Controls on the ³⁶Cl/Cl input ratio of paleo-groundwater in arid environments: New evidence from ⁸¹Kr/Kr data

Measurements of the long-lived ⁸¹Kr and ³⁶Cl radioisotopes in groundwater from the Negev Desert (Israel) were used to assess the ³⁶Cl/Cl input ratios and Cl^- contents for paleorecharge into the Nubian Sandstone Aquifer (NSA). The reconstructed Cl^- content of the recharge flux was on the order of 300-400 mg/L. An initial ³⁶Cl/Cl ratio of 50 × 10^-15 was assessed for the groundwater replenishment in the Negev Desert since the late Pleistocene, in agreement with the ³⁶Cl/Cl ratios in recent local rainwater. This is despite possible changes in the climatic conditions and the ³⁶Cl production rates in the atmosphere over this timeframe. This similarity in values is explained by the major role played by the erosion and weathering of near-surface materials in the desert environment that dominate the hydrochemistry of rains, floods, and the consequent groundwater recharge. Spatial variation in the reconstructed initial ³⁶Cl/Cl ratio is accounted for by the differences in the mineral aerosol sources for specific recharge areas of the NSA. Accordingly, regional variations in the ³⁶Cl/Cl input in groundwater reservoirs surrounding the Mediterranean Sea indicate various processes that govern the ³⁶Cl/Cl system. Finally, the results of this study highlight the great advantage of integrating ⁸¹Kr age information in evaluating the initial ³⁶Cl/Cl and Cl^- input, which is essential for the calibration of ³⁶Cl radioisotope as an available long-term dating tool for a given basin.