GEOCHEMICAL EVIDENCES FROM FOSSIL AQUIFERS OF CENTRAL ARABIA FOR COOLER ENVIRONMENTS DURING AQUIFER RECHARGE

Like many arid regions of the world, the Kingdom of Saudi Arabia (KSA) largely depends on its groundwater resources to sustain its population, and its agricultural and industrial sectors. The primary aquifers within the KSA were largely recharged during previous humid climatic periods throughout the late Pleistocene. Recharge temperatures extracted from noble gas concentrations, stable isotope compositions (O, H), and Cl-36 model ages for groundwater samples from the Late Triassic Minjur aquifer in the Riyadh area and its surroundings provide evidences for humid and cool climatic phases in central Arabian Peninsula during the late Pleistocene. During these humid periods soil temperature was cooler than present by up to 6 ℃ and the isotopic composition of paleo-precipitation now residing in the Minjur Aquifer were depleted (δ²H: -28.1 to -35.4 ‰) compared to modern precipitation over the Minjur Aquifer recharge areas (δ²H: -10 to 40 ‰, weighted mean: 17 ‰). The depleted isotopic compositions and the higher deuterium excess values are consistent with a Mediterranean moisture source and a prevalent cooler climate during aquifer recharge. Cl-36 model ages increase progressively along groundwater flow direction reaching up to 510,000 years some 150 kms from the recharge areas, indicating aquifer recharge during late Pleistocene periods. We hypothesize that the Minjur Aquifer was recharged during humid glacial periods by intensified north westerly wind regimes. Findings provide insights into the nature of climatic conditions that prevailed during the humid late Pleistocene period in the central parts of the Arabian Peninsula where other climatic proxies are scarce.