Solar and anthropogenic imprints on Lake Masoko (southern Tanzania) during the last 500 years

The Masoko crater-lake in southern Tanzania provides a continuous record of environmental changes covering the last 500 years. Multi-proxy studies were performed on a 52 cm sediment core retrieved from the deepest part of the lake. Magnetic, organic carbon, geochemical proxies and pollen assemblages indicate a dry climate during the ‘Little Ice Age’ (AD 1550–1850), confirming that the LIA in eastern Africa resulted in marked and synchronous hydrological changes. However, the direction of response varies between different African lakes (low versus high lake-levels), indicating strong regional contrasts that prevent the clear identification of climate trends over eastern Africa at this time. Inferred changes in Masoko lake-levels closely resemble the record of solar activity cycles, indicating a possible control of solar activity on the climate in this area. This observation supports previous results from East African lakes, and extends this relationship southward. Finally, anthropogenic impact is observed in the Masoko sediments during the last 60 years, suggesting that human disturbance significantly affected this remote basin during colonial and post-colonial times.     

Textural and fluid inclusion constraints on the origin of the banded-iron-formation-hosted gold deposits at Maevatanana,central Madagascar

The Maevatanana deposits consist of gold-bearing quartz–sulphide veins crosscutting banded iron formation (BIF) within a metamorphosed 2.5 Ga greenstone belt. The host rocks are dominated by a sequence of migmatites, gneisses, amphibolites, magnetite-rich quartzites and soapstones, intruded by large granitoid batholiths (e.g. the 0.8 Ga Beanana granodiorite). In the mineralised rocks, pyrite is the dominant sulphide, in addition to accessory chalcopyrite and galena. Outside the immediate ore zone, the BIF is dominated by quartz + magnetite ± hematite, accompanied by cummingtonite, albite and biotite. Gold occurs as globular grains (usually <500 μm) within quartz crystals close to the sulphides and as invisible inclusions within pyrite and chalcopyrite (up to 2,500 ppm Au content). Fluid inclusion textural and microthermometric studies indicate heterogeneous trapping of a low-salinity (∼3.6 wt.% eq. NaCl) aqueous fluid coexisting with a carbonic fluid. Evidence for fluid-phase immiscibility during ore formation includes variable L/V ratios in the inclusions and the fact that inclusions containing different phase proportions occur in the same area, growth zone, or plane. Laser Raman spectroscopy confirms that the vapour phase in these inclusions is dominated by CO₂ but shows that it may contain small amounts of CH₄ (<1 mol%), H₂S (<0.05 mol%) and traces of N₂. Fluid inclusion trapping conditions ranged from 220 to 380°C and averaged 250°C. Pressure was on the order of 1–2 kbar. The abundant CO₂ and low salinity of the inclusions suggest a metamorphic origin for the fluid. Likewise, the presence of H₂S in the fluid and pyritisation of the wall-rock indicate that gold was likely transported by sulphide complexing. Fluid immiscibility was probably triggered by the pressure released by fracturing of the quartzites during fault movements due to competence differences with the softer greenstones. Fracturing greatly enhanced fluid circulation through the BIF, allowing reaction of the sulphide-bearing fluids with the iron oxides. This caused pyrite deposition and concomitant Au precipitation, enhanced by fluid phase separation as H₂S partitioned preferentially into the carbonic phase.     

Synergie entre la télédétection multispectrale et les données de terrain pour la conception d'un nouveau modèle géodynamique d'ouverture du bassin paléozoïque des Jebilet centrales (Maroc)

The geodynamic model of the Palaeozoic basin opening of central Jebilet has been unknown before this study in spite of the abundance in the geological studies carried out in the studied sector using conventional methods. This is due to the scarcity of the key beds and synsedimentary structures. Using the synergy between the image data of the Landsat satellite TM sensor and the ground data, we have highlighted, herein, new structural data allowing the design of a new model of the Palaeozoic basin opening of central Jebilet. This opening could have been made according to dextral submeridian transverse faults with the individualization of subequatorial normal faults. To cite this article: A. El Harti et al., C. R. Geoscience 336 (2004).     

