| Popis: |
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. |
