Assessment of Lateritic Gravelled Materials for Use in Road Pavements in Cameroon

Geotechnical and Geological Engineering - Early surface and structural deterioration of new pavements are becoming increasingly perceptible in Cameroon. This raises serious concerns regarding the...     

Late Paleozoic–Early Triassic magmatism on the western margin of Gondwana: Collahuasi area, Northern Chile

The basement in the ‘Altiplano’ high plateau of the Andes of northern Chile mostly consists of late Paleozoic to Early Triassic felsic igneous rocks (Collahuasi Group) that were emplaced and extruded along the western margin of the Gondwana supercontinent. This igneous suite crops out in the Collahuasi area and forms the backbone of most of the high Andes from latitude 20° to 22°S. Rocks of the Collahuasi Group and correlative formations form an extensive belt of volcanic and subvolcanic rocks throughout the main Andes of Chile, the Frontal Cordillera of Argentina (Choiyoi Group or Choiyoi Granite-Rhyolite Province), and the Eastern Cordillera of Peru.Thirteen new SHRIMP U–Pb zircon ages from the Collahuasi area document a bimodal timing for magmatism, with a dominant peak at about 300 Ma and a less significant one at 244 Ma. Copper–Mo porphyry mineralization is related to the younger igneous event.Initial Hf isotopic ratios for the ~ 300 Ma zircons range from about − 2 to + 6 indicating that the magmas incorporated components with a significant crustal residence time. The 244 Ma magmas were derived from a less enriched source, with the initial Hf values ranging from + 2 to + 6, suggestive of a mixture with a more depleted component. Limited whole rock ¹⁴⁴Nd/¹⁴³Nd and ⁸⁷Sr/⁸⁶Sr isotopic ratios further support the likelihood that the Collahuasi Group magmatism incorporated significant older crustal components, or at least a mixture of crustal sources with more and less evolved isotopic signatures.     

Using a shifted Grünwald-Letnikov scheme for the Caputo derivative to study anomalous solute transport in porous medium

We propose an extension of the shifted Grünwald-Letnikov method to solve fractional partial differential equations in the Caputo sense with arbitrary fractional order derivative α and with an advective term. The method uses the relation between Caputo and Riemann-Liouville definitions, the shifted Grünwald-Letnikov, and the traditional backward and forward finite difference method. The stability of the method is investigated for the implicit and explicit scheme with homogeneous boundary conditions, and a stability criterion is found for the advective-dispersive equation. An application of the method is used to solve contaminant diffusion and advective-dispersive problems. The numerical solution for the fractional diffusion and fractional advection-dispersion is compared with their respective analytical solutions for different time and space grid refinements. The diffusion simulation exhibited a good fit between the analytical and numerical solutions, with the explicit scheme going from stable to unstable as the time and space refinement changes. The fractional advection-dispersion application produced small deviations from the analytical solution. These deviations, however, are analogous to the numerical dispersions encountered in conventional finite difference solutions of the advection-dispersion equation. The new method is also compared with the traditional L2 method. Notably, an example that involves asymmetrical fractional conditions, a fractional diffusivity that depends on time, and a source term show how the methods compare. Overall, this study assesses the quality and easiness of use of the numerical method.     

Deglacial paleoclimate in the southwestern United States: an abrupt 18.6 ka cold event and evidence for a North Atlantic forcing of Termination I

We present a new U-series dated speleothem record (PC-1) from the Great Basin that documents deglacial climate variability between ca 20.1 and 15.6 ka. Our data show an abrupt 18.6 ka cold event preceding Heinrich event 1 that is consistent with expansion of the Laurentide Ice sheet during the ‘binge’ phase of ice growth. This event coincided with dessication of pluvial Lake Mojave suggesting cold and dry conditions in the southern Great Basin at this time. PC-1 δ¹⁸O values before and during Heinrich event 1 are similar, but an increase in stalagmite growth rates suggests wetter conditions that coincided with deposition of spring deposits in southern Nevada. The time interval of our record is consistent with the timing of pluvial conditions in the Great Basin as evident from a comparison to regional wetness proxies. Our new speleothem record, recovered from the recharge area for Devils Hole, does not show a δ¹⁸O increase coincident with the abrupt increase in Devils Hole δ¹⁸O at c. 18 ka, challenging the view that the Great Basin experienced an early Termination I. This hypothesis is supported by two other southwest speleothem records that demonstrate deglaciation was synchronous with forcing from the North Atlantic Ocean. We suggest that Devils Hole speleothem δ¹⁸O values may partly reflect source water changes in the regional aquifer. Further, Devils Hole δ¹³C minima coincide with peak global glacial conditions and weak Asian monsoon periods, suggesting that they constrain better the timing of pluvial conditions in the Great Basin.     

