摘要:

AbstractNahal Paran drains 3600 km2of Egypt's Sinai peninsula and Israel's Negev Desert. Much of the channel is alluvial, but a canyon 10·5 km long has been incised into Late Cretaceous chert and dolomite in the lower portion of the basin. Slackwater deposits and paleostage indicators preserved within the canyon record approximately 10 floods of 200 to 2500 m3s−1over a period of at least 350 years. Step‐backwater simulations of flood‐flow hydraulics indicate extreme variations in stream power per unit area along the length of the canyon, and associated variability in energy expenditure and sediment transport. These variations reflect channel cross‐sectional morphology. The greatest values of stream power occur along the lower half of the study reach, in association with three pronounced knickpoints and an inner channel. The locations of these features reflect the exposure of thick, resistant chert layers along the channel. The presence of several similar, but buried and inactive, knickpoints along the upper study reach indicates that the locus of most active channel incision has shifted with time, probably in response to baselevel changes associated with tectonic activity along the Dead Sea Rift. Thus, the rate and manner of channel incision along the canyon of Nahal Paran are controlled by lithologic variability and tectonic uplift as they influence channel morphology and gradient, which in turn influence hydraulics and sediment t

ISSN:0197-9337    

DOI:10.1002/esp.3290190102

出版商:John Wiley&Sons, Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractThis paper investigates the development of till fabric associated with glaciers overlying deformable beds. First, the till fabrics from both modern and ancient, soft bed and hard bed subglacial environments are investigated, and then soft bedded sites with specific strain histories are examined. The results indicate that lodgement tills had a strong fabric, whilst there is a wide range of much weaker fabric strengths associated with soft bed tills. It is suggested from a theoretical study of soft bed till deposition that longitudinal and vertical processes in the subglacial deforming layer are important in fabric development. Field sites were chosen to isolate these two factors, and fabrics investigated in areas of known relative shear strain, and known relative thickness of the deforming layer. The results indicate that: (1) for a fixed deforming layer, fabric strength does increase with increasing deformation (except at very high strains where the development of a transverse fabric can lead to a reduction in the overall fabric strain); and (2) lodgement tills and soft bed tills associated with a thin deforming layer have a high fabric strength, whilst soft bed tills associated with a thick deforming layer have a low fabric strength. Where the deforming layer is thin, clasts are oriented parallel with the glacier direction either by lodging against a hard bed or ploughing against a soft bed. However, where the deforming layer is thicker and shear strains are high, melt‐out processes encourage a parallel fabric, but the increased shear strain and changes in the deforming layer thickness cause the clasts to develop both a parallel and transverse fabric. These processes combine to reduce the overall fabric strength. Till fabric studies are a further sedimentological technique to separate the depositional processes associated with a homogeneous til

ISSN:0197-9337    

DOI:10.1002/esp.3290190103

出版商:John Wiley&Sons, Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractNearly 200 analyses of meltwaters, ice and snow from three alpine glacial sites in carbonate terrain are summarized and discussed in terms of sources of solutes and kinetic controls on the progress of weathering reactions. Most data derive from the Swiss Glacier de Tsanfleuron which is based on Cretaceous and Tertiary pure and impure limestones. Two other sites (Marmolada, Italian Dolomites and the Saskatchewan Glacier, Alberta) are based on a mixed calcitic‐dolomitic substrate.Most solutes originate from carbonate dissolution; moreover, where pyrite is present its oxidation supplies significant sulphate and forces more dissolution of carbonate. The ratios Sr2+/Ca2+and Mg2+/Ca2+are much higher in Tsanfleuron melt‐waters than local bedrock, a phenomenon that can be reproduced in the laboratory at small percentages of dissolution. These anomalous ratios are attributed to incongruent dissolution of traces of the metastable carbonates Mg‐calcite and aragonite. These phases also provide Na+to solution. K+is argued to originate mainly by ion‐exchange on clays with solute Ca2+. Quartz and very minor feldspar dissolution are also inferred. Locally enhanced input from atmospheric sources is recognized by high Cl−and associated Na+.The progress of weathering reactions has been evaluated by the trends in the data, computer modelling and some simple laboratory experiments. The most dilute samples show a trend towards removal of CO2to low partial pressures (c.10−5.5atmospheres), reflecting initially rapid carbonate dissolution and relatively slow dissolution of gaseous CO2. Later addition of atmospheric CO2or acid from pyrite oxidation allows further carbonate dissolution, but solutions show a wide range of saturations, and CO2pressures as high as 10−2.2where pyrite oxidation is important.In a carbonate terrain, measurement of electroconductivity (corrected to 25°C) and alkalinity in the field allows the following preliminary deductions (where meq stands for milliequivalents):where S is the minimum meq(Ca2++ Mg2+) produced by simple dissolution of carbonate unconnected with pyrite oxidation. As with any proxy method, these deductions do not remove the need for chemical analysis of waters in a gi

