Radiokrypton dating finally takes off

Deep desert aquifers as an archive for Mid- to Late Pleistocene hydroclimate: An example from the southeastern Mediterranean

Many efforts have been made to illuminate the nature of past hydroclimates in semi-arid and arid regions, where current and future shifts in water availability have enormous consequences on human subsistence. Deep desert aquifers, where groundwater is stored for prolonged periods, might serve as a direct record of major paleo-recharge events. To date, groundwater-based paleoclimate reconstructions have mainly focused on a relatively narrow timescale (up to ∼40 kyr), limited by the relatively short half-life of the widely used radiocarbon (5.73 kyr). Here we demonstrate the usage of deep regional aquifers in the arid southeastern Mediterranean as a hydroclimate archive for earlier Mid-to-Late Pleistocene epochs. State-of-the-art dating tools, primarily the 81Kr radioisotope (t1/2 = 229 kyr), were combined with other atmosphere-derived tracers to illuminate the impact of four distinguishable wetter episodes over the past 400 kyr, with differences in climatic conditions and paleo-recharge locations. Variations in stable water isotope composition suggest moisture transport from more proximal (Mediterranean) and distal (Atlantic) sources to different parts of the region at distinct times. Large variability in the computed noble gas-based recharge temperature (NGT), ranging ~15-30 °C, cannot be explained by climate variations solely, and points to different recharge pathways, including geothermal heating in the deep unsaturated zone and recharge from high-elevation (colder) regions. The obtained groundwater record complements and enhances the interpretation of other terrestrial archives in the arid region, including a contribution of valuable information regarding the moisture source origin as reflected in the deuterium-excess values, which is unattainable from the common practice analysis of calcitic cave deposits. We conclude that similar applications in other deep (hundred-m-order) regional groundwater systems (e.g., the Sahara desert aquifers) can significantly advance our understanding of long-term (up to 1 Myr) paleo-hydroclimate in arid regions, including places where no terrestrial remnants, such as cave, lake, and spring sediments, are available.

Feasibility of 129I groundwater dating calibrated by both 81Kr and 4He for the assessment of deep geological repositories in Japan

Iodine-129, which is a promising tracer for dating old groundwater, has been used as a tracer for deep upwelling groundwater. The nuclide is expected to be one of the key factors for site selection for high-level radioactive waste disposal, which is a global societal issue. The pre-anthropogenic 129I/127I ratio for marine iodine is (1.50 ± 0.15) × 10-12, which could be considered the initial value for 129I dating. This study identifies the challenges in groundwater age dating using 129I/127I. We measured the ratios of 129I/127I and 81Kr/Kr and concentration of 4He in groundwater from boreholes on the northern coast of Japan. The 129I dating results were not coincident with the other groundwater dating results. The iodine in the groundwater was inferred to be released in situ from marine organisms in sediments of various ages. We estimated that the primordial iodine ratio originating from seawater was ~ 1 × 10-13 (8 × 10-14 ~ 2 × 10-13). The groundwater age deduced from the 129I/127I ratio using this value agrees with other groundwater dating results.

Developments of a field gas extraction device and krypton purification system for groundwater radio-krypton dating at the IAEA

The long-lived radio-krypton isotope ⁸¹Kr (t_1/2 = 2.29 × 10⁵ yr) is an ideal tracer for old groundwater age dating in the range of 10⁵-10⁶ years which goes beyond the reach of radio-carbon (¹⁴C) age dating. Analytical breakthrough made over the last two decades in Atom Trap Trace Analysis (ATTA) has enabled the use of this isotope with extremely low abundance (⁸¹Kr/Kr = 6 × 10^-13) to be used as a practical dating tool for very old groundwater. The International Atomic Energy Agency aims to provide this new isotope tool for better groundwater resource management of Member States and developed a field sampling device to collect dissolved gas samples from groundwater and a system to separate and purify trace amounts of krypton from the gas samples for the ATTA analysis. The design, setup and performances of our sampling and purification systems are described here. Our system can produce a high purity aliquot of about 5 μL of krypton from 5 L of air sample (recovery yield of >90%). The samples made by our system were confirmed to be acceptable for the ATTA analysis.

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.