A NOBLE GAS PALAEOTEMPERATURE RECORD FROM THE LEDO-PANISELIAN AQUIFER IN BELGIUM W. Aeschbach-Hertig 1, 2 , R. Kipfer 2 , P. C. Blaser 3 , K. Walraevens 4 1 Institute of Environmental Physics, University of Heidelberg, D-69120 Heidelberg, Germany 2 Environmental Isotope Group, W+T, EAWAG and Isotope geology, ETHZ, CH-8600 Dübendorf, Switzerland 3 Climate and Environmental Physics, Physics Institute, University of Bern, CH-3012 Bern, Switzerland 4 Applied Geology and Hydrogeology, Ghent University, B-9000 Gent, Belgium Contact: aeschbach@iup.uni-heidelberg.de The Eocene Ledo-Paniselian Aquifer (LPA) is a sandy, confined coastal aquifer, extending from Flanders in north-western Belgium to the Dutch province of Zealand. The study area is located in Flanders NW of Ghent. The natural hydraulic gradient follows dip from south to north. Flow modelling shows substantial changes in piezometric levels due to exploitation (Walraevens, 1988). 44 samples from 39 wells taken between 1997 and 2001 were analysed for hydrochemistry, radio- carbon, stable isotopes, and noble gases. The LPA is part of an alternating sequen- ce of marine Tertiary clay and sand depo- sits. Recharge takes place in topographi- cally higher regions through the semi- confining cover of Bartonian Clay. Some discharge occurs in the southern outcrop area. In the confined part of the aquifer the main flow direction is northwards. Upward outflow through the Bartonian Clay causes flow velocities to diminish gradually. EAE03-A-14209 Environmental Isotopes Group Dept. of Water Resources and Drinking Wat er Rare Gas Group Inst. of Isotope Geology and Mineral Resources Institute of Environmental Physics University of Heidelberg University of Bern 1. Study Area 6. Conclusions  The NGT-record indicates a glacial cooling of at least 8.5 °C for Belgium  Detailed hydrochemical and isotopic modeling yields reliable 14 C ages  Radiogenic 4 He, mostly non-locally produced, correlates with 14 C ages  Chloride concentrations record a climatic signal (sea-level changes)  The CE-model describes all noble gas data, even degassed samples References Aeschbach-Hertig et al. (2000). Palaeotemperature reconstruction from noble gases in ground water taking into account equilibration with entrapped air. Nature 405, 1040-1044. Blaser (2003). Tracermethoden in der Hydrologie – Kombination verschiedener Methoden und Anwendungen in Aquifersystemen in Belgien und Estland. Diss. Univ. Bern. Leuenberger et al. (1992). Carbon Isotope Composition of Atmospheric CO 2 during the Last Ice Age from an Antarctic Ice Core. Nature 357, 488-490. Marino et al. (1992). Glacial-to-interglacial variations in carbon isotopic composition of atmospheric CO 2 . Nature 357, 461-466. Van der Kemp et al. (2000). Inverse chemical modeling and radiocarbon dating of palaeogroundwaters: The Tertiary Ledo-Paniselian aquifer in Flanders, Belgium. Water Resour. Res. 36(5), 1277-1287. Walraevens (1998). Natural isotopes and noble gases in groundwater of the Tertiary Ledo-Paniselian aquifer in East and West Flanders. Natuurwet. Tijdschr. 78, 246-260. A Ghent Bruges THE NETHERLANDS (Zealand) BELGIUM (Flanders) 10 km 51°13’N 51°02’N 3°42’E Ursel NORTH SEA A’ Western Scheldt Dutch - Belgian frontier Profile A-A’ calculated hydraulic head [m] in natural state (before 1920) recharge area 10.0 2°59’E Eeklo 3.0 5.0 10.0 m TAW SSW NNE 10 km 0 -100 -200 TAW: Belgian reference level (low spring tide ~ -2.3 m asl) ~ Quaternary Oligocene Sands Bartonian Clay Boom Clay Ledo-Paniselian Sands Ypresian Sands Paniselian Clay Ypresian Clay A A’ Spreadsheet, NETPATH and PHREEQC calculations were performed to account for chemical reactions and isotope exchange. The initial δ 13 C and 14 C values of soil CO 2 and dissolved CO 2 at the time of groundwater recharge were calculated for distinct climatic scenarios, according to van der Kemp et al. (2000), but additionally taking temperature effects into account. Log pCO 2 was varied from -1.5 (Holocene) to -3.5 (glacial), temperature from 10 °C (Holocene) to 2 °C (glacial), and the atmospheric CO 2 and δ 13 C history was taken from Leuenberger et al. (1992) and Marino et al. (1992). These adapted input values were then inserted in the usual 14 C-correction models (e.g., Fontes & Garnier). Details are described in Blaser (2003). The concentrations of radio- genic 4 He correlate well with the 14 C model ages, suppor- ting the corrections applied for dating. In or near the recharge area, the He accumulation rate is low and close to the expected rate due to in situ He production. Further downstream, He accumulation is about an order of magnitude stronger. 