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International audience; Hydrological models are important tools for the simulation of water storage and hydrological fluxes in large basins and complex river systems. The hydrological models can compensate the lack of observed data in ungauged basins. In this study, the hydrological model of large basins MGB (for Model of Large Basins in Portuguese) is used to evaluate the hydrological processes of the Ogooué River Basin (ORB), which has been mostly unmonitored for about three decades. Simulations were carried out over an 18-year period from 1998 to 2015 using TRMM 3B42 daily rainfall data from the Tropical Rainfall Measurement Mission (TRMM) as forcing and in situ and altimetry-based river discharges from Envisat, Saral Altika and Jason-2 for calibration and validation. The results of the model were in good agreement with the flows measured at stations upstream and downstream of the Ogooué basin (Nash-Sutcliffe Efficiency (NSE) > 0.56 for all calibration gauges). The MGB model efficiently describes the seasonal and interannual variations of the flow in the Ogooué River and its major tributaries which were found to be highly correlated to the rainfall (r ranging from 0.72 to 0.90 and 0.56 to 0.87 at seasonal and interannual time-scales respectively). Interannual variations of precipitation and river discharge of the ORB are linked to the El Niño Southern Oscillation (ENSO) in the tropical eastern Pacific Ocean and southeastern tropical Atlantic Niño. Also, the Ogooué River discharge was found to be strongly correlated with Sea Surface Temperature (SST) at annual and semi-annual time-scales.

12 pages; International audience; This study interprets 11 years (2006 to 2016) and 6 months (March to August in 2017) of respectively monthly and daily isotopic (δD and δ18O) monitoring of rain at Douala (Cameroon), a humid tropical station in Western Africa. The main scope is to analyze the climate controls on precipitation isotopes at different timescales. Firstly, we examine the annual cycles of δ18O. Over the 11 years of survey, the annual cycle exhibits a W shape that is quite reproducible from year to year, with two minima in spring and autumn periods. Based on back trajectory calculations and remote sensing observations of water vapor isotopic composition, we show that the observed depletion in spring and autumn is due to strong convective activity along air mass trajectories. The same effect of convective activity can be observed at the daily timescale. At seasonal and daily time scales, the isotopic composition is also strongly tied to the convective organization and cloud types. More depleted precipitation is associated with larger areas of high clouds. Very low to low clouds are observed in July-August, mid-level to high clouds are dominant in June and high to very high clouds characterize March-April-May, thus explaining the enriched (depleted) values in summer (spring). Finally, this paper highlights the importance of large scale meteorological conditions controls on precipitation stable isotope composition in the Gulf of Guinea.

We used high-resolution gravity data to analyse crustal structure beneath the Bangui Magnetic Anomaly (BMA) and the Chad Basin within the North-Central African region. Moho depths and effective elastic thickness values, calculated respectively by a regularized non-linear inversion method and by analysis of the coherence function between gravity and topography, have been interpreted with reference to seismic data , previous gravity studies and regional geology . Our aim is to determine the crustal architecture and to analyse its mechanical behaviour. Estimated Moho depths range from 30 to 43 km in the North-Central African region. Beneath the uplift zones (Adamawa-Yade plateau, Ouaddai, Tibesti, Hoggar, Tuareg shield) and the Congo Craton , the Moho depths range between 35 and 37 km. These values can be interpreted by tectonic and seismic events recorded along the regional shear zones; but also to the remobilization of the Precambrian basement. Analysis of the coherence function between the Bouguer gravity and the topography shows two different domains of rigidity: the highest values (100 km e values in Cameroon compared to others regions (Chad, Central African Republic, Niger, Egypt, Algeria) show the magnitude of these events on the crustal architecture. This being defined by the presence of cratonic cores, the Cameroon Volcanic Line (CVL) and hot spots as well as large regional shear zones, explain the fact that thermo-tectonic and seismic events that occurred at the northern edge of the Congo Craton in Cameroon, are more intense than in the Central African Republic. So according to the geodynamic process in Cameroon and Central African Republic, related to the hot spots along the CVL, thermo-tectonic and seismic events recorded along the regional shear zones (Sanaga Shear Zone and the Central Cameroon Shear Zone) and the Congo Craton, the BMA is associated to two different geotectonic contexts. The sources of the BMA are closely linked to the Congo Craton; they bottom should not exceed −37 km depth. The present results agree with previous studies which allow us to understand the evolution of the Saharan metacraton and the preservation of cratonic remnants (Murzuq, Al-Kufrah, Chad); similarly the estimated Moho depths is an important step that would serve as an input to model the sources of the Bangui Magnetic Anomaly.

The Bangui Magnetic Anomaly (BMA) is one of the largest magnetic anomalies in the world whose origin is still not known. This research investigated the crustal thickness , Curie depths and thermal structures in the Central African sub-regions – Cameroon, Central African Republic and adjacent countries – which are largely characterized by the Bangui Magnetic Anomaly. To achieve a better understanding and clearer idea of the location of the possible sources of the BMA, analyses of geothermal structures were conducted. Two potential methods were used: gravity to evaluate the crustal thickness and magnetics for geothermal analysis. Spectral analysis of gravity data shows that crustal thickness range between 14 and 55 km. The highest depths were found in Central African Republic. The lower values of crustal thickness were obtained in South-Chad basin with a minimum of roughly 14 km. Geothermal analysis is carried out using the Curie point depth, thermal gradient and heat-flow evaluations. The results show that the BMA is related to a thick crust of roughly 40 km. Depth to the bottom of possible sources does not exceed the lower crust. The mean Curie point depth estimated is 38 km with an error of ±2 km. Geothermal results also show the difference in the thermal behaviour between the crust in the Pan African and Precambrian domain. The mobile zone which constitutes the Pan African domain is associated with a thin crust of high heat-flow values of 65 mW/m 2. However, the Precambrian domain beneath the BMA is associated with a thick crust with lower heat-flow values (roughly 45 mW/m2). The difference between crustal thickness and Curie point depths shows that all the sources of the BMA are crustal. The present results are in favour of a geological origin for the Bangui Magnetic anomaly.

