap in grey and 5 shows continuous recharge); c Data from the study area (6) showing continuous recharge

2012). The most recent residence times are logically observed near the outcrop areas. For some aquifers (Oligocene and Miocene) the outcrops are also discharge zones where older groundwater is mixed with modern water (Chery and Gadalia 2001). The oldest residence times were measured in the middle Eocene aquifer along the corridor formed by the Garonne River, which constitutes a natural drainage channel for groundwater.

As surprising as it might seem, groundwater sampled in the Upper Cretaceous multi-layer aquifer, which is located beneath the middle Eocene aquifer, shows lower residence times (Fig. 5). The main factor involved lies in the karstic nature of these reservoirs, which favors strong hydraulic velocities when

compared with the sandy deposits of the middle Eocene aquifer. Besides, the geological structures such as the Villagrains-Landiras Anticline or major faults may also influence groundwater circulation leading locally to lower residence times. Overall, the distribution of residence times in the multi-layer aquifer system does not fit Toth's (1995) theoretical concept in which the residence time increases with depth.

In some areas, recent waters have been found among older waters. This is particularly the case near Bordeaux where several wells reach the middle Eocene aquifer. These anomalies may result from local contamination by recent water following faults or from damaged well casing leading to mixing of groundwater
<figure><img src="image_11.png" /><figcaption>Fig. 7 Noble Gas Recharge Temperatures (NGRT) versus the recharge age of groundwater. NGRIP $\delta^{18}$O is from Rasmussen et al. (2014)</figcaption></figure>

coming from various aquifers. The negative correlation between $^{14}$C and $\delta^{13}$C in groundwater observed from the recharge areas to the drainage areas testifies to an important “dilution” of $^{14}$C caused by the geochemical reactions with carbonates. Carbonate dissolution during infiltration also leads to higher $\delta^{13}$C values. As a consequence, the models that take into account chemical and isotopic mixing with isotopic exchange in a dissolved C-soil CO$_2$-CaCO$_3$ solid system (Ingerson and Pearson 1964; Fontes and Garnier 1979; Evans et al. 1979; Eichinger 1983) were considered to be the most appropriate to calculate $A_0$. The outputs of the different models are in good agreement. The differences obtained in residence time, together with the analytical error of $^{14}$C and $\delta^{13}$C measurements were used to estimate the uncertainty associated with the residence time (Table 3). For the old data, an uncertainty was also calculated when the analytic error was available. Taking into account the uncertainty, the picture clearly emerges from the results that there is no gap in the age distribution, which means that the aquifers were recharged continuously (Fig. 6).

Studies on groundwater recharge across Europe give evidence for a recharge gap for many aquifers, which lasted some millennia to two tens of millennia, mostly between ~30 and 10 ka (Jiráková et al. 2011). These gaps are observed in Northern Europe and are thought to have been induced primarily by permafrost during the Last Glacial. The British-Irish and Scandinavian ice sheets also had a large influence on the investigated catchments in England, Denmark and Estonia. In contrast, the new data suggest that the aquifers in southern France may have been recharged continuously during the last 40 ka (Fig. 6).

France is located in an intermediate area where precise estimation of paleopermafrost extent remains difficult and is still a matter of debate (Andrieux et al. 2016). According to isotopic data, an interruption of recharge is observed in the northern part of France (Normandy and Lorraine), whereas such a gap lacks in the southern part (Aquitaine, Provence). The available data set remains limited, however, making it impossible to bring to light any gap lasting one or a few millennia. The new data provided by this study, thanks to their large number (109), leave no doubt about the absence of recharge gap even during the coldest phases of the glacial. This strongly suggests that permafrost, if present, was not widespread enough to have had any significant impact on aquifer recharge.

In the light of this finding, the lack of speleothem growth shown in Villars Cave, which was interrupted between 31 and 15 ka (Genty et al. 2010), is rather surprising. The factors involved may be (1) the existence of local permafrost due to favorable topographic conditions, although the persistence of frozen ground for a long time during the relatively milder parts of MIS 2 remains hardly plausible, (2) a deficit in atmospheric moisture supply on the Villars plateau that is away from any river-fed aquifer, as suggested both by GCMs reconstructions (Jost et al. 2005; Strandberg et al. 2011) and regional eolian sand extension (Sitzia et al. 2015), and (3) a combination of the two factors.

