Particle clusters in gravel‐bed rivers: an experimental morphological approach to bed material transport and stability concepts

Structured gravel river beds clearly exert a major influence on bed stability. Indexing structural stability by field measurements of bed strength neglects the processes operating to entrain and transport bed material in different parts of each structure. This study takes a morphological approach to interpreting the critical processes, using particle tracing to determine the movement of individual cluster particles over a range of flood event magnitudes and durations. The experiment was carried out on the River South Tyne, UK; it uses flow hydrographs measured nearby and also benefits from previous studies of historical development, channel morphology and sediment transport at the same site. More than 30 clusters were monitored over a seven‐month period during which clusters occupied 7–16 per cent of the bed. Threshold flows delimiting three apparently contrasting bed sediment process regimes for cluster particles are tentatively set at 100 m³ s^?1 and 183 m³ s^?1; durations of flow at these levels are critical for cluster development, rather than flow peak values. Wake particles are transported most easily. Flow straightening in the wandering channel planform reduces the stability of clusters, since mechanical strength is markedly reduced by this change of direction. The overall area covered by clusters between significant transport events varies little, implying a dynamic equilibrium condition. Copyright © 2005 John Wiley & Sons, Ltd.     

Bimodal wave spectra in lower Chesapeake Bay, sea bed energetics and sediment transport during winter storms

The transition zone separating estuarine environments from the coastal ocean is characterized not only by distinctive morphological and sedimentary trends but by unique hydrodynamic forces as well. Lower Chesapeake Bay, a large coastal estuary within the Mid-Atlantic Bight of the U.S. East Coast, experiences complex wave and current-induced forces produced during winter storms. Wave and current measurements made near Thimble Shoal Light over five winter seasons show that most storms simultaneously produce both ocean and bay-generated wave trains that appear as distinct bimodal peaks in directional spectra. Analysis of selected storm wave records reveal that lower-frequency ocean waves, although nominally lower in amplitude than higher-frequency bay waves, are roughly equivalent to bay waves in terms of energy expended on beds of fine- to medium-grained sand at either end of the Thimble Shoal Channel. Grain-friction energy dissipation estimates calculated for waves and currents suggest that waves provide more net energy capable of transporting bottom sediment than currents, although strong barotropic flows briefly encountered during a major storm on 13–14 March 1993, exceeded wave energy expended at the bed by almost an order of magnitude. From analyses of wave orbital velocity spectra, it is shown that dual wave trains characterized by differences in peak frequency and direction may assist each other through interactions that increase their combined contribution to frictional energy dissipation and inferred sediment transport at the bed.     

Sulfur diffusion in rhyolite melts

 Diffusion rates for sulfur in rhyolite melt have been measured at temperatures of 800–1100° C, water contents of 0–7.3 wt%, and oxygen fugacities from the quartz-fayalite-magnetite buffer to air. Experiments involved dissolution of anhydrite or pyrrhotite into rhyolite melt over time scales of hours to days. Electron microprobe analysis was used to measure sulfur concentration profiles in the quenched glasses. Regression of the diffusion data in dry rhyolite melt gives D_sulfur=0.05·exp{−221±80RT}, which is one to two orders of magnitude slower than diffusion of other common magmatic volatiles such as H₂O, CO₂ and Cl^-. Diffusion of sulfur in melt with 7 wt% dissolved water is 1.5 to 2 orders of magnitude faster than diffusion in the anhydrous melt, depending on temperature. Sulfur is known to dissolve in silicate melts as at least two different species, S²⁻ and S⁶⁺, the proportions of which vary with oxygen fugacity; despite this, oxygen fugacity does not appear to affect sulfur diffusivity except under extremely oxidizing conditions. This result suggests that diffusion of sulfur is controlled by one species over a large range in oxygen fugacity. The most likely candidate for the diffusing species is the sulfide ion, S²⁻. Re-equilibration between S²⁻ and S⁶⁺ in oxidized melts must generally be slow compared to S²⁻ diffusion in order to explain the observed results. In a silicic melt undergoing degassing, sulfur will tend to be fractionated from other volatile species which diffuse more rapidly. This is consistent with analyses of tephra from the 1991 eruption of Mount Pinatubo, Philippines, and from other high-silica volcanic eruptions. Received: 26 April 1995 / Accepted: 1 November 1995     

Study of (010)[101] and (001)[110]/2 dislocations in K-feldspars by HRTEM and modelling

Dislocations in K-feldspars were studied by high resolution transmission electron microscopy (HRTEM) and HRTEM images were further submitted to a filtering in order to improve their interpretation. (010)[101] dislocations appear to be dissociated with (001)[001]/2 planar defect, whereas (001)[110]/2 dislocations are perfect. Structural models of planar defects in (010) and (001) planes were investigated. The energy estimation of these models was performed using the Keating potential. The structural analysis agreed with the experimental result in that dislocations can be dissociated in the (010) plane, whereas they cannot be dissociated in the (001) plane.     

