Exploring the 36Cl/Cl input in arid environments: New insights gained by 81Kr groundwater dating in the Negev Desert, Israel

   ³⁶Cl and ⁸¹Kr (half-lives of 301 and 229 kyr, respectively) are among a very few age tracers with dating capabilities in the 10⁴–10⁶ yr timescale. Although widely applied since the 1980s in various hydrological studies, the ³⁶Cl/Cl system has been found complex as an effective dating tool. In contrast, ⁸¹Kr has become a practical tool only recently and is considered to be an ideal dating tool due to the inert properties of the noble gas. In the present study, simultaneous measurements of both radioisotopes were used to assess the ³⁶Cl/Cl input ratios and the Cl^- content for paleorecharge into the deep, transboundary Nubian Sandstone Aquifer (NSA) which stretches below the hyperarid deserts of the Sinai Peninsula (Egypt) and the Negev (Israel).

   By means of ⁸¹Kr data, reconstructed Cl^- content of recharge that occurred during the late Pleistocene was found to be 300–400 mg/L with an initial ³⁶Cl/Cl ratio of 50 × 10^-15. This latter value is in agreement with the ³⁶Cl/Cl ratio in recent local rainwater, indicating constancy over prolonged periods with possible variable climatic conditions. This similarity in values suggests a process that is rather insensitive to atmospheric ³⁶Cl fallout rates. Erosion and weathering of near-surface materials in the desert environment could dominate the hydrochemistry of rains, floods, and the consequent groundwater recharge. This near-surface Cl^- reservoir integrates various sources and processes, including marine and terrestrial Cl^-, cosmogenic ³⁶Cl fallout, and cosmogenic ³⁶Cl production in the shallow unsaturated zone, all of which are active over long timescales and accumulate on the land surface and in the epigene zone.  Spatial differences in the reconstructed initial ³⁶Cl/Cl ratio are attributed to differences in the mineral aerosol sources for specific recharge areas of the NSA. The results of this study highlight the potential of integrating ⁸¹Kr age information in evaluating the initial ³⁶Cl/Cl and Cl^- input, which is essential for the calibration of ³⁶Cl radioisotope as a long-term dating tool for a given basin.