The new data set for northern Aquitaine (Fig. 6) also clearly shows a contrast between the $\delta^{18}$O of groundwater during the periods ~40–35 ka mod. BP (i.e. Marine Isotopic Stage (MIS) 3), ~35–15 ka mod. BP (late MIS 3 and MIS 2), and ~15–0 ka mod. BP (Lateglacial/Holocene, MIS 1). For the period 35–15 ka mod.
BP, groundwater is generally strongly depleted in $^{18}$O when compared to both older water and Lateglacial/Holocene water. This is particularly true for groundwater sampled in the middle Eocene aquifer, which provides the best record of paleoclimate variations. Darling et al. (1997, 2004) already found that sandstone aquifers tend to be better archives than limestone aquifers because large scale porosity in the latter due to karstification leads to greater mixing of groundwaters. The $\delta^{18}$O of the Holocene samples is similar to that of present-day recharge as shown in Fig. 4 (Millot et al. 2010; Négrel and Petelet-Giraud 2011). Depletion in $\delta^{18}$O and $\delta^{2}$H is stronger for water with low $^{14}$C activity (i.e. yielding an age older than 15 ka mod. BP). This is assumed to reflect mostly temperature lowering during the glacial period.

Greater distance to the source of moisture due to westward retreat of the coastline during the LGM sea level lowstand (ca. -120 m) may also have been responsible to some extent for depletion due to isotopic fractionation between water vapor and liquid precipitation. Since the distance did not exceed 100 km, this continental effect is assumed to have been insignificant. Similarly, the altitude effect (ca. 0.28‰/100 m for $\delta^{18}$O, Poage and Chamberlain 2001) cannot explain the observed depletion range. It can be noticed that such a contrast between Holocene and late Pleistocene groundwater was also clearly observed in the UK (East Midlands) and in eastern France (Lorraine) as well, but is not as clearly identifiable in other European aquifers (Fig. 6).

## Paleotemperatures

Associating noble gases recharge temperature (NGRT) with groundwater residence times makes it possible to reconstruct paleotemperatures since 40 ka (Fig. 7). According to the data, the mean NGRT during the Holocene is 11.3 ± 1.2 °C. This value is in good agreement with the mean modern temperature of near surface aquifers in southwest France, which was estimated by Blavoux et al. (1993) to range between 11–13 °C. Similarly, the study carried out by Huneau (2000) in southeast France gave a mean modern temperature of 12.7 °C.

Over the period 25–18 mod. ka BP (MIS 2), the NGRTs estimated in this work are quite homogeneous, and lead to a mean temperature equal to 5.9 ± 0.9 °C. Assuming that the present temperature of near-surface aquifers is 11.3 °C, the temperature decrease during the coldest period of MIS 2 relative to present is estimated at 5.4 °C. This variation is coherent with previous NGRT studies made in Europe, i.e. 5–7 °C in southwest France (Blavoux et al. 1993), 7 °C in southeast France (Huneau 2000), 3–9 °C in northeast France (Kloppmann et al. 1998), 4–7 °C in England (Bath et al. 1996), 5–7 °C in Germany (Rudolph et al. 1984; Bertleff et al. 1993), 6 °C in Austria (Andrews et al. 1985), >5 °C in Switzerland (Beyerle et al. 1998) and 5–7 °C in the Czech Republic (Corcho Alvarado et al. 2011). Blaser et al. (2010) and Stute and Deák (1989) observed a more pronounced

temperature drop respectively in Belgium (>9.5 °C) and Hungary (9 °C).

For the MIS 3 samples (ages >27 mod. ka BP), the NGRT rises by approximately 2.6 °C. According to the Greenland ice core isotopic record (Rasmussen et al. 2014), the latest part of MIS 3 is typified by rapid climate variation, with an alternation of short (a few hundred years long) warm interstadials and longer (in order of a millennium) cold stadials. Blavoux et al. (1993) in southwest France and Blaser et al. (2010) in Belgium also observed a similar NGRT evolution through the last 40 ka, showing a minimum during MIS 2 (in the Belgian record, just before and after a gap between 21 and 14 ka), followed by a rapid increase during the Lateglacial.

The mean reconstructed NGRT for the coldest period of MIS 2 (5.9 ± 0.9 °C) indicate that near-surface ground thermal conditions were significantly over 0 °C and, therefore, not adequate for long-term permafrost development in southwest France, as already suggested by the lack of recharge gap in the aquifers. The lowest NGRT value found in Leognan in southwest France (5.0 ± 0.4 °C) at 23.4 ± 0.8 ka mod. BP, also suggests that permafrost, even in favorable contexts (sporadic permafrost), was unlikely to occur during the coldest phases.

