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

Measurements of the long-lived ⁸¹ Kr and ³⁶ Cl radioisotopes in groundwater from the Negev Desert (Israel) were used to assess the ³⁶ Cl/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 ³⁶ Cl/Cl ratio of 50 × 10 ^-15 was assessed for the groundwater replenishment in the Negev Desert since the late Pleistocene, in agreement with the ³⁶ Cl/Cl ratios in recent local rainwater. This is despite possible changes in the climatic conditions and the ³⁶ Cl 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 ³⁶ Cl/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 ³⁶ Cl/Cl input in groundwater reservoirs surrounding the Mediterranean Sea indicate various processes that govern the ³⁶ Cl/Cl system. Finally, the results of this study highlight the great advantage of integrating ⁸¹ Kr age information in evaluating the initial ³⁶ Cl/Cl and Cl ^- input, which is essential for the calibration of ³⁶ Cl radioisotope as an available long-term dating tool for a given basin.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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