This chapter describes the variability of rainfall and river discharges in the Ogooué River basin (ORB) in recent decades (since 1940). Due to its location crossing the Equator, the ORB receives abundant precipitation that maintains one of the world’s best-preserved ecosystems. In contrast to neighboring forest basins that have been severely degraded because of deforestation, mining resources extraction, extensions of agricultural areas, and river transport, which is a crucial alternative to the cruel lack of road infrastructures, the ORB is experimenting with an exceptional conservation policy in the region. For example, the rural penetration rate in Gabon is about 1 inhabitant per km² and many studies report a deforestation rate close to 0%, with even full natural regeneration. However, the fluctuations of the standardized anomaly index of rainfall in the ORB show three main phases of variations: the first wet phase was characterized by abundant precipitations from 1940 to 1970, the second phase of the long-term mild drought was extended in the 1970s and 1980s and the final third phase presented a slight return of abundance in precipitation. Even though drought severity in the ORB was mainly weak, its effects in river discharges were very sensitive on seasonal and inter-annual scales. The pure equatorial regime of the ORB characterized by equal maximum floods in spring and autumn changed significantly from the difference between both maximum discharges of 13.5 % during the 1960s to 27.0 %, 38.4 %, 33.9 %, and 26.7 % for the 1970s, 1980s, 2000s and 2010s respectively. A brief comparison between the ORB and the Congo River basin showed that changes in the ORB are part of a regional process that Central Africa is undergoing with some spatial heterogeneities.

The Hellenic arc, where the African (Nubian) slab subducts beneath the Aegean and Anatolian microplates, has emerged as a type-locality for understanding subduction dynamics, including slab tear, slab fragments, drips, and transfer zones. Based on field evidence and geophysical, tectonics, and geochemical studies, it has been recognized that the subducting African slab is a primary driver for extension in the Aegean and Anatolian microplates and plays a significant role in accommodating present-day westward extrusion of the Anatolian microplate. Thus, understanding the Hellenic arc subduction zone initiation (SZI) age is critical in deciphering ancient mantle flow, how plate tectonics is maintained, and the mechanisms involved in triggering the onset of subduction. The SZI for the Hellenic arc has two disparate ages based on different lines of evidence. A Late Cenozoic (Eocene-Pliocene) SZI is proposed using the analysis of topography combined with estimates of slab age and depth, paleomagnetism, the timing of metamorphism, and volcanic activity, and timing of sedimentation within its accretionary wedge, the Mediterranean Ridge. This age follows an induced-transference SZI model, where a new subduction zone initiates following the jamming of an older subduction zone by buoyant crust due to regional compression, uplift, and underthrusting. A Late Cretaceous-Jurassic SZI age has also been proposed using reconstructions of images of subducted slabs seen using tomography and timing of obducted ophiolite fragments thought to be related to the system. In this case, the induced-transference SZI model fails, and a single subduction zone persists. As a result, continental lithospheric fragments and the ancient oceans between them become incorporated into the overall system without creating a new subduction zone. The presence of a long-lived subduction zone has implications for understanding Earth’s mantle dynamics and how plate tectonics operates. This paper describes and summarizes the evidence for both models in the Aegean-Western Anatolia region.

Jeff Apple Benowitz1, Michael Everett Mann21 GeoSep Services, 1521 Pine Cone Road, Moscow, ID, USA2 Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI, USACorresponding author: Jeff Apple Benowitz ([email protected])AbstractLong-lived magmatic arcs theoretically should migrate large trench perpendicular distances as convergent margin configurations and slab geometry vary over time, however many arc-magmatic belts are spatially localized over 10’s of millions of years. We document, by compiling published crystallization geochronology data for southern Alaska (6485 total bedrock and single grain detrital ages combined), that since ca. 100 Ma, arc magmatism has been localized along the Alaska Range suture zone, at times over 500-km inboard. However, since ca. 100 Ma incoming subducting slab characteristics, beneath mobile southern Alaska and convergent margin configurations, varied greatly and include both normal oceanic plate and oceanic plateau subduction, plate vector changes, oroclinal bending and reconfiguration of trench shape, terrane accretion, long distance translation and a Paleocene slab break off/slab window event. Therefore, it is inferred that inherited upper-plate lithospheric shape and heterogeneity must control in part the geometry of the subducting slab below a mobile southern Alaskan margin through hydrodynamic (viscous) mantle wedge “suction” forces. Additionally, crustal thickness heterogeneity may preferentially focus magma ascent through melt ponding along Moho offsets, and upper-plate lithospheric-scale strike-slip faults may be acting as passive and active conduits for arc magmatism. Inherited upper-plate controls on slab geometry could be a factor localizing arc magmatism along other long-lived convergent margin settings.

