Discriminating between pore‐filling load and bed‐structure load: a new porosity‐based method,exemplified for the river Rhine

Sediments contained in the river bed do not necessarily contribute to morphological change. The finest part of the sediment mixture often fills the pores between the larger grains and can be removed without causing a drop in bed level. The discrimination between pore‐filling load and bed‐structure load, therefore, is of practical importance for morphological predictions. In this study, a new method is proposed to estimate the cut‐off grain size that forms the boundary between pore‐filling load and bed‐structure load. The method evaluates the pore structure of the river bed geometrically. Only detailed grain‐size distributions of the river bed are required as input to the method. A preliminary validation shows that the calculated porosity and cut‐off size values agree well with experimental data. Application of the new cut‐off size method to the river Rhine demonstrates that the estimated cut‐off size decreases in a downstream direction from about 2 to 0·05 mm, covariant with the downstream fining of bed sediments. Grain size fractions that are pore‐filling load in the upstream part of the river thus gradually become bed‐structure load in the downstream part. The estimated (mass) percentage of pore‐filling load in the river bed ranges from 0% in areas with a unimodal river bed, to about 22% in reaches with a bimodal sand‐gravel bed. The estimated bed porosity varies between 0·15 and 0·35, which is considerably less than the often‐used standard value of 0·40. The predicted cut‐off size between pore‐filling load and bed‐structure load (D_c,p) is fundamentally different from the cut‐off size between wash‐load and bed‐material load (D_c,w), irrespective of the method used to determine D_c,p or D_c,w. D_c,w values are in the order of 10^?1 mm and mainly dependent on the flow characteristics, whereas D_c,p values are generally much larger (about 10⁰ mm in gravel‐bed rivers) and dependent on the bed composition. Knowledge of D_c,w is important for the prediction of the total sediment transport in a river (including suspended fines that do not interact with the bed), whereas knowledge of D_c,p helps to improve morphological predictions, especially if spatial variations in D_c,p are taken into account. An alternative to using a spatially variable value of D_c,p in morphological models is to use a spatially variable bed porosity, which can also be predicted with the new method. In addition to the morphological benefits, the new method also has sedimentological applications. The possibility to determine quickly whether a sediment mixture is clast‐supported or matrix‐supported may help to better understand downstream fining trends, sediment entrainment thresholds and variations in hydraulic conductivity.     

Sedimentary evolution of a Late Pleistocene temperate red algal reef (Coralligène) on Rhodes,Greece: correlation with global sea‐level fluctuations

Autochthonous red algal structures known as coralligène de plateau occur in the modern warm‐temperate Mediterranean Sea at water depths from 20 to 120 m, but fossil counterparts are not so well‐known. This study describes, from an uplifted coastal section at Plimiri on the island of Rhodes, a 450 m long by 10 m thick Late Pleistocene red algal reef (Coralligène Facies), interpreted as being a coralligène de plateau, and its associated deposits. The Coralligène Facies, constructed mainly by Lithophyllum and Titanoderma, sits unconformably upon the Plio‐Pleistocene Rhodes Formation and is overlain by a Maerl Facies (2 m), a Mixed Siliciclastic‐Carbonate Facies (0·2 m) and an Aeolian Sand Facies (2·5 m). The three calcareous facies, of Heterozoan character, are correlated with established members in the Lindos Acropolis Formation in the north of the island, while the aeolian facies is assigned to the new Plimiri Aeolianite Formation. The palaeoenvironmental and genetic‐stratigraphic interpretations of these mixed siliciclastic‐carbonate temperate water deposits involved consideration of certain characteristics associated with siliciclastic shelf and tropical carbonate shelf models, such as vertical grain‐size trends and the stratigraphic position of zooxanthellate coral growths. Integration of these results with electron spin resonance dates of bivalve shells indicates that the Coralligène Facies was deposited during Marine Isotope Stage 6 to 5e transgressive event (ca 135 to 120 ka), in water depths of 20 to 50 m, and the overlying Maerl Facies was deposited during regression from Marine Isotope Stage 5e to 5d (ca 120 to 110 ka), at water depths of 25 to 40 m. The capping Aeolian Sand Facies, involving dual terrestrial subunits, is interpreted as having formed during each of the glacial intervals Marine Isotope Stages 4 (71 to 59 ka) and 2 (24 to 12 ka), with soil formation during the subsequent interglacial periods of Marine Isotope Stages 3 and 1, respectively. Accumulation rates of about 0·7 mm year^?1 are estimated for the Coralligène Facies and minimum accumulation rates of 0·2 mm year^?1 are estimated for the Maerl Facies. The existence of older red algal reefs in the Plimiri region during at least Marine Isotope Stages 7 (245 to 186 ka) and 9 (339 to 303 ka) is inferred from the occurrence of reworked coralligène‐type lithoclasts in the basal part of the section and from the electron spin resonance ages of transported bivalve shells.     

