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651.
Keith Walters John J. Hutchens Jr. Eric T. Koepfler James O. Luken 《Aquatic Sciences - Research Across Boundaries》2010,72(3):309-324
Anthropogenic alteration of terrestrial shorelines can have pronounced effects on marine environments at the upland-marsh
boundary. Possible terrestrial development effects on several physical and biological variables of high-marsh habitats were
examined along developed and undeveloped shorelines in an ocean-dominated, southeastern US estuary. Analyses of sediment characteristics
suggested development of the upland boundary affected physical conditions within the high-marsh. For example, pore water salinities
were greater along undeveloped shorelines during a non-drought period even after rain events. Significant floral and faunal
differences also existed between shoreline treatments. Black needle rush stems were significantly taller and marsh periwinkle
densities significantly greater, but eastern coffee bean snail densities were significantly reduced along developed shorelines.
Benthic infaunal community abundance and composition also were significantly different between shoreline treatments with sand
fly larvae, human pest precursors, either only present or present in greater densities along developed shorelines. Sediment
respirometry experiments indicated significant differences in heterotrophic and autotrophic processes occurring between shoreline
treatments. Greater sediment surface temperatures along developed shorelines provided one possible mechanism driving high-marsh
responses to boundary alteration. The history and extent of shoreline development along with a tendency in ocean-dominated
southeastern marshes to resist change likely influenced current ecological conditions within our high-marsh study areas. A
greater understanding of the driving mechanisms producing localized effects on salt marshes and recognizing regional differences
in marsh resistance to change will facilitate predictions of shoreline development consequences and help in proposing effective
management strategies for coastal boundaries. 相似文献
652.
Mollusc death assemblages were recovered in 98 subtidal sampling stations on the seafloor of the shallow Pertuis Charentais Sea (Atlantic coast of France). Taxonomic composition and spatial distribution of death assemblages were investigated, as well as their response to sediment grain size (field data), bottom shear stress (coupled tide and wave hydrodynamic modelling), and sediment budget (bathymetric difference map). Results showed that molluscs are likely to be reliable paleoenvironmental indicators since death assemblages were able to acquire ecological changes within years (decadal-scale taphonomic inertia), and live–dead agreement inferred from existing data on living benthic communities was high, except close to river mouths and intertidal mudflats that provide terrestrial and intertidal species to subtidal death assemblages, respectively. Taxonomic composition of these within-habitat death assemblages strongly depended on sediment grain size and bottom shear stress, similarly to living subtidal communities. Post-mortem dispersal of shells, owing to relatively low bottom shear stress in the area, was only of a few 10s to 100s of meters, which shows that death assemblages preserved environmental gradients even at a fine spatial scale. Sediment budget had also a significant influence on death assemblages. Thick-shelled epifaunal species were correlated with erosion areas on one side, and thin-shelled infaunal species with deposition on the other, showing that mollusc fossil assemblages could be used as indicators of paleo-sedimentation rate. This new proxy was successfully tested on a previously published Holocene mollusc fossil record from the same area. It was possible to refine the paleoenvironmental interpretation already proposed, in accordance with existing stratigraphic and sedimentological data. 相似文献
653.
654.