The Congo River provides potential for socio-economic growth at the regional scale, but with limited information on the river dynamics it is difficult for basin countries to benefit from this potential, and to invest in the development of water resources. In recent years, the number of hazards related to navigation and flooding has sharply increased, resulting in high loss of human lives as well as economic losses. Associated problems of river management in the Congo also include inefficiency in hydropower production, an increase in rate of river sedimentation and land use changes. Accurate information is needed to support adequate management strategies such as prediction of navigation water levels and sediment movement, and assessment of environmental impacts and engineering implications of water resources infrastructure. Modelling approaches and space observations have been used to understand the Congo River dynamics, but their effective application has proved difficult due to a lack of ground-based observational data for validation. Recent developments in data capture with acoustic Doppler technologies have considerably improved measurements of river dynamics. As well measuring river discharge, they also allow the analysis of the multiple hydrodynamic features occurring in fluvial systems. This paper presents the results of field measurement campaigns carried out in the middle reach of the Congo River and the Kasai tributary using state of the art measurement technology (ADCP, Sonar, GNSS) for investigation of large rivers. The measurements relate to river flow at multiple transects, river bathymetry, static and continuous water surface elevation, and targeted sediment sampling along the river. The paper provides a descriptive summary of the measurement results, a discussion on the application and performance of the equipment used in the Congo River, and lessons for future use of this equipment for measurements of large rivers in a data scarce environment such as the Congo Basin.

The spatiotemporal evolution of droughts in the Congo River Basin (CRB) from 1981–2018 was investigated using the Standardised Precipitation Index (SPI) and Standardised Precipitation–Evapotranspiration Index (SPEI) to assess the roles of precipitation and potential evapotranspiration. The results confirmed a notable trend toward drier conditions, particularly in parts of the northern and central basin, as well as in the south of the CRB, which was associated with increases in potential evapotranspiration and declining rainfall. Global outputs of the Lagrangian model FLEXPART were used to model air masses over four important climatological regions considered to be the main sources of precipitation in the CRB, and their contributions to precipitation over the basin were computed. These analyses confirmed that moisture in the CRB is ~60% self-sourced; African lands were the next greatest contributor, followed by the Indian and Atlantic Oceans. It was found that a reduction in contributions of the sources prevailed during 53 meteorological drought episodes that affected the CRB during the study period and it could be inferred that a reduction in moisture supplied from the Atlantic and Indian Oceans played an important role in the onset of drought episodes. It was also observed that the contribution of moisture from all sources to the CRB decreased during the study period, especially over the northern half of the basin, where the main humid forest of the CRB is located, confirming the importance of water transport and local hydroclimatological dynamics on the hydrological conditions, ecosystems, and local communities of the CRB.