Routes of Agulhas rings in the southeastern Cape Basin

Using weekly sea surface height data, Agulhas rings from the period October 1992 to December 2006 are detected and tracked, from their formation dates and throughout the Cape Basin. While 102 of them formed at the Agulhas Current retroflection, their subsequent subdivisions and junctions led to 199 trajectories. The rings geographical probability of presence shows two maxima. One, related to numerous ring passages, lies in the submarine bight formed by the Erica seamount, the Schmitt-Ott seamount, and the northeastern tip of the Agulhas Ridge. The other one, to be ascribed to topographic blocking of the eddies, is southeast of the latter obstacle. On the basis of topographic effects three routes for Agulhas rings are distinguished, a Northern route for rings that enter the south-Atlantic northeast of the Erica seamount, a Central one for those passing westward between this seamount and the tip of the Agulhas Ridge, and a Southern one farther south. Despite its bathymetric obstacles, the central route is the dominant one, both in terms of percentage of eddy crossings at its definition segment, and in terms of conveyed volume transport. Specific behaviours of rings along each route are described, referring to observations in previous studies. Some rings from the Northern route interact with the flow regime of the South African continental slope. The southernmost trajectories of the Central route are thought to settle the location of the climatological Subtropical Front in that region. The rings of the Southern route experience important core property alteration as they transit through the subantarctic domain.     

On the need to test hydrological models under changing conditions

Abstract

The ability of hydrological models to deal with changing conditions should not be taken for granted: it is an unfortunate but well-known problem of hydrology that the model structure and/or parameters optimized for certain conditions may not be transferable in time. Consequently, it is essential that, for application under changing conditions (e.g. in climate change impact studies), models be thoroughly assessed for their extrapolation capacity using adequate protocols. This editorial provides an overview of the Special Issue of Hydrological Sciences Journal compiled after a workshop on this theme held during the General Assembly of the International Association of Hydrological Sciences (IAHS) in Gothenburg (Sweden) in 2013. The Workshop participants had been invited to apply a calibration and evaluation protocol to their own models on a given set of changing basins. The results show that this protocol is an appropriate and instructive way of assessing the suitability of hydrological models to be applied under changing conditions. This special issue also includes papers following alternative testing methodologies, as well as an opinion paper on the definition of non-stationarity.     

Combined hillslope diffusion and sediment transport simulation applied to landscape dynamics modelling

We present a new numerical approach for simulating geomorphic and stratigraphic processes that combines open‐channel flow with non‐uniform sediment transport law and semi‐empirical diffusive mass wasting. It is designed to facilitate modelling of surface processes across multiple space‐ and time‐scales, and under a variety of environmental and tectonic conditions. The physics of open‐channel flow is primarily based on an adapted Lagrangian formulation of shallow‐water equations. The interaction between flow and surface geology is performed by a non‐uniform total‐load sediment transport law. Additional hillslope processes are simulated using a semi‐empirical method based on a diffusion approach. In the implementation, the resolution of flow dynamics is made on a triangulated grid automatically mapped and adaptively remeshed over a regular orthogonal stratigraphic mesh. These new methods reduce computational time while preserving stability and accuracy of the physical solutions. In order to illustrate the potential of this method for landscape and sedimentary system modelling, we present a set of three generic experiments focusing on assessing the influence of contrasting erodibilities on the evolution of an active bedrock landscape. The modelled ridges morphometrics satisfy established relationships for drainage network geometry and slope distribution, and provide quantitative information on the relative impact of hillslope and channel processes, sediment discharge and alluviation. Our results suggest that contrasting erodibility can stimulate autogenic changes in erosion rate and influence the landscape morphology and preservation. This approach offers new opportunities to investigate joint landscape and sedimentary systems response to external perturbations. The possibility to define and track a large number of materials makes the implementation highly suited to model source‐to‐sink problems where material dispersion is the key question that needs to be addressed, such as natural resources exploration and basin analysis. Copyright © 2014 John Wiley & Sons, Ltd.     

Lateral stirring of large-scale tracer fields by altimetry

Ocean surface fronts and filaments have a strong impact on the global ocean circulation and biogeochemistry. Surface Lagrangian advection with time-evolving altimetric geostrophic velocities can be used to simulate the submesoscale front and filament structures in large-scale tracer fields. We study this technique in the Southern Ocean region south of Tasmania, a domain marked by strong meso- to submesoscale features such as the fronts of the Antarctic Circumpolar Current (ACC). Starting with large-scale surface tracer fields that we stir with altimetric velocities, we determine ‘advected’ fields which compare well with high-resolution in situ or satellite tracer data. We find that fine scales are best represented in a statistical sense after an optimal advection time of ～2 weeks, with enhanced signatures of the ACC fronts and better spectral energy. The technique works best in moderate to high EKE regions where lateral advection dominates. This technique may be used to infer the distribution of unresolved small scales in any physical or biogeochemical surface tracer that is dominated by lateral advection. Submesoscale dynamics also impact the subsurface of the ocean, and the Lagrangian advection at depth shows promising results. Finally, we show that climatological tracer fields computed from the advected large-scale fields display improved fine-scale mean features, such as the ACC fronts, which can be useful in the context of ocean modelling.     

