The concentration of redox sensitive trace metals (RSTEs) and their isotopic composition preserved in Precambrian marine sediments, are critical for the reconstruction of ocean-atmosphere oxygenation history. Particularly, the concentration of Fe, its redox speciation, and isotopic distribution, have gained widespread use for inferring the biogeochemical processes that controlled Fe cycling in Precambrian oceans linked to the reconstruction of Earth surface redox budget. However, questions remain about the biotic and abiotic processes involved in Fe cycling in these ancient oceans, including the impact of post-depositional alterative processes on the reliability of the Fe redox proxy. Here we present a multi-proxy mineralogical and geochemical study of the 1.1 Ga Atar and El Mreiti strata of the Taoudeni Basin in Mauritania, to better constrain pathways involved in Fe cycling, linked to Fe mineralogy, redox speciation, isotopic ratios during this time and metamorphism. We compare unmetamorphosed sedimentary deposits with facies metamorphosed by dolerite sill intrusion. The results reveal the occurrence of diagenetic Fe minerals in the basal unmetamorphosed samples associated with light S56Fe signatures, reflecting dominant anoxic conditions that promoted microbial dissimilatory Fe reduction. Notably, S56Fe composition of these rocks reveal several fluctuations in evolving seawater redox state from oxic to anoxic/sulfidic conditions associated with changes in sea level stand and periods of full bottom water oxygenation and redox stratification. Overall, Ce anomalies suggest a general up sequence increase in seawater oxygen content. Metamorphosed rocks display heterogeneous S56Fe distribution, consisting of light and heavy signatures associated with secondary Fe-bearing minerals produced by metamorphic and metasomatic overprinting of carbonated rocks by hot circulating fluids. The results thus indicate metamorphic overprinting of primary seawater S56Fe promoted by increased mobility of reactive Fe during post-depositional metamorphic transformation. They show that post-depositional metamorphic/metasomatic overprinting complicates direct reconstruction of seawater biogeochemical Fe cycling and redox state using S56Fe systematics. (c) 2024 International Association for Gondwana Research. Published by Elsevier B.V. All rights are reserved, including those for text and data mining, AI training, and similar technologies.

The formation of chondrite materials represents one of the earliest mineralogical processes in the solar system. Phyllosilicates are encountered at various stages of the chondrule formation, from the initial stages (IDP agglomerates) to the final steps (chondrule internal alteration). While typically linked to aqueous alteration, recent studies reveal that phyllosilicates could precipitate directly from residual fluids in post-magmatic or deuteric conditions and under a wide range of temperatures, pressures, water/rock ratios, and H2/H2O ratio conditions. This study re-examined the formation of hydrated phyllosilicates in chondrules and associated fine-grained rims (FGRs) using published petrographical, mineralogical, and chemical data on carbonaceous chondrites. Given that chondrules originate from the melting of interplanetary dust particles, the water liberated by the devolatilization of primary phyllosilicates, including clay minerals or ice melting, reduces the melting temperature and leads to water dissolution into the silicate melt. Anhydrous minerals (e.g., olivine and diopside) form first, while volatile and incompatible components are concentrated in the residual liquid, diffusing into the matrix and forming less porous FGRs. Serpentine and cronstedtite are the products of thermal metamorphic-like mineral reactions. The mesostasis in some lobated chondrules is composed of anhydrous and hydrous minerals, i.e., diopside and serpentine. The latter is probably not the alteration product of a glassy precursor but rather a symplectite component (concomitant crystallization of diopside and serpentine). If so, the symplectite has been formed at the end of the cooling process (eutectic-like petrographical features). Water trapped inside chondrule porosity can lead to the local replacement of olivine by serpentine without external water input (auto-alteration). In the absence of water, hydrated phyllosilicates do not crystallize, forming a different mineral assemblage.

The present study focuses on the hydrogeological system of the Batha basin (Chad). This area is symptomatic of the functioning of an endoreic basin in an arid to semi-arid climate, of its water storage capacity related with the climatic fluctuations, and of the problems associated with assessing resources in this Sahelian zone. This basin forms the eastern part of the Lake Chad basin, which extends over 2.38 Mkm2. It is a vast sedimentary basin, filled by detrital and fluvial-lacustrine deposits of Eocene (Continental Terminal) to Quaternary age, and bordered by Precambrian crystalline formations. The study aims to assess the aquifer potential of the groundwater system and its dynamics using a combined geology-hydrogeology-hydrochemistry approach. The lithology defined an heterogeneous and multi-layered system. The piezometric map reveals the dynamics of the general groundwater flow direction SE-NW, suggesting the interconnection of the different parts of the aquifer system. Based on the concentrations of major ions in solution, the hydrochemical results have identified four main types of facies: calcium bicarbonate (dominant), sodium bicarbonate, sulphate-sodium and mixed. The mineralization of these facies appears to be essentially controlled by water-rock interaction and to a lesser extent by base exchange and evaporation process. Calculations of saturation indices indicate that these waters are close to equilibrium with the calcite-Mg phases, gaylussite and gypsum. Nevertheless, given the low and medium ionic contents, it appears that the groundwater in the Batha basin is moderately evolved. On the other hand, stable isotope analyses (δ2H and δ18O) of groundwaters show the existence of three compositional groups corresponding to: (A) ancient water disconnected from current meteoric influences, (B) mixtures of current meteoric water and older water affected by evaporation during infiltration, (C) mixtures of current and ancient meteoric water more impacted by the evaporation process. In addition, tritium (3H) analysis of these waters shows the existence of three composition groups corresponding to current rainwater, modern water and post-modern water. Taken together, these results show that the hydrogeochemical differentiations obtained from both ionic and isotopic analyses cannot be strictly associated with the different aquifers (Quaternary, Pliocene, CT and basement). This confirms the interconnection of the Batha system and suggests that the heterogeneity observed is mostly controlled by lithological and climatic variations.