Time-related information of pre-eruptive magmatic processes is locked in the chemical profile of compositionally zoned minerals and can be retrieved by means of elemental diffusion chronometry. However, only the timescale of the outermost rim is commonly resolved, limiting our knowledge of timescales to those directly preceding the eruption. A major obstacle is the need to accurately constrain temperatures at which diffusion occurred. This is particular difficult for multiple zoned minerals where the different compositional boundaries indicate multiple physicochemical changes of melt environments during the lifetime of a crystal. Here, we argue that elemental diffusion chronostratigraphy can be fully resolved for crystals that have spent their lifetime in hot storage. Under this condition, crystals will be kept at the temperature of the eruptible magma(s), and diffusion timescales approximate the storage of the crystal in question in different melt environments. We further argue that hot storage conditions are typical of open-conduit systems in steady-state and are driven by the regular supply of fresh hot magmas determining the constant presence of eruptible magma. Fe-Mg interdiffusion in pyroxenes from Stromboli and Popocatepetl volcanoes are used as examples to reconstruct the time-dependent elemental diffusion chronostratigraphy of single crystals and discuss magma dynamics implications. Uncertainties introduced by temperature estimates and other input data, including experimentally derived values for the activation energy E and the pre-exponential factor D0, have large effects on the accuracy of modelled timescales, which need to be correctly evaluated and mitigated. Elemental diffusion chronostratigraphy is an extremely powerful tool to obtain time-related temporal information on the dynamics and histories of volcanic plumbing systems, which can lead to an in-depth knowledge of the magmatic system far beyond late-stage pre-eruptive processes. Combined with monitoring data and other petrological, geological and geophysical constraints at active volcanoes, they can greatly enhance our capability to inform volcanic hazard assessments.

Iron is present in magmas at concentrations ranging from less than 1 wt% to more 8 than 10 wt% in two valence state. In general, Fe2+ is a network modifier in the melt structure while Fe3+ is a weak network former. The ratio Fe3+/(Fe3+ + Fe2+) depends on temperature, pressure, oxygen fugacity and melt composition. Parametric models allow its calculation, but the complex links between melt composition, iron oxidation state and coordination can be further rationalized using a ionic-polymeric model. Constraining concentration and oxidation state of iron is critical for determining magma density and viscosity, which drive exchanges of matter and heat in the Earth. At high pressures, changes in the coordination of elements, including iron, yield a stiffening and densification of magmas, potentially influencing dynamic and geochemical processes. Near surface, crystallization of Fe-bearing phases changes the residual melt composition, including iron content and oxidation state as well as volatile concentration, ultimately driving large changes in density and viscosity of magmas, and, hence, in the dynamic of fluid flow in volcanic systems. The complex interplay between magma iron content and oxidation state, major element chemistry, crystal and volatile content thus can play a large role on the dynamic of volcanic systems.

Anthropogenic subsurface urban heat islands (SUHIs) in groundwater under cities are known worldwide. SUHIs are potentially threats to springs because much spring fauna, like trout, amphipods, and rare plants, is cold stenothermal. The city of Minneapolis, Minnesota, USA, has a SUHI documented by the temperature of an underground spring, dubbed “Little Minnehaha Falls,” inside Schieks Cave, which is located 23 m below the central core of the city. In 2000 the temperature of that spring was elevated 11°C above regional background groundwater temperatures (8°C) at this latitude (45°N). A thermometric survey of the cave and nearby tunnel seepages in 2007 found that an abandoned drill-hole through the bedrock ceiling of the cave was discharging groundwater with a temperature of 17.9°C. By comparison, groundwater in the deep water-table below the cave was closer to natural background temperatures for the region. The unusually warm groundwater was thereby localized to the strata above the cave. This is the strongest signal of anthropogenic groundwater warming in the state of Minnesota and is attributed to vertical heat conduction from basements and pavements. Minneapolis is unique among SUHIs in that a cave forms a natural collection gallery deep below the city surface, whereas the literature is almost exclusively based on data from observation wells.