C.?Ian?SchipperEmail author Jonathan?M.?Castro Hugh?Tuffen Fabian?B.?Wadsworth Debra?Chappell Andres?E.?Pantoja Mark?P.?Simpson Eric?C.?Le Ru 《Bulletin of Volcanology》2015,77(5):34
Cristobalite is a low-pressure high-temperature polymorph of SiO2 found in many volcanic rocks. Its volcanogenic formation has received attention because (1) pure particulate cristobalite can be toxic when inhaled, and its dispersal in volcanic ash is therefore a potential hazard; and (2) its nominal stability field is at temperatures higher than those of magmatic systems, making it an interesting example of metastable crystallization. We present analyses (by XRD, SEM, EPMA, Laser Raman, and synchrotron μ-cT) of representative rhyolitic pyroclasts and of samples from different facies of the compound lava flow from the 2011–2012 eruption of Cordón Caulle (Chile). Cristobalite was not detected in pyroclasts, negating any concern for respiratory hazards, but it makes up 0–23 wt% of lava samples, occurring as prismatic vapour-deposited crystals in vesicles and/or as a groundmass phase in microcrystalline samples. Textures of lava collected near the vent, which best represent those generated in the conduit, indicate that pore isolation promotes vapour deposition of cristobalite. Mass balance shows that the SiO2 deposited in isolated pore space can have originated from corrosion of the adjacent groundmass. Textures of lava collected down-flow were modified during transport in the insulated interior of the flow, where protracted cooling, additional vesiculation events, and shearing overprint original textures. In the most slowly cooled and intensely sheared samples from the core of the flow, nearly all original pore space is lost, and vapour-deposited cristobalite crystals are crushed and incorporated into the groundmass as the vesicles in which they formed collapse by strain and compaction of the surrounding matrix. Holocrystalline lava from the core of the flow achieves high mass concentrations of cristobalite as slow cooling allows extensive microlite crystallization and devitrification to form groundmass cristobalite. Vapour deposition and devitrification act concurrently but semi-independently. Both are promoted by slow cooling, and it is ultimately devitrification that most strongly contributes to total cristobalite content in a given flow facies. Our findings provide a new field context in which to address questions that have arisen from the study of cristobalite in dome eruptions, with insight afforded by the fundamentally different emplacement geometries of flows and domes. 相似文献
655.
656.
The role of soil in vegetated gravelly river braid plains: more than just a passive response?
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This paper reviews the role of alluvial soils in vegetated gravelly river braid plains. When considering decadal timescales of river evolution, we argue that it becomes vital to consider soil development as an emergent property of the developing ecosystem. Soil processes have been relatively overlooked in accounts of the interactions between braided river processes and vegetation, although soils have been observed on vegetated fluvial landforms. We hypothesize that soil development plays a major role in the transition (speed and pathway) from a fresh sediment deposit to a vegetated soil‐covered landform. Disturbance (erosion and/or deposition), vertical sediment structure (process history), vegetation succession, biological activity and water table fluctuation are seen as the main controls on early alluvial soil evolution. Erosion and deposition processes may not only act as soil disturbing agents, but also as suppliers of ecosystem resources, because of their role in delivering and changing access (e.g. through avulsion) to fluxes of water, fine sediments and organic matter. In turn, the associated initial ecosystem may influence further fluvial landform development, such as through the trapping of fine‐grained sediments (e.g. sand) by the engineering action of vegetation and the deposit stabilization by the developing aboveground and belowground biomass. This may create a strong feedback between geomorphological processes, vegetation succession and soil evolution which we summarize in a conceptual model. We illustrate this model by an example from the Allondon River (Switzerland) and identify the research questions that follow. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
657.
Wim Clymans Eric Struyf An Van den Putte Christoph Langhans Zhengang Wang Gerard Govers 《地球表面变化过程与地形》2015,40(9):1171-1181
Amorphous silica (ASi) carried in suspension by rivers is an important component in the global Si budget. Water erosion processes in cultivated catchments are likely to drive ASi delivery to the river system. However, no studies have investigated the controls on ASi mobilization by erosional processes in croplands. Rainfall experiments were performed on split fields (i.e. a part conventionally ploughed and a part under reduced tillage) to simulate ASi mobilization by inter‐rill erosion in croplands, and identify its dependency on soil, field and rainfall characteristics. The ASi content of the soil and the inter‐rill erosion rate were determined as the major controls on ASi mobilization. Variables such as tillage technique and crop type did not have a consistent direct or indirect effect. Inter‐rill erosion is clearly selective with respect to ASi, indicating association of ASi with the fine soil fraction and with soil organic carbon. Our experiments demonstrate that erosion increases due to human perturbation will increase the delivery of reactive Si to aquatic systems. We estimate that globally, c. 7% of all reactive Si that enters aquatic systems is derived from erosion of agricultural soils. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
658.