Humid equatorial regions are recognized as the least documented in term of hydrogeological functioning of aquifers despite the fact that they house a lot of developing countries and that groundwater is often the main water resource. Regarding this aspect, a study was conducted in sub-Saharan Africa, focusing on the Mio-Pliocene aquifer in Douala megacity (Cameroon) which is the rainiest city in West-Africa (about 4000 mm/year) with one of the greatest demographic growth rate of the African continent. Firstly, groundwater recharge rate has been calculated through water balance and Water Table Fluctuation methods. Results show that the aquifer is characterized by a high recharge of 600–760 mm/year. Then infiltration process and groundwater flow conditions have been examined by combining hydrogeological and isotopic methods. Rainwater infiltrated is recycled in the vadose zone through plants roots transpiration and groundwater flows with a Darcy velocity of 5 m/day. From the recharge area to the estuary, the mineralization increases controlled by anthropogenic activities and water-rocks interactions which are amplified by the residence time and accelerated by the hot and humid climate of Douala. The paper ends with the determination of natural background levels (NBLs) and threshold values (TV) of chemical components in groundwater to assess the contamination for different flow paths. This multi-proxy study and the establishment of NBLs and TV can be beneficial to improve groundwater resources management. Moreover, the conceptual model provided in this study could be used as a reference for porous aquifers submitted to high rainfall amount.

Major and trace element composition of iron duricrusts including clayey material samples and biostratigraphy of the Missole I outcrop from the Paleocene–Eocene N’Kapa Formation in the Douala sub-basin of Cameroon were used to infer the palaeoenvironment and relative age of the iron duricrusts. Iron duricrusts and clayey materials are essentially kaolinitic and smectitic and are generally siliceous and ferruginous (iron duricrusts) or siliceous and aluminous (clayey materials). These materials have high Chemical Indices of Alteration (CIA = 86.6–99.33%). The negative Eu anomalies with high (La/Yb) N shown by iron duricrusts and clayey sediments are essentially derived from silicic or felsic parent rocks when fractionated chondrite-normalized REE patterns also indicate felsic or silicic parent rocks. The Missole I iron duricrusts have a post-Thanetian age according to the relative age of claystones (Thanetian) and were formed after the deposition of sedimentary materials in an anoxic low-depth marine environment with eutrophication of surface water, and may have been exhumed and oxidized under arid climate.

The aim of this study is to determine the depth of deep tectonic structures observed in the Adamawa-Yadé zone (central part of Cameroon) and propose a new structural map of this area. The horizontal gradient associated with upward continuation and the 3D Euler deconvolution methods have been applied to the Earth Magnetic Anomaly Grid 2 (EMAG2) data from the study area. The determination of the maximum magnitude of the horizontal gradient of the total magnetic intensity field reduced to the equator, in addition to the main contacts deducted by Euler solution, allowed the production of a structural map to show the fault systems for the survey area. This result reveals the existence of two structural domains which is thus confirmed by the contrast of magnetic susceptibility in the Central Cameroon Zone. The suggested depths are in the range of 3.34 km to 4.63 km. The structural map shows two types of faults (minors and majors) with W-E, N-S, NW-SE, NE-SW, ENE-WSW, WNW-ESE, NNE-SSW, and NNW-SSE trending. The major faults which are deepest (3.81 km to 4.63 km) with NE-SW, W-E, and N-S direction are very represented in the second domain which includes the Pangar-Djerem zone. This domain which recovers many localities (Ngaoundéré, Tibati, Ngaoundal, Yoko Bétaré-Oya, and Yaoundé) is associated with the Pan-African orogeny and the Cameroon Volcanic Line.

The Wouri estuary is located in the Gulf of Guinea on the Atlantic coast of Cameroon’s coastline plain (3°49′ and 4°04′ north latitude and 9°20′ to 9°40′ east longitude), and is strongly influenced by coastal dynamics that have remained unquantified over a long period of time. This study analyzed the historical evolution of the Wouri estuarine coastline between 1948 and 2012. Variations in the estuarine evolution of the Wouri were studied from (i) minute topographic extracts from 1948, (ii) 1996−1999 nautical charts, and (iii) 2012 spatial map vectors. The net temporal spatial variation rates were calculated using the statistical methods of the Digital Shoreline Analysis System (DSAS). These change rates were also calculated over two time intervals (1948−1996 and 1996−2012) and over a 64-year period (1948−2012). The study reveals highly disparate results. Indeed, kinematics show that the Wouri estuary was dominated by erosion in its downstream section, with 262.83 ha for −3.2 m/year and 110.56 ha for −5.8 m/year between 1948−1996 and 1996−2012 respectively, and by accretion on the other hand in its upstream section, with 239.17 ha for 4.3 m/year in zone 5 between 1948−1996 and 150.82 ha for 12.6 m/year in zone 4 between 1996−2012. Thus, over the 64-year period (1948−2012), we have a dominance of variation by erosion downstream and conversely by accretion upstream, marked by the presence of amplifying factors (anthropogenic pressure and climate change) of the rate of variation of morphological evolution at the beginning of the 21st century, as compared to the middle-20th century. The observed development of sediment loss and accumulation, both influences and will influence, the sediment regime along the Wouri estuarine coastline. There is a need to develop a systematic sub-regional coastal surveillance activity to effectively manage Cameroon’s coastline system.