Radiokrypton unveils dual moisture sources of a deep desert aquifer
Abstract
Understanding the past water availability during colder and warmer periods provides clues on how regional water cycles respond to projected climate change. In this context, direct records of paleo-precipitation are highly valuable. In arid regions where the subsurface infiltration of rainwater is intermittent, the timing of groundwater recharge corresponds directly to regional humid periods. Furthermore, the recharged water from humid periods constitutes a valuable water resource in such regions. However, the use of groundwater as paleoclimate proxy beyond Last Glacial Maximum (LGM) has been hampered by the complex hydrology and the lack of appropriate chronometers to determine the recharge time without complication. Recent advances in Atom Trap Trace Analyses and associated sample preparation methods enabled high throughput of 81Kr analyses. Applying the novel tracer, we investigated the paleo-hydroclimate and subsurface water storage properties of the Nubian Sandstone Aquifer in the Negev Desert, Israel.
The 81Kr abundance from 22 production wells in the eastern Negev Desert d id not show a systematic trend of increasing groundwater age from the putative recharge area in the southern Sinai to the discharge site south of the Dead Sea, but stable isotopes and the abundance of 81Kr were clearly correlated. Based on this correlation and the spatial distribution of the data, we resolved subsurface mixing and identified two distinct moisture sources and periods of the recharge. A recent ( <38 ky ago) recharge occurred from the small outcrops in the Negev, and the other about 360 ky ago recharged from the outcrops in southern Sinai. The recent recharge provided by the moisture from Mediterranean cyclones can be attributed to the southward shift of the storm track during the LGM, 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 81Kr, groundwater can now serve as a direct record of paleo-precipitation over land and of subsurface water storage from the mid-Pleistocene and onward.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.H43C..01Y
- Keywords:
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- 1804 Catchment;
- HYDROLOGY;
- 1829 Groundwater hydrology;
- HYDROLOGY;
- 1871 Surface water quality;
- HYDROLOGY;
- 1886 Weathering;
- HYDROLOGY