Many rivers systems of the world are super-saturated in dissolved CO2 (pCO2) relative to equilibrium with the atmosphere — why? Here we compare the coupled organic matter and pCO2 dynamics of the world’s two largest river systems, the Amazon and Congo, where data sets enable insights into the overall functioning of the respective basins. Discharge is the primary control on particulate (POC) and dissolved organic carbon (DOC) export in both the Amazon and Congo Rivers. Total suspended sediments (TSS) yield from the Amazon is twenty times greater per unit area than the Congo. However, despite low TSS concentrations, the Congo has a POC content approximately five times higher than the Amazon. The organic rich character of both watersheds is reflected in the DOC export, with the Amazon exporting ~ 11% and the Congo ~ 5% of the global land to ocean flux, based on measurements from the last discharge gauging stations. But care should be taken when describing estimates of TSS and carbon to the ocean. Processing and sequestration in tidal and coastal areas can significantly alter TSS and carbon delivery, and last discharge gauging stations are typically hundreds of kilometers from the sea. pCO2 in the Amazon mainstem ranges from 1,000 to 10,000 μatm, with floodplain lakes ranging from 20 to 20,000 μatm. Concentrations in the Congo mainstem are lower, with maximum values of ~5,000 μatm observed. The elevated level of pCO2 even as far as the mouth of such major rivers as the Amazon and Congo, up to thousands of kilometers from CO2-rich small streams, poses a most interesting question — what set of processes maintains such high levels? The answer is presumably some combination of instream metabolism of organic matter of terrestrial and floodplain origin, and/or injection of very high pCO2 water from local floodplains or tributaries.

The Tropical Rainfall Measuring Mission (TRMM) daily (3B42) and monthly (3B43) rainfall products are evaluated relative to synoptic weather station observations in Cameroon and according to the main agro-climatic regions. In order to achieve this goal, deterministic and categorical metrics were used, as well as inter annual variability and seasonnal distributions. Outcomes of the comparison showed that synoptic weather station data are strongly correlated with the TRMM 3B43 data and that rainfall distribution is characteristic for each agro-climatic region. The highest skill scores were observed in the Sudano-sahelian, High Savannah, and Western Highlands zones, while the Uni-modal Equatorial zone displayed the lowest correspondence scores between TRMM rainfall estimates and station-based observations. Daily TRMM 3B42 showed good performance in detecting rainy events, especially for light and moderate intensity rainfall events. TRMM 3B42 overestimates rainfall intensities except in the uni-modal region where rainfall intensities are underestimated. Rainfall seasonnality, as well convective zone are well reproduced by the TRMM datasets. Overall, the skill of TRMM 3B42 decreases for increasing precipitation intensities.

The dynamic topography of the core-mantle boundary (CMB) provides important constraints on dynamic processes in the mantle and core. However, inferences on CMB topography are complicated by the uneven coverage of data with sensitivity to different length scales and strong heterogeneity in the lower mantle. Particularly, a trade-off exists with density variations, which ultimately drive mantle flow and are vital for determining the origin of mantle structures. Here, I review existing models of CMB topography and lower mantle density, focusing on seismological constraints. I develop average models and vote maps with the aim to find model consistencies and discuss what these may teach us about lower mantle structure and dynamics. While most density models image two areas of dense anomalies beneath Africa and the Pacific, their exact location and relationship to seismic velocity structure differs between studies. CMB topography strongly influences the retrieved density structure, which helps to resolve differences between recent studies based on Stoneley modes and tidal measurements. Current CMB topography models vary both in pattern and amplitude and a discrepancy exists between models based on body-wave and normal-mode data. As existing models feature elevated topography below the Large-Low-Velocity Provinces (LLVPs), very dense compositional anomalies may currently be ruled out as possibility. To achieve a similar consistency as observed in lower mantle models of S-wave and P-wave velocity, future studies should combine multiple data sets to break existing trade-offs between CMB topography and density. Important considerations in these studies should be the choice of theoretical approximation and parameterisation. Efforts to develop models of CMB topography consistent with body-wave, normal-mode and geodetic data should be intensified, which will aid in narrowing down possible explanations for the LLVPs and provide additional insights into mantle dynamics.