This finding is obviously in contradiction with the data provided by the analysis of periglacial features, which shows that ground ice bodies (i.e. composite wedges) were locally able to grow in northern Aquitaine (Andrieux et al. 2016). It cannot be excluded that smoothing of the temperature signal by the processes that took place within the aquifers, such as gas diffusion or water mixing, has filtered the high-frequency climate variability, so that the “cold phases” as recorded by noble gases are actually not representative of the coldest events, which may have lasted no more than a few hundred years. A similar “warm bias” seems also to emerge from the work of Blaser et al. (2010) in northern Belgium. The coldest NGRT found in this region is 0.7 ± 0.3 °C at ~24 ka, which is well above expected temperatures for widespread permafrost as reconstructed by Vandenberghe et al. (2014) and Andrieux et al. (2016). In current arctic environments, the mean annual ground temperature at the level of zero annual amplitude (a few meters below the ground surface) is typically lower than -2 °C (usually between -2 °C and -5 °C) in the zone of widespread permafrost in northern Alaska and Canada (Smith et al. 2010), which translates into much colder mean annual air temperatures (MAATs) due to the thermal effect of vegetation and snow, and to the physical properties of the ground. Another plausible bias that may explain the discrepancies between reconstructed NGRTs and the temperature estimates derived from field proxies (periglacial features) is a decoupling between NGRTs and MAATs in cold environments. According to Singleton and Moran (2010) and Utting et al. (2012), NGRTs are strongly influenced by the temperature at the time of snow and ground ice melt, which may be significantly higher than the average yearly value in periglacial regions. In a permafrost environment from Canada, Utting et al. (2012) actually showed that NGRTs ranged
between 0 and 5 °C, whereas the MAAT was ca. -8.9 °C. The studied aquifer was of limited extent, however, and such a large bias should not have affected the northern Aquitaine basin, which encompasses large recharge areas. Better evaluation of this bias in northern Aquitaine would need further investigations.

## Conclusion

The new water samples retrieved from multi-layer aquifers in northern Aquitaine, southwest France, in addition to previously published data, provide the basis for the determination of residence times and the reconstruction of recharge temperatures through ¹⁴C dating, analysis of water stable isotopes (δ¹⁸O and δ²H), and the measurement of dissolved noble gases (He, Ne, Ar, Kr, Xe). In total 120 samples have been used to obtain modeled ages, 138 for δ¹⁸O and δ²H measurement, and 14 for noble gases.

The groundwater samples coming from the middle Eocene aquifer, which lies above the Upper Cretaceous multi-layer aquifer, yielded larger residence times. This anomaly is assumed to result from the complexity of the systems, namely the heterogeneity in lithology and permeability, and the geological structures. The data on residence time do not show any recharge gap of the aquifers for the last 40 ka. Late Pleistocene water and Holocene water clearly exhibit contrasted ¹⁸O content, the former being significantly depleted when compared to present-day recharge.

During the coldest periods of MIS 2 (27 to -18 ka mod. BP), the mean near-surface ground temperature estimated from NGRT was 5.9 ± 0.9 °C, implying that stable permafrost was unable to develop in the study region, which is consistent with the lack of recharge gap shown by the distribution of water ages. The lowest temperature recorded, close to 5.0 °C at 23.4 ± 0.8 ka mod. BP, strongly suggests that during the coldest phases of MIS 2, sporadic permafrost was also unlikely to occur even in favorable contexts. This finding does not agree with the data provided by the analysis of periglacial features such as thermal contraction cracks, which showed that ground ice bodies formed locally in northern Aquitaine. A plausible explanation lies in the smoothing of the paleoclimate signal in the aquifers due to diffusion and water mixing, making it impossible to detect short (i.e. a few hundred years) events that had nevertheless a significant impact on the landscape. Another possible bias lies in the fact that NGRTs in cold regions do not reflect annual means but rather the ground temperature during the thaw months. Further investigation on current NGRTs of large aquifers in a permafrost context could provide useful data to better estimate this bias. Holocene groundwaters indicate a mean NGRT of 11.3 ± 1.1 °C, consistent with the mean modern temperature of near-surface aquifers in south France.

**Acknowledgements** The sampling campaign has been realized with the help of the Conseil départemental de la Gironde and SUEZ Environment who gave us access to their wells.

**Funding information** This work was financed by Adour-Garonne Water Agency and OASU (Observatoire Aquitain des Sciences de l'Univers) through the Special Program "Organics and Inorganics Contaminants".

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