Nd and Sr isotopic systematics of central Australian granulites: chronology of crustal development and constraints on the evolution of lower continental crust

Nd and Sr isotopic data are reported for a granulite terrain in the Proterozoic Arunta Block of Central Australia. Sm-Nd data from a wide range of rock types define a crust formation age of 2,070±125 Ma and provide further evidence for voluminous crustal growth in the Proterozoic. An _Nd value of +1.5±0.8 indicates a depleted mantle source for this crustal segment and there is no evidence for a large component of significantly older sialic crust. Field relationships, geochemistry and Rb-Sr data for mafic and felsic granulites indicate that intracrustal differentiation and polyphase deformation were followed by granulite facies metamorphism (Rb depletion) at 1,800 Ma. Rb-Sr data for strongly retrogressed granulites define an age of 1,700 Ma which is interpreted as the time of retrograde biotite growth. Partial melting at the presently exposed crustal level and anatexis at deeper crustal levels were broadly coeval with the retrograde metamorphism. Sm-Nd and Rb-Sr isotopic systematics of minerals indicate that the terrain cooled slowly, did not experience significant uplift until 1,000 Ma and remained at temperatures above 320° C until the late Palaeozoic. The mineral data are consistent with geological relationships and petrological evidence for a prolonged period of isobaric cooling followed by uplift late in the metamorphic evolution of the terrain. The granulite protoliths appear to have formed in a rift which closed within 280 Ma of initial separation. Deformation and granulite facies metamorphism at 1,800 Ma are interpreted to be a consequence of collision between the continental blocks which defined the rift. Regional retrogression and granitoid magmatism at 1,700 Ma are attributed to underthrusting of the granulites by lower grade rocks in the final stages of collision. Subsequent events in the cooling and uplift history appear to have been controlled by the presence of long-lived major faults in the crust and a prolonged history of episodic compression in the continental lithosphere. The results of this study suggest that granulite terrains, in general, cannot be equated with lower continental crust but instead represent assemblages of (mainly) supracrustal rocks which in some instances have been involved in major collision events.Abbreviations Opx orthopyroxene - Cpx clinopyroxene - Plag plagioclase - Hb hornblende - Ox opaque oxide - Bi biotite - Ap apatite - Zir zircon - Gt garnet - 2°CA secondary clinoamphibole - Qtz quartz - Ol olivine - Sp spinel - Se serpentine - Ep epidote - Kf alkali feldspar - Mz monazite - Sph sphene - Sill sillimanite - Rut rutile - Cd cordierite - Sphal sphalerite - Ms muscovite - Act actinolite - Cc calcite - Scap scapolite - Cor corundum - Xen xenotime - Sapph sapphirine - Ged gedrite - All allanite - Clh clinohumite - Dol dolomite - Mt magnetite - Ghn gahnite - Gal galena     

Enhancing the credibility of ecology: Is interdisciplinary research for land use planning useful?

Interdisciplinary approaches involving inputs from both the natural and social sciences represent one operational and social response to the complexity of today's environmental and land use problems. The resulting amalgam can be useful to both science and society, given the right mix of ingredients. Important steps in the planning and conduct of interdisciplinary research include problem indentification; definition of a precise scientific theme derived from the social demand; choice of relevant disciplines; selection of study area and spatial scale; ensuring that the project is adaptable to changing circumstances; obtaining the involvement of scientists, planners and local people; developing a continuum of actions from basic and applied research through to training and information diffusion; early and explicit definition of the criteria for evaluation. Though interdisciplinary approaches have shown their worth, they founder easily. Obstacles include the behavioural and psychological characteristics of individual scientists, differences in scientific method, and bottlenecks of a more administrative, institutional, political and financial nature.The views expressed are those of the authors and not necessarily those of the authors' employers.     

Enhancing the credibility of ecology: Can research be made more comparable and predictive?

A sounder theoretical basis for extrapolation and prediction, typologies of what is and what is not comparable, and representative natural areas for long-term field research are three geographically-based ingredients of comparative research in ecology. Two types of conceptual model allow assessment of the effects of phylogenesis and environment on ecosystem structure and functioning. The first is based on fixing of genetic variables and study of biological performance along a gradient of environmental conditions — an approach of sequential analysis of environmental gradients. The second is based on fixing of environmental variables and comparison of performance of different phylogenetic stocks in non-contiguous geographical areas — an approach entailing matching of disjunct ecological analogues. The two approaches are important complements in examining hypotheses of convergent and divergent evolution, in helping to define what really can be compared in comparative ecological research, and in contributing to ecology becoming a more predictive and credible science.     

Nd and Sr isotopic crustal contamination patterns in an Archaean meta-basic dyke from northern Labrador

Samples from the core to the margin of a 20 cm wide meta-dolerite dyke are sequentially enriched in K, Rb, Sr, and the light REE's. Rb-Sr and Sm-Nd compositional and isotopic profiles in the dyke are interpreted to be the result of selective contamination with components of country rock derivation, rather than the result of simple bulk mixing. ${}^{87}\text{Rb}{}^{86}\text{Sr}$ ratios are higher at the edge of the dyke than at its centre, although they are somewhat irregular, due probably to the effects of subsequent alteration. This profile and one shown by unsupported ⁸⁷Sr are both consistent with contamination of the dyke by a fluid phase derived by the breakdown of biotite. Common Sr shows a parallel, albeit weaker, contamination profile which is interpreted to reflect the contribution of a Sr-bearing phase such as plagioclase. ${}^{147}\text{Sm}{}^{144}$Nd ratios and ¹⁴⁴Nd concentrations increase and decrease respectively from the margin to the core of the dyke. In addition, the margin of the dyke is significantly less radiogenic than the interior. This contrasts with the relatively radiogenic character of an adjacent pegmatite vein. As this sample does not lie on an anticipated contamination profile between the Uivak gneisses and the dyke it is concluded that the REE contamination of the dyke occurred by the addition of a REE-enriched fluid phase which gained access to the dyke by flow along the dyke-pegmatite interface. If it is assumed that both the Rb-Sr and Sm-Nd contamination profiles are the result of diffusion limited processes, then the observations of scale made in this paper suggest that the rate of diffusion of Nd is an order-of-magnitude slower than that for Sr. In view of the scale and nature of these profiles, ages obtained from isotopic data for such mafic dykes must be interpreted with some care. Nevertheless, in spite of these limitations the ?_Nd values for the least contaminated specimens provide a clear indication that the Saglek dykes were derived from a depleted mantle source with ?_Nd? +2.