Trends of thought concerning meteorological research

Summary Certain problems of synoptic meteorology relating to the nature of the large quasi-stationary cyclonic and anticyclonic centers aloft are examined from the standpoint of an hypothesis which seeks to connect their maintenance with that of the smaller transient disturbances. The suggested link between these two scales of eddies is in many ways similar to that operating in the general circulaton in order to account for the sustained existence of the circumpolar mean zonal motions, according to the more modern concepts. Newer views concerning the dynamcis of large convective processes, and the essential nonlinearity of the laws governing motions in the atmosphere, figure importantly in the arguments advanced.

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Zusammenfassung Gewisse Probleme der synotischen Meteorologie im Zusammenhang mit der Natur von grossen halb-stationären zyklonischen und antizyklonischen Zentren in der höheren Atmosphäre werden von einem hypothetischen Standpunkt aus untersucht; und es wird versucht, deren Bestehen in Zusammenhang zu bringen mit dem Vorhandensein von kleineren, vorübergehenden Störungen. Der vorgeschlagene Zusammenhang zwischen diesen beiden Arten von Wirbel-Systemen ähnelt in vielen Beziehungen der allgemeinen Zirkulation, die verantwortlich ist für das fortwährende Bestehen der zirkumpolaren durchschnittlichen zonalen Strömungen, wie sie zufolge modernerer Auffassungen erklärt werden. Neuere Standpunkte, die die Dynamik von grossen konvektiven Vorgängen in der Atmosphäre und die nicht-linearen Gesetze im Hinblick auf die atmosphärischen Strömungs-Gesetze betreffen, werden hauptsächlich in der folgenden Abhandlung untersucht.

     

Structure and tectonic history of the foreland basins of southernmost South America

The common elements and differences of the neighboring Austral (Magallanes), Malvinas and South Malvinas (South Falkland) sedimentary basins are described and analyzed. The tectonic history of these basins involves Triassic to Jurassic crustal stretching, an ensuing Early Cretaceous thermal subsidence in the retroarc, followed by a Late Cretaceous–Paleogene compressional phase, and a Neogene to present-day deactivation of the fold–thrust belt dominated by wrench deformation. A concomitant Late Cretaceous onset of the foreland phase in the three basins and an integrated history during the Late Cretaceous–Cenozoic are proposed. The main lower Paleocene–lower Eocene initial foredeep depocenters were bounding the basement domain and are now deformed into the thin-skinned fold–thrust belts. A few extensional depocenters developed in the Austral and Malvinas basins during late Paleocene–early Eocene times due to a temporary extensional regime resulting from an acceleration in the separation rate between South America and Antarctica preceding the initial opening of the Drake Passage. These extensional depocenters were superimposed to the previous distal foredeep depocenter, postdating the initiation of the foredeep phase and the onset of compressional deformation. Another pervasive set of normal faults of Paleocene to Recent age that can be recognized throughout the basins are interpreted to be a consequence of flexural bending of the lithosphere, in agreement with a previous study from South Malvinas basin. Contractional deformation was replaced by transpressive kinematics during the Oligocene due to a major tectonic plate reorganization. Presently, while the South Malvinas basin is dominated by the transpressive uplift of its active margin with minor sediment supply, the westward basins undergo localized development of pull-apart depocenters and transpressional uplift of previous structures. The effective elastic thickness of the lithosphere for different sections of each basin is calculated using a dynamic finite element numerical model that simulates the lithospheric response to advancing tectonic load with active sedimentation.     

2.5D seismic velocity modelling in the south-eastern Romanian Carpathians Orogen and its foreland

The DACIA-PLAN (Danube and Carpathian Integrated Action on Processes in the Lithosphere and Neotectonics) deep seismic reflection survey was performed in August–September 2001, with the objective of obtaining new information on the deep structure of the external Carpathians nappe system and the architecture of the Tertiary/Quaternary basins developed within and adjacent to the Vrancea zone, including the rapidly subsiding Focsani Basin. The DACIA-PLAN profile is about 140 km long, having a roughly WNW–ESE direction, from near the southeast Transylvanian Basin, across the mountainous south-eastern Carpathians and their foreland to near the Danube River. A high resolution 2.5D velocity model of the upper crust along the seismic profile has been determined from a tomographic inversion of the DACIA-PLAN first arrival data. The results show that the data fairly accurately resolve the transition from sediment to crystalline basement beneath the Focsani Basin, where industry seismic data are available for correlation, at depths up to about 10 km. Beneath the external Carpathians nappes, apparent basement (material with velocities above 5.8 km/s) lies at depths as shallow as 3–4 km, which is less than previously surmised on the basis of geological observations. The first arrival travel-time data suggest that there is significant lateral structural heterogeneity on the apparent basement surface in this area, suggesting that the high velocity material may be involved in Carpathian thrusting.     

