Krypton-81 dating of the deep Continental Intercalaire aquifer with implications for chlorine-36 dating

Abstract

Deep groundwater samples from the Continental Intercalaire (CI) aquifer in the Northern Tunisian Sahara have been analyzed for noble gases 3He, 4He, Ne and 81Kr, and for 14C to better constrain the groundwater residence time of this large transboundary aquifer. Its significant radiogenic 4He content and background-level 14C both indicate water older than a few tens of thousands of years. Distinct helium concentrations and Ne/He ratios suggest different groundwater flow paths through the Tozeur and Kebili regions. 81Kr is applied for the first time on the CI aquifer, providing more direct evidence for the presence of old groundwater with 81Kr/Kr ratios at 63% – 15% of the atmospheric value, corresponding to residence times of 150 – 630 ka. These newly obtained 81Kr ages enable a re-evaluation of the previously reported data from the Tozeur region in order to better constrain the input values of 36Cl/Cl and Cl. The initial 36Cl/Cl ratio is found to be 4 – 6 times larger than previously assumed, and the initial Cl concentration 6 – 15 times smaller. The results of 81Kr dating and the recalibrated 36Cl dating define an eastward age progression across >500 km of flow path from the Algerian to Tunisian sections. This study exemplifies the utility of 81Kr not only as an independent and conservative chronometer, but also as a supplemental measurement to better calibrate the parameters required for other age tracers, including cosmogenic 36Cl and radiogenic 4He.