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

Deep groundwater samples from the Continental Intercalaire (CI) aquifer in the Northern Tunisian Sahara have been analyzed for noble gases He-3, He-4, Ne and Kr-81, and for C-14 to better constrain the groundwater residence time of this large transboundary aquifer. Its significant radiogenic He-4 content and background-level C-14 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. Kr-81 is applied for the first time on the CI aquifer, providing more direct evidence for the presence of old groundwater with Kr-81/ Kr ratios at 63% - 15% of the atmospheric value, corresponding to residence times of 150 - 630 ka. These newly obtained Kr-81 ages enable a re-evaluation of the previously reported data from the Tozeur region in order to better constrain the input values of Cl-36/Cl and Cl. The initial Cl-36/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 Kr-81 dating and the recalibrated Cl-36 dating define an eastward age progression across >500 km of flow path from the Algerian to Tunisian sections. This study exemplifies the utility of Kr-81 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 Cl-36 and radiogenic He-4. (C) 2020 Elsevier B.V. All rights reserved.