Dissolved noble gases are ideal tracers for deconvoluting hydroclimatic signals preserved in groundwater. First, radioactive and radiogenic noble gas isotopes serve as proxies for groundwater residence time, and second, stable noble gas abundances and isotope ratios are sensitive indicators for past temperatures and water-table fluctuations [e.g., 1–3]. In this talk, I will present the application of state-of-the-art noble gas isotope measurement techniques for reconstructing the hydroclimatic signal preserved in the deep Nubian Sandstone Aquifer (NSA) of the Sinai (Egypt) and Negev (Israel) deserts.

Application of the long-lived ⁸¹Kr radioisotope (t_1/2=229 kyr) revealed a wide age range of 40 to 600 kyr in the aquifer. Two major paleo-recharge episodes were reconstructed by combining ⁸¹Kr and water stable isotope (d¹⁸O and d²H) data, each with a distinct moisture source. Furthermore, extremely high amounts of dissolved noble gases (excess Ne of ~300%) hint at large-scale (tens of m) water-table rise during past pluvial epochs. We attempted to reconstruct paleotemperatures based on Ne, Ar, Kr, and Xe contents, and discovered noble gas recharge temperatures (NGTs) up to 10 °C higher than the current mean annual surface temperature (MAST) over the recharge area. The NGT–MAST discrepancy is even more significant if considering recharge during glacial epochs [4]. In theory, the ultimate gas entrapment and equilibration take place at the unsaturated zone and the water-table interface. A hundreds-of-m-deep water table characterizes the NSA, and therefore, this gap could be at least partially explained by a geothermal heating effect. In the last part of the talk, I will present the project's current stage, in which we attempt to apply ultra-high precision Kr and Xe stable isotope measurements [3] to reconstruct paleo- water-table depths. In theory, gravitational settling fractionates noble gases, increasing heavy-to-light isotope ratios with unsaturated zone depth. With this complete suite of tracers, including noble gas bulk contents, stable isotope ratios, and radioisotope activities, we aim to further illuminate the NSA’s paleo-hydroclimatic archive.

[1] Lu et al. (2014), Earth Sci. Rev. 138, 196–214.

[2] Aeschbach-Hertig et al. (2000), Nature 405, 1040–1044.

[3] Seltzer et al. (2019), Earth Planet. Sci. Lett. 514, 156–165.

[4] Yokochi et al. (2019), Proc. Natl. Acad. Sci. 116, 16222–16228.