XFEM,strong discontinuities and second-order work in shear band modeling of saturated porous media

We investigate shear band initiation and propagation in fully saturated porous media by means of a combination of strong discontinuities (discontinuities in the displacement field) and XFEM. As a constitutive behavior of the solid phase, a Drucker–Prager model is used within a framework of non-associated plasticity to account for dilation of the sample. Strong discontinuities circumvent the difficulties which appear when trying to model shear band formation in the context of classical nonlinear continuum mechanics and when trying to resolve them with classical numerical methods like the finite element method. XFEM, on the other hand, is well suited to deal with problems where a discontinuity propagates, without the need of remeshing. The numerical results are confirmed by the application of Hill’s second-order work criterion which allows to evaluate the material point instability not only locally but also for the whole domain.     

Plume Generation Zones at the margins of Large Low Shear Velocity Provinces on the core–mantle boundary

Large Igneous Province (LIP) eruption sites of the past 300 My lie vertically above 1% slow shear wave velocity (V_s) contours bounding the African and Pacific Large Low Shear Velocity Provinces (LLSVPs) at the core–mantle boundary (CMB), or in the cases of the Siberian and Columbia River LIPs, bounding one or other of two smaller, Low Shear Velocity Provinces (LSVPs). Steep gradients in V_s at the CMB coincide with those 1% slow contours. The sites of 24 active hotspot volcanoes project down to the same narrowly defined borders of the LLSVPs at the CMB. Plumes that have generated LIPs and major hotspot volcanoes have risen only from the immediate neighbourhoods of the 1% slow V_s contours at the CMB which thus define Plume Generation Zones (PGZs). PGZs projected vertically upward approximately match the + 10 m elevation contour of the geoid showing that the LLSVPs are a dominant control on the positively elevated geoid. Minima in the frequency distribution of shear wave velocities in the lowermost mantle near V_s = ? 1% indicate that regions with more negative velocities, forming ～ 2% of total mantle mass, are likely to be of material compositionally different from the rest of the mantle. Because all LIP eruption sites with ages younger than 300 Ma lie above the borders of LLSVPs or LSVPs at the CMB, PGZ footprints are inferred to have remained in the same places for the past 300 My. Because no plumes have risen from the interior of the LLSVPs and because no lithospheric slabs have penetrated those bodies the volumes of the LLSVPs are inferred to have also remained unchanged for the past 300 My. Because the LLSVPs are the dominant control on the positively elevated areas of the geoid those too must have remained as they now are since 300 Ma. The LLSVPs are not rising buoyant objects but stable features of the deep mantle. LIPs have been erupted throughout the past 2.5 Gy indicating that PGZs comparable to those of the past 0.3 Gy and LLSVPs (of which PGZs mark the margins at the CMB) have also existed for at least that long. LLSVPs could thus form the isolated reservoir invoked by some to explain the distinctive isotopic compositions of terrestrial rocks. PGZs lie at places where the boundaries of: (i) The outer core, (ii) one of the LLSVPs or LSVPs, and (iii) the seismically faster part of the deep mantle meet. Horizontal temperature gradients across the steeply inclined margins to the LLSVPs, the interiors of which are hotter than the surrounding mantle, at the CMB are key controls for the generation of plumes. Near the CMB the association of the high temperature of the outer core with an inclined thermal boundary layer at the margins of LLSVPs facilitates the generation of mantle plumes in the PGZs.     

Groundwater isoscapes in a montane headwater catchment show dominance of well‐mixed storage

We conducted an integrated groundwater–surface water monitoring programme in a 3.2‐km² experimental catchment in the Scottish Highlands by sampling all springs, seepages, and wells in six, spatially extensive synoptic surveys over a 2‐year period. The catchment has been glaciated, with steep hillslopes and a flat valley bottom. There is around 70% glacial drift cover in lower areas. The solid geology, which outcrops at higher elevations, is granite and metamorphic schist. The springs and seepages generally occur at the contact between the solid geology and drift or at breaks of slopes in the valley bottom. Samples were analysed for stable isotopes, Gran alkalinity and electrical conductivity. Despite the surveys encompassing markedly different antecedent conditions, the isotopic composition of groundwater at each location exhibited limited temporal variability, resulting in a remarkable persistence of spatial patterns indicating well‐mixed shallow, groundwater stores. Moreover, line‐conditioned excess values derived from the isotope data indicated no evidence of fractionation affecting the groundwater, which suggests that most recharge occurs in winter. The alkalinity and electrical conductivity of groundwater reflected geological differences in the catchment, being highest where more weatherable calcareous rocks outcrop at higher altitudes in the catchment. Springs draining these areas also had the most variable isotope composition, which indicated that they have shorter residence times than the drift covered part of the catchment. The study showed that even in geologically heterogeneous upland catchments, groundwater can be characterized by a consistent isotopic composition, reflecting rapid mixing in the recharge zone. Our work, thus, emphasizes the critical role of groundwater in upland catchments and provides tracer data that can help constrain quantitative groundwater models.     

