Sediment yield and transport process assessment from reservoir monitoring in a semi‐arid mountainous river

Soil loss, fluvial erosion, and sedimentation are major problems in semi‐arid environments due to the high associated costs of decreasing services such as provisioning and regulating water resources. The objective of this research is to analyse sediment yield in a mountainous semi‐arid basin, paying special attention to the sources of sediment, the associated uncertainties, and the transport processes involved. The segregation hypothesis along a reservoir of the sediment coming from hillslopes or fluvial systems is also evaluated. For this purpose, bottom‐set and deltaic deposits of a reservoir (110 hm³ ) in southern Spain have been measured and compared with basin erosion and fluvial transport monitoring over a 12‐year period. The volume of sediment stored at the bottomset of the reservoir shows a relative match with parametric predictions based on the Revised Universal Soil Loss Equation/Modified Universal Soil Loss Equation hillslope models and rating curves, estimated as being between 7 and 13 t·ha^?1·year^?1. Similarly, the measured volume of deltaic deposit fits the average value of stochastic simulations from different bedload transport equations. These contributions represent 50–65% of the total volume measured regarding suspended sediment inputs, way above that described in previous works. This highlights the importance of considering bedload when estimating the useful life of reservoirs in semi‐arid environments. The major differences in sediment grain size between hillslopes and river systems, and the size fractions measured along the reservoir, support the assertion of segregation hypothesis. Nonetheless, based on the processes observed and the uncertainty related to modelling, that assertion has to be taken with caution. At basin scale, a specific sediment yield of between 19 and 24 t·ha^?1·year^?1 has been estimated, which includes hillslopes and fluvial contributions. This rate is in the range of sediment yield reported for Mediterranean mountain areas of a torrential character. The pulse‐like nature of the system and the spatial heterogeneity of fluvial and hillslope erosion rates points out the importance of considering mid to long‐term and process‐based approaches and emphasizes the limitations of annual estimations for management purposes.     

Comparison of the main characteristics of the daily zonally averaged surface air temperature as represented by reanalysis and seven CMIP3 models

This paper analyses the ability exhibited by seven coupled global climate models of the Climate Model Inter-comparison Project 3 used in the Fourth Assessment Report of the Intergovernmental Panel on Climate Change to simulate current zonally averaged surface air temperature (ZASAT) meridional profiles. The expansion in second order of ZASAT profiles by means of Legendre polynomials was compared with the same expansion carried out over the ZASAT profiles provided by the ERA40 and National Centers for Environmental Prediction reanalysis from 1961 to 1998. According to the theoretical support provided by the one-dimensional energy balance models (1D-EBMs), the Legendre coefficients corresponding to the ZASAT profile can be qualitatively interpreted as the independent modes that represent the meridional energy flux from the equator to the poles. We find that three models, MIROC3.2-MR, MIROC3.2-HR and MPI-ECHAM5 may be considered as the models that best reproduce the meridional structure of current ZASAT, although the differences between the models are not really large. Consequently, the results shown in this paper support the accuracy of the models in representing the poleward meridional heat fluxes and global thermal inertia under the qualitative interpretation provided by the 1D-EBM approach.     

Cytological and Population Genetic Changes in Northwestern Iberian Mussels After the Prestige Oil Spill

The ecological catastrophe produced by the Prestige oil spill (November 2002) caused severe damage in both North Spanish and French coastal communities. Wild mussel populations of Mytilus galloprovincialis in a zone with marginal introgression of Mytilus edulis were affected at all levels, from high DNA damage to increased polycyclic aromatic hydrocarbon content in tissues. In this article, we describe cytological and population genetic changes of wild mussel populations from the northwestern Iberian coast following the catastrophe. The micronucleus test was employed as an indicator of cytological damage, and the Barcoding mitochondrial cytochrome oxidase I (COI) and the nuclear Glu-5′ genes were analyzed for determining the species and assessing population genetic diversity. Immediate increase of micronuclei counts after the oil spill was found, with a further decrease in consecutive months although the counts did not recover pre-Prestige levels. Reduced variation at mitochondrial sequences in the most exposed areas and reduction of M. edulis traces in the regional genetic pool also suggest long-term impact that may result in evolutionary changes. These results highlight the need of adopting more strict measures in order to prevent this type of accidents and avoid long-term effects on wild populations.     

Fossil birds from the Late Cretaceous Los Alamitos Formation,Río Negro Province,Argentina

In this note we report new avian remains from the Late Cretaceous Los Alamitos Formation (Campanian-Maastrichtian) at the Los Alamitos locality, Río Negro Province, Argentina. Isolated remains referable to indeterminate Aves, ?Patagopterygiformes, indeterminate Ornithurae, cf. Hesperornithes and cf. Neornithes are described and discussed. The new genus and species Alamitornis minutus is erected to include a minute-sized and gracile bird, probably related to the non-volant ratite-like bird Patagopteryx. If correctly identified, the record of Hesperornithes may be the first for this group in the Southern Hemisphere. The Los Alamitos paleoavifauna represents one of the most diverse fossil bird assemblage from the Mesozoic of Gondwana known to date.     

