Analyses for 81 Kr and noble gases on groundwater from the deepest aquifer system of the Baltic Artesian Basin (BAB) were performed to determine groundwater ages and uncover the flow dynamics of the system on a timescale of several hundred thousand years. We find that the system is controlled by mixing of three distinct water masses: Interglacial or recent meteoric water (δ 18 O ≈ −10.4‰) with a poorly evolved chemical and noble gas signature, glacial meltwater (δ 18 O ≤ −18 ‰) with elevated noble gas concentrations, and an old, high-salinity brine component (δ 18 O ≥ −4.5‰, ≥ 90 g Cl − /L) with strongly depleted atmospheric noble gas concentrations. The 81 Kr measurements are interpreted within this mixing framework to estimate the age of the endmembers. Deconvoluted 81 Kr ages range from 300 ka to 1.3 Ma for interglacial or recent meteoric water and glacial meltwater. For the brine component, ages exceed the dating range of the ATTA-3 instrument of 1.3 Ma. The radiogenic noble gas components 4 He* and 40 Ar* are less conclusive butalso support an age of > 1 Ma for the brine. Based on the chemical and noble gas concentrations and the dating results, we conclude that the brine originates from evaporated seawater that has been modified by later water-rock interaction. As the obtained tracer ages cover several glacial cycles, we discuss the impact of the glacial cycles on flow patterns in the studied aquifer system. Virbulis et al., 2013), the latter of which estimated the hydraulic age of groundwater in the CAS to be on the order of several hundreds of ka to 1 Ma. In the light of such long residence times, it is crucial to consider the effect of repeated glacial cycles on the long-term evolution of groundwater composition and flow. Sampling the deeper parts of the CAS on a regional scale for chemistry, noble gases, and multiple dating tracers ( 81 Kr, 85 Kr, 39 Ar, 14 C, 4 He, 40 Ar) allows us to elucidate the evolution of the brine, mixing proportions of the different groundwater components, and the flow dynamics over the last 1 Ma.

In arid regions, groundwater is a vital resource that can also provide a long-term record of the regional water cycle. However, the use of groundwater as a paleoclimate proxy has been limited by the complex hydrology and the lack of appropriate chronometers to determine the recharge time without complication. Applying 81Kr, a long-lived radioisotope tracer, we investigate the paleohydroclimate and subsurface water storage properties of the Nubian Sandstone Aquifer in the Negev Desert, Israel. Based on the spatial distributions of stable isotopes and the abundance of 81Kr, we resolve subsurface mixing and identify two distinct moisture sources of the recharge: one recent (<38 ky ago) from the Mediterranean and the other 361 ± 30 ky ago from the tropical Atlantic, both of which occurred under conditions of low orbital eccentricity comparable to that of the present. The recent recharge provided by the moisture from Mediterranean cyclones can be attributed to the southward shift of the storm track during the Last Glacial Maximum, and the earlier recharge can be attributed to moisture from the Atlantic delivered as tropical plumes under a climate colder than the present. Furthermore, the residence time of the latter reveals that tectonically active terrain can store groundwater for an unexpectedly long period, likely due to strongly attenuated groundwater flow across the fault zones. With this tracer, groundwater can now serve as a direct record of paleoprecipitation over land and of subsurface water storage from the mid-Pleistocene and onward.