3. Radiocarbon dating and He accumulation The marine cations Na + , K + and Mg 2+ were replaced by Ca 2+ in the order of increasing affinity to the clay. This cation exchange induced a pronounced chromatographic pattern upflow of the fresh/salt-water interface, resulting in a typical sequence of NaCl, NaHCO 3 , MgHCO 3 and CaHCO 3 watertypes. Ledo-Paniselian Aquifer 0.01 0.10 1.00 10.00 100.00 0 5 10 15 20 25 30 Distance from recharge [km] Concentration [meq/L] Na + Cl - HCO3 - SO4 2- K + Ca 2+ Mg 2+ The marine influence is still visible in the downstream part of the investigated flow line, characterised by high concentrations of Cl - , Na + , and TDS. The noble gases also reveal a zonation of the aquifer. Most of the wells in the recharge area show low He accumu- lation. Degassed samples occur mainly in a narrow band further downstream and in the southern outcrop. 2. Hydrochemical evolution Inverse modeling of the observed noble gas concentrations was used to interpret the data in terms of recharge temperature and excess air. The closed-system equilibration (CE) model was used to describe the excess air component (Aeschbach-Hertig et al., 2000). The model assumes equilibration between groundwater and gas bubbles. ( ) ( ) * * 1 1 , , i i i i i C FAz Az F P S T C C + − + = values of the fractionation para- meter F < 1 describe excess air, whereas F > 1 corresponds to degassing by equilibration with a secondary gas phase (e.g. CH 4 ). The parameter A describes the initial concentration of trapped air. 4. Interpretation of noble gas concentrations The noble gas temperature (NGT) record from the LPA indicates strong glacial cooling. Holocene samples yield NGTs of 8 to 10 °C, in agreement with modern air temperatures. At 14 C-ages of about 20 kyr, NGTs near the freezing point (< 2 °C) occur. Cl- rich samples older than 30 kyr show intermediate NGTs. A certain gap exists between the latter two groups of samples. The stable isotope ratios show little climate sensitivi- ty in the LPA. δ 18 O correla- tes only weakly with NGTs. In contrast, Cl - concentra- tions exhibit a significant correlation with NGTs (excluding the downstream region with elevated Cl - ). In coastal aquifers, Cl - records changes in sea-salt input due to sea-level variations. 5. Paleoclimate indicators 0 2 4 6 8 10 12 0 10000 20000 30000 40000 normal degassed high Cl, TDS NGT [°C] 14 C model age [yr] age gap? ∆T = 8.5 °C 0 0.2 0.4 0.6 0.8 1 1.2 1.4 -7.5 -7 -6.5 -6 -5.5 -5 0 2 4 6 8 10 12 Cl - normal Cl - degassed δ 18 O normal δ 18 O degassed Cl - [meq l -1 ] δ 18 O [‰] NGT [°C] normal low He degassed high Cl main flow line recharge area N 0.2 0.4 0.6 0.8 1 1.2 1.4 Ne Ar Kr Xe concentration relative to equilibrium Noble Gas A = 0.001, F = 0 A = 0.04, F = 0.7 A = 0.1, F = 0.9 A = 0.1, F = 1.2 A = 0.1, F = 1.8 A = 0.1, F = 3.5 symbols: data lines: model excess air degassing The LPA forms a model example of a freshening aquifer, where the initial marine pore- water is flushed by meteoric recharge. The same model can also descri- be the phenomenon of degassing, as shown here for some samples. In the model equation The absence of samples with ages between 23 and 29 kyr may be due to inhibition of groundwater recharge by permafrost conditions during the Last Glacial Maximum. 0 1 10 -6 2 10 -6 3 10 -6 4 10 -6 5 10 -6 0 10000 20000 30000 40000 He rad [cm 3 STP g -1 ] 14 C model age [yr] Observed He accumulation rate: 1 . 10 -10 cm 3 STP g -1 yr -1 Estimated in situ accum. rate: 5 . 10 -12 cm 3 STP g -1 yr -1 View publication stats View publication stats