Palaeoenvironmental significance of Late Permian palaeosols in the South‐Eastern Iberian Ranges,Spain

The Late Permian (Wuchiapingian) Alcotas Formation in the SE Iberian Ranges consists of one red alluvial succession where abundant soil profiles developed. Detailed petrographical and sedimentological studies in seven sections of the Alcotas Formation allow six different types of palaeosols, with distinctive characteristics and different palaeogeographical distribution, to be distinguished throughout the South‐eastern Iberian Basin. These characteristics are, in turn, related to topographic, climatic and tectonic controls. The vertical distribution of the palaeosols is used to differentiate the formation in three parts from bottom to top showing both drastic and gradual vertical upwards palaeoenvironmental changes in the sections. Reconstruction of palaeoenvironmental conditions based on palaeosols provides evidence for understanding the events that occurred during the Late Permian, some few millions of years before the well‐known Permian‐Triassic global crisis.     

Carbonate aeolianites of western Saurashtra,India: experimental decipherment of the depositional mechanisms

Granular carbonate deposits of Late Pleistocene to Early Holocene age, commonly referred to as ‘miliolite limestone’, occur in a linear belt, parallel to the southern coast of Saurashtra, India. In the present study area these carbonate deposits are found in select valleys between ridges and mounds of pyroclastic material present in the Deccan trap plateau. Two different depositional histories have been proposed for these sediments. The presence of marine bioclasts led to the postulation of a marine origin for these deposits. The second school of thought propounded redeposition of the coastal sediments by aeolian processes. Although a few features could not be explained by the proposed aeolian model, critical comparison of these two views favoured the aeolian origin. The mode of occurrence, lithological and structural attributes, and microscopic evidence presented here, also support a possible aeolian origin for these deposits. Experimental observation indicates that these carbonate aeolianites represent backflow deposits, which accumulated because of the flow separation caused by the presence of topographic highs. The conspicuous concave‐up geometry of the deposit conformed to the shape of the separation bulb. In view of the inferred depositional mechanism, the disposition of the deposits and the signature of the palaeoflow direction suggest that the carbonate particles were derived from the north‐western coast of Saurashtra by strong south‐easterly winds. Massive granular carbonates with outsized basement clasts appear to be the product of avalanching of granular material from the higher contours because of oversteepening of the primary deposit.     

Morphological analysis of the drainage system in the Eastern Alps

We study the morphology of the major rivers draining the Eastern Alps to test whether the active tectonics of this part of the orogen is reflected in the shape of channel profiles of the river network. In our approach we compare channel profiles measured from digital elevation models with numerically modelled channel profiles using a stream power approach. It is shown that regions of high stream power coincide largely with regions of highest topography and largest uplift rates, while the forelands and the Pannonian Basin are characterised by a significantly lower stream power. From stream power modelling we conclude that there is young uplift at the very east of the Eastern Alps, in the Bohemian Massif and in the Pohorje Range. The impact of the Pleistocene glaciations is explored by comparing properties of rivers that drain in proximal and distal positions relative to the ice sheet during the last glacial maximum. Our analysis shows that most knick points, wind gaps and other non-equilibrium features of catchments covered by ice during the last glaciations (Salzach, Enns) can be correlated with glacial processes. In contrast the ice free catchments of the Mur and Drava are characterized by channels in morphological equilibrium at the first approximation and are showing only weak evidence of the strong tectonic activity within these catchments. Finally, the channel profiles of the Adige and the divide between the upper Rhine and Danube catchments differ significantly from the other catchments. We relate this to the fact that the Adige and the Rhine respond to different base levels from the remainder of the Eastern Alps: The Adige may preserve a record from the Messininan base level change and the Rhine is subject to the base level lowering in the Rhine Graben.     