A sequential inversion methodology for combining geophysical data types of different resolutions is developed and applied to monitoring of large-scale CO2 injection. The methodology is a two-step approach within the Bayesian framework where lower resolution data are inverted first, and subsequently used in the generation of the prior model for inversion of the higher resolution data. For the application of CO2 monitoring, the first step is done with either controlled-source electromagnetic (CSEM) or gravimetric data, while the second step is done with seismic amplitude-versus-offset (AVO) data. The Bayesian inverse problems are solved by sampling the posterior probability distributions using either the ensemble Kalman filter or ensemble smoother with multiple data assimilation. A carefully designed parameterization is used to represent the unknown geophysical parameters: electric conductivity, density, and seismic velocity. The parameterization is well suited for identification of CO2 plume location and variation of geophysical parameters within the regions corresponding to inside and outside of the plume. The inversion methodology is applied to a synthetic monitoring test case where geophysical data are made from fluid-flow simulation of large-scale CO2 sequestration in the Skade formation in the North Sea. The numerical experiments show that seismic AVO inversion results are improved with the sequential inversion methodology using prior information from either CSEM or gravimetric inversion.

Due to the awareness of degrading groundwater quality in Florida’s freshwater 7 springs and beginning in the early 1990s, the state’s water management districts, the Florida 8 Department of Environmental Protection, and the U.S. Geological Survey began efforts to 9 coordinate monitoring of Florida’s first-and second-magnitude springs. This study investigates 10 changes in spring discharge and the concentrations of two saline indicators sodium (Na +) and 11 chloride (Cl-) from 1991 through 2020 (30 years) in the Floridan aquifer system (FAS). Data were 12 obtained from 32 major springs and three additional discharge gaging stations. Spring discharge 13 was observed to decrease, while concentrations of sodium and chloride increased. As a group, the 14 FAS springs experienced passive saline encroachment. Not only did encroachment occur along 15 Florida’s coasts, but also in the interior. Median concentrations of sodium and chloride increased 16 by an estimated range of 7 to 11% per decade. Evidence suggests the major driver is decreasing 17 rainfall and subsequent declines in recharge to the FAS, followed by sea-level rise. The sources 18 of the saline water are from salt water near Florida’s coasts and relict sea water from the deeper 19 portions of the FAS. The observed changes agree with those predicted by the Ghyben-Herzberg 20 principle for coastal, carbonate aquifers. 21

The study aims to assess the local response of the regional climate model version 4.6 (RegCM4.6) to the coupling of ocean-atmosphere interaction in Central Africa. The ability of the model is evaluated over six years (first January 2001, to thirty-first December 2006) by conducting two different experiments with the Grell convective scheme. The experiments are carried out monthly with a spatial resolution of 40 km. The model was forced by ERA-Interim reanalyses and validated by GPCP (Global Precipitation Climatology Project) observational data, ERA 5 and ERA-Interim reanalyses. To evaluate the influence of the slab-ocean, we carried out two different experiments: The first experiment is designed to produce the climatology and force the surface limits of RegCM with the sea surface temperature. The second experiment is designed to couple RegCM with the slab-ocean, which provides mutual interaction between the ocean and the atmosphere. Using statistical tools, we evaluated the model’s ability to simulate precipitation, surface temperature and wind. Both experiments reasonably reproduce the main characteristics of the rainfall regime, temperature and wind. A comparative analysis of the different experiments reveals that the performances of the experiments are similar in Central Africa and in the different homogeneous sub-regions as far as rainfall is concerned, but there are subtle differences. Slab-ocean improvement varies from season to season and from the sub-region to sub-region. However, we note a significant improvement in temperature and rainfall over the Indian Ocean.