Element partitioning between magnesium silicate perovskite and ferropericlase: New insights into bulk lower-mantle geochemistry

In this study, we investigated iron–magnesium exchange and transition-metal trace-element partitioning between magnesium silicate perovskite (Mg,Fe)SiO₃ and ferropericlase (Mg,Fe)O synthetised under lower-mantle conditions (up to 115 GPa and 2200 K) in a laser-heated diamond anvil cell. Recovered samples were thinned to electron transparency by focused ion beam and characterized by analytical transmission electron microscopy (ATEM) and nanometer-scale secondary ion mass spectroscopy (nanoSIMS). Iron concentrations in both phases were obtained from X-ray energy dispersive spectroscopy measurements and nanoSIMS. Our results are the first to show that recently reported spin-state and phase transitions in the lower mantle directly affect the evolution of Fe–Mg exchange between both phases. Mg-perovskite becomes increasingly iron-depleted above 70–80 GPa possibly due to the high spin–low spin transition of iron in ferropericlase. Conversely, the perovskite to post-perovskite transition is accompanied by a strong iron enrichment of the silicate phase, ferropericlase remaining in the Fe-rich phase though. Nanoparticles of metallic iron were observed in the perovskite-bearing runs, suggesting the disproportionation of ferrous iron oxide, but were not observed when the post-perovskite phase was present. Implications on the oxidation state of the Earth and core segregation will be discussed. Transition trace-element (Ni, Mn) concentrations (determined with the nanoSIMS) show similar trends and could thus be used to trace the origin of diamonds generated at depth. This study provides new results likely to improve the geochemical and geophysical models of the Earth's deep interiors.     

No significant post-Eocene rotation of the Moesian Platform and Rhodope (Bulgaria): Implications for the kinematic evolution of the Carpathian and Aegean arcs

The region located between the Carpathian–Balkan and Aegean arcs, the Moesian Platform and Bulgarian Rhodope, is generally assumed to have been stably attached to the East European craton during the Cenozoic evolution of these arcs. The kinematic evolution of this region is, however, poorly constrained by paleomagnetic analysis. In this paper we provide new paleomagnetic data (800 volcanic and sedimentary samples from 12 localities) showing no significant post-Eocene rotation of the Moesian platform and Rhodope with respect to Eurasia, therefore confirming the stability of this region. We compare this result to a provided review of paleomagnetic data from the South Carpathians (Tisza block) and the Aegean region. The Tisza block underwent 68.4 ± 16.7° of middle Miocene ( 15–10 Ma) clockwise rotation with respect to the Moesian Platform, in line with previous rotation estimates based on structural geology. The stability of the Moesian platform during middle Miocene eastward emplacement of the Tisza block into the Carpathian back-arc supports dextral shear along the Southern Carpathians recorded by 13–6 Ma clockwise strike-slip related rotations in foreland deposits. The new reference direction for the Moesian platform and Rhodope allows accurate quantification of the rotation difference with the west Aegean domain at 38.0 ± 7.2° occurring between 15 and 8 Ma. To accommodate this rotation, we propose that the pivot point of the west-Aegean rotation was located approximately in the middle of the rotating domain rather than at the northern tip as previously proposed. This new scenario predicts less extension southeast of the pivot point, in good agreement with estimates from Aegean structural geology. Northwest of the pivot point, the model requires contraction or extrusion that can be accommodated by the coeval motion of the Tisza Block around the northwestern edge of the Moesian platform.     

Automatic correction and simplification of geological maps and cross-sections for numerical simulations

Incorporating prior geological knowledge in geophysical process models often meets practical meshing challenges and raises the question of how much detail is to be included in the geometric model. We introduce a strategy to automatically repair and simplify geological maps, geological cross-sections and the associated meshes while preserving elementary consistency rules. To identify features breaking validity and/or the thin features potentially problematic when generating a mesh, we associate an exclusion zone with each model feature (horizon, fault). When these zones overlap, both the connectivity and the geometry of the geological layers are automatically modified. The output model enforces specific practical quality criteria on the model topology and geometry that facilitates the generation of a mesh with lower bounds on minimum angles and minimum local entity sizes. Our strategy is demonstrated on an invalid geological cross-section from a real-case study in the Lorraine coal basin. We further explore the impacts of the model modifications on wave propagation simulation. We show that the differences on the seismograms due to model simplifications are relatively small if the magnitude of simplifications is adapted to the physical problem parameters.