ISSN:0197-9337    

DOI:10.1002/esp.3290190104

出版商:John Wiley&Sons, Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractBased on interpolation of thermoluminescence dates and the mean accumulation rate of 0.034 mm yr−1, four cycles of pedogenic CaCO3accumulation are found within the Loveland Loess: 415–325 ka, 325–250 ka, 250–195 ka and 195–95 ka. The four CaCO3peaks correspond chronologically to marine oxygen isotope stages 11, 9, 7 and 5, respectively. The early Wisconsin (95–70 ka) was characterized by sand dune activity. The reddish pedocomplex was formed from 70 to 35 ka under relatively warm and moist climatic conditions with a very slow rate of silt accumulation (0·016 mm yr−1). The Gilman Canyon pedocomplex, enriched in organic matter and dated at 35–20 ka, was formed under a strong physical weathering regime and a relatively high rate of silt accumulation (0·15 mm yr−1), indicating a windy, relatively moist, probably cool environment. It developed when the Laurentide ice sheet was advancing and dust content in Greenland ice core was low. The Peoria Loess was accumulated at a rate of 0·3 mm yr−1in central Kansas under cold dry conditions when the ice sheet fluctuated around its maximum position and the dust content in the Greenland ice core was the highest. Even the warm substage around 13 ka has some corresponding evidence in the central Great Plains. The well‐developed Brady Soil, dated at 10·5–8·5 ka, indicates that the early Holocene was the optimal time for soil development since 20 ka. The poorly weathered Bignell Loess might have been deposited during the Altithe

ISSN:0197-9337    

DOI:10.1002/esp.3290190105

出版商:John Wiley&Sons, Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractCoastal marsh loss in Louisiana is attributed to plane dieback caused by processes that stress vegetation, and a common landscape pattern is broken marsh that expands at the expense of surrounding unbroken marsh. We tested the hypothesis that vegetation is more stressed in broken marsh than in adjacent unbroken marsh, as indicated by vegetation aboveground biomass, species diversity and soil Eh, on transects that extended from broken marsh to unbroken marsh at Marsh Island, Louisiana. Soil Eh, vegetation above‐ground biomass and species diversity did not differ between broken marsh and unbroken marsh, and above‐ground biomass was similar to that reported from other marshes. Thus, we rejected the hypothesis that marsh loss is related to vegetation stress. Two factors were related to vegetation vigour: soil drainage and soil bulk density. Surprisingly, significant soil drainage occurred in broken marsh but not in unbroken marsh. Above‐ground biomass of the dominant plant,Spartina patens(Aiton) Muhl., was lowest where soil bulk density was less than 0‐08 gcm−3, which illustrated the importance of mineral matter accumulation in submerging coastal marshes. The mechanism of marsh loss appeared to be erosion below the living root zone, as indicated by the vertical and often undercut marsh‐water interface, and by the separation of sod clasts. This is different from more rapid marsh loss associated with plant stress which we observed in other Louisiana marshes only 135 km away, indicating that marsh loss mechanisms can vary spatially even within a relatively s

ISSN:0197-9337    

DOI:10.1002/esp.3290190106

出版商:John Wiley&Sons, Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractLandsliding in eastern Scotland results from high‐magnitude rainstorms generated under either cyclonic or anticyclonic conditions, particularly during the summer. Data from Aviemore indicated that cyclonic storms produce higher rainfall totals than anticyclonic storms, as well as being of longer duration and lower intensity. The distribution of rain during individual storms also varies with the synoptic conditions under which the storms are produced. These different rainfall characteristics produce different geomorphic responses, which can be investigated in detail using physical based modelling. In this paper, a physically based coupled hydrology–stability model is used to assess the significance of these rainfall characteristics to soil moisture response and slope instability for mature podsols. The results provide evidence that rainstorms of different synoptic origin produce varying hydrological response, involving both the extent and the timing of moisture content change. This affects the depth and timing of slope failure, with anticyclonic storms promoting a large, rapid response in the factor of safety at shallow depths within the soil. Cyclonic storms produce a more gradual response, with the region of probable failure being deeper. Futhermore, each of these storm types is associated with different rainfall distributions, and this is also shown to have a significant effect on the timing and depth of slope instabil

ISSN:0197-9337    

DOI:10.1002/esp.3290190107

出版商:John Wiley&Sons, Ltd    

年代:1994

数据来源: WILEY

ISSN:0197-9337    

DOI:10.1002/esp.3290190101

出版商:John Wiley&Sons, Ltd    

年代:1994

数据来源: WILEY