Eric Bardou Guillaume Favre‐Bulle Claire Faucheux Nicolas Jeannée Pascal Ornstein 《地球表面变化过程与地形》2015,40(9):1191-1201
Complex para‐glacial systems may show signs of destabilization leading to frequent and potentially hazardous debris‐flows. Understanding creeping permafrost displacement over a period of time is therefore crucial for hazard management and risks assessment. This paper presents our methodology for estimating creeping permafrost displacement based upon data derived from various survey methods and demonstrates its relevance on the Glacier Bonnard system in southern Switzerland. Geostatistical processing allowed estimation of the displacement intensity over the area of interest, as well as assessment of the interpolation quality. Although the local measurement network needs to be refined, the results largely improve the understanding of the Bonnard para‐glacial system, though highlighting the need to locally refine the measurement network. In the present case, the destabilized front advances at ~1 m/a and the upstream creeping part at ~0.4 m/a. Variance analysis also provide objective thresholds that could be used to distinguish underlying physical processes. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
659.
Yongcai Tang Yongshun John Chen Yuanyuan V. Fu Haiyang Wang Shiyong Zhou Eric Sandvol Jieyuan Ning Yongge Feng Mian Liu 《地震科学(英文版)》2010,23(6):549-553
Located at the northeastern margin of the Tibetan plateau,the Ordos block is a stable tectonic unit in North China.With its active boundary fault zones,the Ordos block played an important role in the eastward extrusion mechanism of the Tibetan plateau.Peking University deployed a linear array of 15 portable broadband seismometers across the western Weihe graben during September 2005 to August 2006 and later a 2-D seismic array(Southwest Ordos Array) of 14 portable broadband seismometers during 2007-2008 at its southwestern boundary.Analyses of shear wave splitting of SKS and SKKS phases at these stations show that the fast directions trend ~110° with an average delay time of 0.9 s in the southwestern margin of the Ordos block.The agreement between the lithosphere deformation indicated by GPS data and Quaternary fault slip-rate observations and the mantle flow represented by shear wave splitting implies that accordant deformation patterns from lithosphere to asthenosphere in relation to the eastward extrusion of the Tibetan plateau could extend at least to 200 km depth.Spatial distribution of splitting polarization directions indicates that the mantle flow driven by the Tibetan plateau is blocked by the Ordos block and locally restricted in a narrow channel along the Qinling-Dabie fault zones between the Ordos block and Sichuan basin. 相似文献
660.
Lydia A. Olaka Eric O. Odada Martin H. Trauth Daniel O. Olago 《Journal of Paleolimnology》2010,44(2):629-644
Sequences of paleo-shorelines and the deposits of rift lakes are used to reconstruct past climate changes in East Africa.
These recorders of hydrological changes in the Rift Valley indicate extreme lake-level variations on the order of tens to
hundreds of meters during the last 20,000 years. Lake-balance and climate modeling results, on the other hand, suggest relatively
moderate changes in the precipitation-evaporation balance during that time interval. What could cause such a disparity? We
investigated the physical characteristics and hydrology of lake basins to resolve this difference. Nine closed-basin lakes,
Ziway-Shalla, Awassa, Turkana, Suguta, Baringo-Bogoria, Nakuru-Elmenteita, Naivasha, Magadi-Natron, Manyara, and open-basin
Lake Victoria in the eastern branch of the East African Rift System (EARS) were used for this study. We created a classification
scheme of lake response to climate based on empirical measures of topography (hypsometric integral) and climate (aridity index).
With reference to early Holocene lake levels, we found that lakes in the crest of the Ethiopian and Kenyan domes were most
sensitive to recording regional climatic shifts. Their hypsometric values fall between 0.23–0.29, in a graben-shaped basin,
and their aridity index is above unity (humid). Of the ten lakes, three lakes in the EARS are sensitive lakes: Naivasha (HI
= 0.23, AI = 1.20) in the Kenya Rift, Awassa (HI = 0.23, AI = 1.03) and Ziway-Shalla (HI = 0.23, AI = 1.33) in the Main Ethiopian
Rift (Main Ethiopian Rift). Two lakes have the graben shape, but lower aridity indices, and thus Lakes Suguta (HI = 0.29,
AI = 0.43) and Nakuru-Elmenteita (HI = 0.30, AI = 0.85) are most sensitive to local climate changes. Though relatively shallow
and slightly alkaline today, they fluctuated by four to ten times the modern water depth during the last 20,000 years. Five
of the study lakes are pan-shaped and experienced lower magnitudes of lake level change during the same time period. Understanding
the sensitivity of these lakes is critical in establishing the timing or synchronicity of regional-scale events or trends
and predicting future hydrological variations in the wake of global climate changes. 相似文献