Reconstructing the iron cycle from the horizontal distribution of metals in the sediment of Baldeggersee

In Baldeggersee, the distributions of solid phase Fe, Mn, V, Cr, As and Mo were determined in different sediment strata, deposited under various deep-water oxygen conditions. Iron concentrations are correlated with water depth when an anoxic sediment is in contact with an oxic water column. Benthic redox gradients trigger iron transport towards the deepest site (geochemical focusing) and loss of iron from the shallower parts through the outflow. Fe cycling in the lake is inhibited by oxygen penetration into the sediment. Vanadium and arsenic can be used as tracers for the internal Fe cycle. Their distribution patterns are highly correlated with iron. In case of a stable oxycline in the deep water, Mo is enriched in the sediment and correlates with Mn. The horizontal distribution patterns of Fe, V, As and the correlation of Fe and Mn with trace metals are promising proxy indicators for the reconstruction of deep-water oxygen conditions during deposition.     

Medium-scale maps after survey from space—The “Metric-Camera”—Experiment

Airphotos have been used for topographic survey since the Twenties. The “European Space Agency” (ESA), the “Deutsche Forschungs- und Versuchsanstalt für Luft- und Raumfahrt” (DFVLR) and “Carl Zeiss” have modified this successful technique for spacecraft and recently have tested the system over four continents. Each of these satellite photos covers a region of 189×189 km to the scale of 1∶800000 and may be enlarged to 1∶250000 or 1∶100000. Between the altitudes of 200 and 300 km, mapmaking or-revision seems to be economic in zones or in countries, where existing maps are of minor quality or outdated. In this paper, examples from deserts and semideserts in the Sudan are presented.     

中德联合冬给措纳湖地区寒区环境变迁研究进展

青藏高原腹地的内陆湖及其湖相沉积物能够较真实地反映气候变化特征,在高原环境变迁研究中备受中外科学家的关注.2006年8月15日~9月10日,中德联合考察小组对青藏高原东北部的冬给措纳湖一带进行了第四纪冻土、地貌和地质学考察,并对湖岸阶地做了初步的数字高程分析.调查了冬给措纳湖底地形、地貌,采集了湖相沉积物样本.在湖的东岸和西岸发现多年冻土,湖泊西北山谷观察到古冰川活动遗迹.初步推测该湖是由断陷沉降形成.为进一步分析湖区气候和环境变迁,联合小组拟定2007年3月份钻取湖芯.     

Ammoniten-Ökologie am Beispiel des Oberjura

Zusammenfassung In vielen Fällen beeinflussen äußere Faktoren das Vorkommen der Ammoniten. Im Oberjura spielt die Wassertiefe eine große Rolle, daneben tritt die Wassertemperatur. Voraussetzungen der ökologischen Analyse sind bodenbezogenes Leben der Tiere und postmortale Autochthonie der Schalen. Methoden und Fehlerquellen werden besprochen. Aus der ökologischen Bindung vieler Ammoniten ergeben sich Folgen, vor allem für Stratigraphie, Phylogenie und Ontogenie.

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In many cases external factors determine the occurence of the ammonites. In the Upper Jurassic the depth of the sea is of great importance, as well as the temperature of the water. Ecological analysis requires life of the animals close to the sea floor, and that the shells are autochthonous.Methods and sources of errors are discussed. The dependence on environment of many ammonites has consequences, especially for stratigraphy, phylogeny and ontogeny.

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Résumé Souvent des facteurs extérieurs déterminent la présence des ammonites. Dans le Jurassique Supérieur la profondeur de la mer joue un grand rôle, aussi bien que la température de l'eau. L'analyse écologique doit supposer que les animaux vivent près du fond de la mer et que les coquilles sont autochtones. Les méthodes et les sources d'erreurs sont discutées. La dépendance des ammonites de leur milieu a des conséquences avant tout pour la stratigraphie, la phylogénie et l'ontogénie.

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