The Cape Verde archipelago is a group of Ocean Islands in the Central Atlantic that forms two chains of islands trending Northwest and Southwest. Several of the islands are considered to be volcanically active, with frequent eruptions on Fogo. We examine the mineral chemistry and thermobarometry of the southern islands; Santiago, Fogo and Brava together with the Cadamosto Seamount. Our objective is to explore the magmatic storage system and implications for volcanic eruptions and associated hazards at Cape Verde. The volcanic rocks at Cape Verde are alkaline and dominantly mafic, whereas the island of Brava and the Cadamosto Seamount are unusually felsic. Clinopyroxene compositions range from 60 to 90 Mg# at Santiago and Fogo. In contrast, at Brava and the Cadamosto Seamount the clinopyroxene compositions are 5 to 75 Mg#. Mineral chemistry and zonation records fractional crystallization, recharge, aggregation of crystals, magma mixing and variations in thermal conditions of the magma at temperatures from 925 to 1250C. Magma storage depths at Santiago, Fogo, Brava and the Cadamosto Seamount are between 12 and 40 km, forming deep sub-Moho magma storage zones. Transient magma storage in the crust is suggested by fluid inclusion re-equilibration and pre-eruption seismicity. A global compilation of magma storage at Ocean Islands suggests deep magma storage is a common feature and volcanic eruptions are often associated with rapid magma ascent through the crust. Shallow magma storage is more variable and likely reflects local variations in crustal structure, sediment supply and tectonics. Petrological constraints on the magma plumbing system at Cape Verde and elsewhere are vital to integrate with deformation models and seismicity in order to improve understanding and mitigation of the volcanic hazards.

The viscosity of the mantle affects every aspect of the thermal and compositional evolution of Earth's interior. Radial variations in viscosity can affect the sinking of slabs, the morphology of plumes, and the rate of convective heat transport and thermal evolution. Below the mantle transition zone, we detect changes in the long-wavelength pattern of lateral heterogeneity in global tomographic models, a peak in the the depth-distribution of seismic scatterers, and changes in the dynamics plumes and slabs, which may be associated with a change in viscosity. We analyze the long-wavelength structures, radial correlation functions, and spectra of four recent global tomographic models and a suite of geodynamic models. We find that the depth-variations of the spectral slope in tomographic models are most consistent with a geodynamic model that contains both a dynamically significant phase transition and a reduced-viscosity region at the top of the lower mantle. We present new inferences of the mantle radial viscosity profile that are consistent with the presence of such a feature.

Electrical and electromagnetic (EM) techniques are used to map the electrical resistivity of the subsurface. The injection of carbon dioxide (CO2) is likely to form a lenticular, slab or wedge-like bodies of increased resistivity, and therefore resistive thin-layer and sphere models are used for detectability analyses. Thin resistive layer models are used to determine the maximum depth at which resistive layer be detected, and the maximum depth at which time changes in the layer could be monitored. Sphere models are used for quantitatively studying the basic properties of the secondary fields produced on the surface by a finite body at variable depth and in a variety of source fields, and for order-of magnitude rules for depth and size determination of a confined volume filled with CO2. Both surface based EM and DC resistivity methods have limitations for detecting and delineating flat lying resistive features. Configurations with EM sources at depth, which induce vertical currents that are sensitive to the transverse (vertical) resistivity show great promise for detecting and monitoring the emplacement of tabular zones or bodies of resistive CO2.

The rainfall reduction in the 1970s, less marked in Central Africa than in West Africa, still had a major impact on the hydrological regimes of the region’s large rivers. The study of the hydropluviometric behavior of the Ubangi at Mobaye has the advantage of studying a basin excluding anthropogenic impact. Forest cover and population density have not changed since at least 1970. Statistical analysis of the breaks in the long rainfall time series from Ubangi at Mobaye (1935-2015) confirms a long period of drought from 1969 to 2006 corresponding to a reduction of -8% in rainfall. And the study of the corresponding hydrological series indicates a second downward break in 1981, few years after the rainfall increase. This period points an exceptional hydrological drought period until 2013, which is the first year with an increase of flows. The statistical study of the annual rainfall/flow series of the upstream basins over the period 1951-1995 (the Kotto at Kembé and Bria, the Mbomu at Bangassou and Zémio, the Uélé + Bili hydrographic system) highlights different hydrological behaviors related to the vegetation cover. The savanna basins show a continuous hydrological deficit marked by a runoff coefficient (CE) that fell to 5% only from the 1990s. On the other hand, the basins under forest show a runoff increase since 1990 marked by CE above 10%. Under savannah, the part of the flow infiltrating to recharge the aquifer would have decreased faster than under forest, which results in a runoff coefficient CE very significantly negatively correlated with the savanna area present in the studied watershed.