Benthic Phosphorus Dynamics in the Gulf of Finland, Baltic Sea

Benthic fluxes of soluble reactive phosphorus (SRP) and dissolved inorganic carbon (DIC) were measured in situ using autonomous landers in the Gulf of Finland in the Baltic Sea, on four expeditions between 2002 and 2005. These measurements together with model estimates of bottom water oxygen conditions were used to compute the magnitude of the yearly integrated benthic SRP flux (also called internal phosphorus load). The yearly integrated benthic SRP flux was found to be almost 10 times larger than the external (river and land sources) phosphorus load. The average SRP flux was 1.25?±?0.56?mmol?m^?2?d^?1 on anoxic bottoms, and ?0.01?±?0.08?mmol?m^?2?d^?1 on oxic bottoms. The bottom water oxygen conditions determined whether the SRP flux was in a high or low regime, and degradation of organic matter (as estimated from benthic DIC fluxes) correlated positively with SRP fluxes on anoxic bottoms. From this correlation, we estimated a potential increase in phosphorus flux of 0.69?±?0.26?mmol?m^?2?d^?1 from presently oxic bottoms, if they would turn anoxic. An almost full annual data set of in situ bottom water oxygen measurements showed high variability of oxygen concentration. Because of this, an estimate of the time which the sediments were exposed to oxygenated overlying bottom water was computed using a coupled thermohydrodynamic ocean?Csea and ecosystem model. Total phosphorus burial rates were calculated from vertical profiles of total phosphorus in sediment and sediment accumulation rates. Recycling and burial efficiencies for phosphorus of 97 and 3%, respectively, were estimated for anoxic accumulation bottoms from a benthic mass balance, which was based on the measured effluxes and burial rates.     

Hydrochemistry of Lakes of the Patagonian Province of Tierra del Fuego (Argentina)

Chemical studies on 19 lakes of the Argentine sector of the Patagonian Tierra del Fuego Island were performed. These lakes are placed in two geological provinces, Extra Andine (northern) and Fueguian Cordillera (southern). Range of the results obtained, pH (5.80–8.60), TDS (29.5–10,232 mg/l), ionic composition (from calcium bicarbonate to sodium chloride type), Secchi disc depth (0.05–12 m), total inorganic nitrogen (20–237 μg N/l), orthophosphate (5–498 μg P/l), total phosphorus (5–3,800 μg P/l) and chlorophyll a (0.1–63.0 μg/l), indicated the presence of a wide variety of aquatic ecosystems. According to Gibbs model, the mechanisms that control the water chemistry are mainly rock dominance and evaporation–crystallization processes.     

A preliminary mass balance model of primary productivity and dissolved oxygen in the Mississippi River Plume/Inner Gulf Shelf Region

A deterministic, mass balance model for phytoplankton, nutrients, and dissolved oxygen was applied to the Mississippi River Plume/Inner Gulf Shelf (MRP/IGS) region. The model was calibrated to a comprehensive set of field data collected during July 1990 at over 200 sampling stations in the northern Gulf of Mexico. The spatial domain of the model is represented by a three-dimensional, 21-segment water-column grid extending from the Mississippi River Delta west to the Louisiana-Texas border, and from the shoreline seaward to the 30–60 m bathymetric contours. Diagnostic analyses and numerical experiments were conducted with the calibrated model to better understand the environmental processes controlling primary productivity and dissolved oxygen dynamics in the MRP/IGS region. Underwater light attenuation appears relatively more important than nutrient limitation in controlling rates of primary productivity. Chemical-biological processes appear relatively more important than advective-dispersive transport processes in controlling bottom-water dissolved oxygen dynamics. Oxidation of carbonaceous material in the water column, phytoplankton respiration, and sediment oxygen demand all appear to contribute significantly to total oxygen depletion rates in bottom waters. The estimated contribution of sediment oxygen demand to total oxygen-depletion rates in bottom waters ranges from 22% to 30%. Primary productivity appears to be an important source of dissolved oxygen to bottom waters in the region of the Atchafalaya River discharge and further west along the Louisiana Inner Shelf. Dissolved oxygen concentrations appear very sensitive to changes in underwater light attenuation due to strong coupling between dissolved oxygen and primary productivity in bottom waters. The Louisiana Inner Shelf in the area of the Atchafalaya River discharge and further west to the Texas border appears to be characterized by significantly different light attenuation-depth-primary productivity relationships than the area immediately west of the Mississippi Delta. Nutrient remineralization in the water column appears to contribute significantly to maintaining chlorophyll concentrations on the Louisiana Inner Shelf.