REE and Y distribution and speciation in fossil water: The northwestern African Continental Intercalaire aquifer, Algerian Sahara

The Continental Intercalaire aquifer is a part of the Northwestern Sahara Aquifer System which, extending through Algeria, Tunisia and Libya with over one million km2, is one of the largest fossil aquifers in the world, with an age of about 775,000 years, water resource estimated at ~31000×109 m3 and negligible recharge rate at 1×109 m3/year. The fossil water of the Algerian Sahara, part of the Continental Intercalaire aquifer, was examined, for the first time, for its REE+Y composition, speciation, and geochemical significance in order to constrain the processes affecting the REE+Y distribution and fractionation pattern in a peculiar aquatic system. The fossil water of the Algerian Sahara has total mineralization in the 1170 and 2309 mg/l range and the prevailing hydrochemical facies is Na-Cl-SO4. The recorded REE+Y concentrations increase downstream, along with the dissolved silica content and temperature, as a function of water flow. The water samples are in equilibrium with 2:1 and 1:1 clay minerals with the exception of few upstream samples, characterized by alkaline pH values, that are in equilibrium with Na- and K-feldspars. The REE+Y speciation is mainly controlled by carbonate species. The Ln(CO3)+ species increase and the Ln(CO3)2 - species decrease downstream as the carbonate rocks of the aquifer dissolve and the PCO2 equilibrating pressure increases. The LnSO4 + species are mostly LaSO4 + downstream the aquifer and, more in general, LaSO4 + largely prevails up to pH between 6.9 and 7.2. The low negative cerium anomalies affecting most of the samples might be explained by the CeO2 precipitation following Ce3+ transformation to Ce4+. Ce oxidation is probably promoted by Mn-minerals by oxidative scavenging, whereas positive Ce anomaly could be due to the preferential desorption of Ce3+ over Ce4+. Finally, the pronounced negative samarium anomaly observed in some samples may reflect depletion of organic matter and Sm adsorption onto clay minerals.