How hydrofractures become arrested

Fluids in the earth's crust are commonly transported by hydrofractures, such as dykes and mineral veins, many of which become arrested at various crustal depths. Hydrofractures are commonly arrested – some showing blunt tips – at contacts between soft (low Young's modulus) and stiff (high Young's modulus) layers. For example, many dyke tips are arrested at contacts between soft pyroclastic rocks and stiff basaltic lava flows, and vein tips at contacts between soft marl and stiff limestone. Theoretical models indicate that overpressured, buoyant hydrofractures in homogeneous, isotropic host rocks should normally reach the surface. In layered host rocks, however, abrupt changes in Young's moduli, horizontal discontinuities, and layers with unusually high fracture-perpendicular stresses encourage hydrofracture arrest. It is proposed that for layer-parallel loading, stiff layers favour hydrofracture arrest during active compression but soft layers during extension. It is concluded that for hydrofracture propagation to occur, the stress field along its potential pathway must be essentially homogenous.     

The 410,000 year terrestrial age of eucrite Río Cuarto 001

Abstract— We have measured a surprisingly long terrestrial age of 410,000 ±_45,00^20,000 yr (410 ±₂₀⁴⁵ka) for basaltic eucrite Río Cuarto 001 using accelerator mass spectrometry of ²⁶Al, ³⁶Cl, and ⁴¹Ca. Though many meteorites are known to have survived for tens or hundreds of ka in Antarctica or hot deserts, the mean annual precipitation of 815 mm in Río Cuarto, Cordoba Province, Argentina, makes the long survival of this meteorite remarkable. We propose two explanations for the exceptional preservation of Río Cuarto 001. First, the meteorite contains only trace amounts of metal, so the weathering and oxidation of metallic Fe, which commonly destroys chondrites, is ineffective in this case. Second, the meteorite was found in a relatively young deflation basin, and may have been exhumed only recently from beneath a protective layer of soil. Insofar as the survival on Earth of Río Cuarto 001 is due to environmental factors, there may be other meteorites with comparably long terrestrial ages still to be discovered in the vicinity.     

Stratigraphy and paleomagnetism of a 2.9-km composite lava section in Eyjafjördur,Northern Iceland: a reconnaissance study

Eight mountainside profiles in lava flows south of the fjord Eyjafjördur, Northern Iceland, were sampled for paleomagnetic studies. The sampling was concurrent with pilot stratigraphic mapping of the lava sequences in these and several supplementary profiles. The eight profiles are correlated with minor overlaps so that they form a composite section of 2.9 km thickness, estimated to cover the age range between about 9 and 5 million years ago (Hardarson et al. 1999). Paleomagnetic measurements made on 319 lavas generally yield primary remanence directions of high stability and within-unit consistency. Evidence for at least 17 reversals of the geomagnetic field is seen, as well as numerous field excursions. Frequent clustering of directions in successive lavas indicates that the volcanism was episodic.     

The effects of layering and local stresses in composite volcanoes on dyke emplacement and volcanic hazards

Since most volcanic eruptions are fed by dykes, any assessment of volcanic hazards in an area must include an evaluation of the probability of injected dykes either reaching the surface or becoming arrested. Composite volcanoes are normally composed of alternating stiff (high Young's modulus) and soft (low Young's modulus) layers. Numerical models indicate that during unrest periods with magma-chamber inflation, the local stresses in composite volcanoes commonly prevent dyke-fed eruptions: while the stresses in the stiff layers may favour dyke propagation and seismogenic faulting, the local stresses in the soft layer remain seismically quiet and favour dyke arrest. Geodetic and field studies also indicate that most dykes never reach the surface, and that only a small fraction of the magma volume injected from a chamber erupts at the surface. I propose that for a dyke-fed eruption to occur, all the layers along the potential pathway of the dyke must have local stresses that favour magma-driven extension-fracture propagation. Thus, the stress field along the pathway must be homogenised. To cite this article: A. Gudmundsson, C. R. Geoscience 337 (2005).     

On the conditions of sheet injections and eruptions in stratovolcanoes

Nearly all eruptions in stratovolcanoes (composite volcanoes, central volcanoes) are supplied with magma through fractures. Consequently, a primary physical condition for an eruption to occur in a stratovolcano is that a magma-driven fracture is able to propagate to the surface. Magma-filled fractures, frozen or fluid, are referred to as sheet intrusions. More specifically, they are named dykes when subvertical, and inclined (or cone) sheets when inclined. Field observations indicate that most sheet intrusions do not reach the surface to feed eruptions but rather become arrested at various crustal depths. For this reason periods of volcanic unrest with sheet injections are much more common than volcanic eruptions. Whether a sheet intrusion becomes arrested or, alternatively, propagates to the surface depends primarily on the stress field in the stratovolcano. A stratovolcano normally consists of layers of contrasting mechanical properties, such as soft (low Youngs modulus) pyroclastic units and stiff (high Youngs modulus) lava flows. We present numerical models indicating that volcanoes composed of such layers commonly develop stress fields encouraging sheet and dyke arrest. The models indicate that a necessary condition for a sheet intrusion to reach the surface and feed a volcanic eruption is that the stress field along the sheet pathway becomes homogenised. We propose that much of the activity in a stratovolcano during a volcanic cycle encourages stress-field homogenisation. Field studies show that the sheet intrusions in individual stratovolcanoes have various dips: some are vertical dykes, others inclined sheets, and still others horizontal sills. Analytical models indicate that the dip of a sheet reaching the surface can have great effects on the magma transport during an eruption. This effect is normally greater for a flat volcano such as a collapse caldera than for a stratovolcano that forms a topographic high. We conclude that the shallower the dip of a sheet intrusion, the less will be its volumetric magma transport to the surface of a stratovolcano.Editorial responsibility: D Dingwell