What controls dead-ice melting under different climate conditions? A discussion

In the geological record, hummocky dead-ice moraines represent the final product of the melt-out of dead-ice. Processes and rates of dead-ice melting in ice-cored moraines and at debris-covered glaciers are commonly believed to be governed by climate and debris-cover properties. Here, backwasting rates from 14 dead-ice areas are assessed in relation to mean annual air temperature, mean summer air temperature, mean annual precipitation, mean summer precipitation, and annual sum of positive degree days. The highest correlation was found between backwasting rate and mean annual air temperature. However, the correlation between melt rates and climate parameters is low, stressing that processes and topography play a major role in governing the rates of backwasting. The rates of backwasting from modern glacial environments should serve as input to de-icing models for ancient dead-ice areas in order to assess the mode and duration of deposition.A challenge for future explorations of dead-ice environments is to obtain long-term records of field-based monitoring of melt progression. Furthermore, many modern satellite-borne sensors have high potentials for recordings of multi-temporal Digital Elevation Models (DEMs) for detection and quantification of changes in dead-ice environments. In recent years, high-accuracy DEMs from airborne laser scanning altimetry (LiDAR) are emerging as an additional data source. However, time series of high-resolution aerial photographs remain essential for both visual inspection and high-resolution stereographic DEM production.     

Church-based social capital, networks and geographical scale: Katrina evacuation, relocation, and recovery in a New Orleans Vietnamese American community

This research examines the role of social capital and networks to explain the evacuation, relocation, and recovery experiences of a Vietnamese American community in New Orleans, Louisiana in the aftermath of Hurricane Katrina. As the single largest community institution, the parish church’s complex bonding and bridging social capital and networks proved particularly critical in part because of its historically based ontological security. The process of evacuation, but especially relocation and recovery, was dependent on deploying co-ethnic social capital and networks at a variety of geographical scales. Beyond the local or community scale, extra-local, regional, and national scales of social capital and networks reproduced a spatially redefined Vietnamese American community. Part of the recovery process included constructing discursive place-based collective-action frames to successfully contest a nearby landfill that in turn engendered social capital and networks crossing ethnic boundaries to include the extra-local African American community. Engaging social capital and networks beyond the local geographical scale cultivated a Vietnamese American community with an emergent post-Katrina cultural and political identity.     

Influence of Structural Non-Stationarity of Surface Roughness on Morphological Characterization and Mechanical Deformation of Rock Joints

Summary Structural non-stationarity of surface roughness affects accurate morphological characterization as well as mechanical behaviour of rock joints at the laboratory scale using samples with a size below the stationarity threshold. In this paper, the effect of structural non-stationarity of surface roughness is investigated by studying the scale dependence of surface roughness and mechanical behaviour of rock joints. The results show that the structural non-stationarity mainly affects the accurate characterization of the surface roughness of the fracture samples. It also controls the amount and location of the contact areas during shear tests, which in turn affects the mechanical properties and asperity degradation of the samples. It is concluded that for accurate determination of the morphological and mechanical properties of rock joints at laboratory and field scales, samples with size equal to or larger than the stationarity threshold are required. Author’s address: Nader Fardin, Rock Mechanics Group, Department of Mining Engineering, Faculty of Engineering, University of Tehran, P.O. Box: 11365/4563, Tehran, Iran     

南极冰退与深圳湾北岸海平面变化的地质记录

南极冰层的冰量为２４．５×１０６ｋｍ３，占地球总冰量的９０％以上，南极冰川进退控制着全球海平面变化和气候波动。酉部南极乔治王岛第四纪冰碛与湖积剖面记录了近１２０００ａ来南极曾于距今１１０００ａ，９０００ａ和６１００ａ出现过３次快速的冰消过程，近６０００ａ来是一小幅度冷暖交替的气候波动过程。深圳湾北岸潮间带堆积是在近６０００ａ以来发育的，堆积物的环境记录表明，海平面呈周期性升降变化，波动周期平均为６７０ａ，低海面时期发生于距今５５００—４９００ａ，３９００—３６００ａ，２４００—２２００ａ和１３００—１２００ａ。相邻低海面间则是海面上升时期，每一升降周期的海平面变化幅度为８０ｃｍ左右，近１００ａ的现代增温，海平面处于上升阶段，上升速率为２—３ｍｍ·ａ（－１）。