This study investigates the Batha endorheic basin in Chad, situated east of the Lake Chad basin in the arid to semi-arid Sahelian zone. This region has not yet undergone comprehensive geological and hydrogeological studies. More broadly, the transition zone between semi-arid and arid climates has been minimally explored. This research aims to evaluate the resources and dynamics of this multi-layered system using a combined geology-hydrogeology-hydrochemistry-isotopes approach. The multilayer system includes sedimentary layers (Quaternary, Pliocene, and Eocene) over a crystalline basement. A piezometric investigation of the system shows a general SE–NW groundwater, indicating an interconnection between all layers. Hydrochemical analyses identifies four main facies (calcium-bicarbonate, sodium-bicarbonate, sulphate-sodium, and mixed), primarily controlled by water–rock interaction with secondary influences from base-exchange and evaporation. Saturation indices indicate that these waters are close to equilibrium with the calcite-Mg phases, gaylussite and gypsum. Stable isotopes (oxygen-18 and deuterium) categorize groundwater into three groups: ancient water, recent and older meteoric water mixtures affected by evaporation, and mixtures more heavily impacted by evaporation. Tritium contents reveal three groups: current rainwater, modern water, and sub-modern water. These results indicate that ionic and isotopic differentiations cannot be strictly linked to specific layers, confirming the interconnected nature of the Batha system. The observed heterogeneity is mainly influenced by lithological and climatic variations. This study, though still limited, enhances significantly the understanding of the basin’s functioning and supports the rational exploitation of its vital resources for the Batha area’s development. Future investigations to complete the present study are highlighted.

Water pollution is an increasing global concern affecting millions of people around the world, especially in arid to semi-arid areas. Management of water resources has become a crucial requirement for present times and specifically for future decades due to its detrimental impact on populations. The eastern High Atlas represents a demonstrative situation to highlight the problems in water-stressed areas. The eastern High Atlas of Morocco has a semi-arid climate and low rainfall (< 200 mm), irregular in time and space. The hydrology of the studied basin is characterized by intermittent flows and frequent, sometimes intense, limited floods. The main aquifer corresponds to the Plio-Quaternary deposits containing an unconfined groundwater of low thickness, from 3 to more than 30 m. This aquifer is controlled by the Paleozoic basement—Mesozoic to Quaternary cover interface. The hydrogeological system is located in a structurally complex area. The fractured and weathered surface part of the basement, directly in contact with the cover deposits, is also permeable. This system is mainly recharged by the depletion of the seasonal groundwater which take place in the Mesozoic landforms during rainy periods. In this semi-arid area, we show that the water resources and their qualities are highly dependant on the climatic conditions and water–rock interaction. The 2H and 18O isotopes indicate that the recharge of the Plio-Quaternary aquifer is mainly through precipitation on the summits of the NW part of the basin. The 3H isotope reveals that the waters in the basin are less than 50 years old, implying a rapid turnover. The reduced potential of the groundwater and its quality of medium to high alkalinity, limit its use for drinking and irrigation.

The origins of climate change have an increasingly important impact on the management of water resources on a local or global scale. This issue is particularly crucial in arid areas and oases, which are among the most immediately affected in Morocco. The study area is part of the oases eastern High Atlas, extending from the southeastern foothills of the Atlas Mountains to the western edge of the Sahara Desert. The High Atlas highlands receive most of the rainfall and represent the regional recharge area. The geology of the basin consists mainly of jurassic and cretaceous formations. The structure of the basin is strongly affected by the atlasian tectonics. This results in large successive synclines separated by major W-E faults. The objective of this study is to understand the hydrodynamic behavior in the face of meteorological phenomena of Bouanane watershed, the data processing is based on the digital terrain model (DTM) that have been able to define the main characteristic parameters of the basin and trace the watershed boundary and the state of evolution of the watershed by contribution of hypsometry.

The eastern part of Morocco, surrounded by the High Atlas chain close to Errachidia, represents a very sensitive area in a semi-arid context. Average annual rainfall does not exceed 300 mm/year. Most of the regional water resources are linked to the aquifer systems in the Atlas area. The study site coincides with the Eastern Upper Guir Basin, one of the main rivers in Eastern Morocco. This basin is located close to the eastern border of the Moroccan High Atlas. The geology of the basin consists mainly of Jurassic and Cretaceous formations. The structure of the basin is strongly affected by the Atlasian tectonics. This results in successive large synclines separated by major W-E faults. The hydrogeological system of the basin is accordingly constituted by sub-basins associated with each syncline and separated by faulted anticlines, whose overlapping faults act as semi-impermeable barriers. This compartmentalization causes the very complex functioning of the whole system. The study undertaken, based on structural, meteorological, hydrogeological, hydrochemical and isotopic data, allowed us to advance significantly in the understanding of this system. The compartments behave as hydrogeological sub-units but remain interdependent on each other. The recharge of the aquifer system is essentially associated with the rains and snowfalls at high altitudes. This is consistent with the stable isotope results. Important development projects are planned in this region, associated with a drastic increase in water demands. Groundwater suitability was assessed using the WHO standards for drinking purposes and the SAR (sodium adsorption ratio) for irrigation. Complementary investigations should be considered to further the results presented here and move towards a quantitative assessment and management of the water resources.