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191.
Submarine external levées are constructional features that develop outside slope channel systems, and are a volumetrically significant component of continental margins. However, detailed observations of their process sedimentology and depositional architecture are rare. Extensive exposures of external levées at multiple stratigraphic intervals and well‐constrained palaeogeographic positions in the Fort Brown Formation, Karoo Basin, South Africa, have been calibrated with research boreholes. This integrated data set permits their origin, evolution and anatomy to be considered, including high‐resolution analysis of sedimentary facies distribution and characterization of depositional sub‐environments. An idealized model of the stratigraphic evolution and depositional architecture of external levées is presented, and variations can be attributed to allogenic (for example, sediment supply) and autogenic (for example, channel migration) factors. Initiation of external levée construction is commonly marked by deposition of a basal sand‐rich facies with sedimentary structures indicating rapid deposition from unconfined flows. These deposits are interpreted as frontal lobes. Propagation of the parent channel, and resultant flow confinement, lead to partial erosion of the frontal lobe and development of constructional relief (levées) by flow overspill and flow stripping. Overall fining‐upwards and thinning‐upwards profiles reflect increased flow confinement and/or waning flow magnitude through time. Identification of a hierarchy of levée elements is not possible due to the absence of internal bounding surfaces or sharp facies changes. The down‐slope taper in levée height and increasing channel sinuosity results in increasing numbers of crevasse lobe deposits, and is reflected by the increased occurrences of channel avulsion events down‐dip. External levées from the Fort Brown Formation are silt‐rich; however their stratigraphic evolution and the distribution of many components (such as sediment waves and crevasse lobe) share commonalities with mud‐rich external levées. This unique integrated data set has permitted the first high‐resolution characterization of external submarine levée systems. 相似文献
192.
Arnold Kogelnig Johannes Hübl Emma Suriñach Ignasi Vilajosana Brian W. McArdell 《Natural Hazards》2014,70(3):1713-1733
Rapid mass movements such as avalanches, debris flows, and rock fall are periodic or episodic phenomena that occur in alpine regions. Recent studies have shown that debris flows generate characteristic signals in the low-frequency infrasonic spectrum (4–15 Hz). Infrasound can travel thousands of kilometers and can still be detectable. This characteristic provides a basis for the development of wide area automated monitoring systems that can operate in locations unaffected by the activity of the process. This study focuses on the infrasound vibrations produced by a debris flow at the Lattenbach torrent, Tyrol (Austria), and by two events at the Illgraben torrent, Canton of Valais (Switzerland). The Lattenbach torrent is a very active torrent, which is located in the west of Tyrol in a geologic fault zone between the Silvrettakristallin and the Northern Limestone Alps. It has a large supply of loose sediment. The Illgraben torrent, which is well known for its frequent sediment transport and debris flow activity, has been equipped with instruments for debris flow monitoring since the year 2000. This study shows that debris flow emits low-frequency infrasonic signals that can be monitored and correlated with seismic signals. During the passage of the debris flow, several surges were identified by ultrasonic gauges and detected in the time series and the running spectra of infrasonic data. 相似文献
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194.
Hydrogeological and geophysical investigations demonstrated the existence of an epikarstic zone in a carbonate aquifer of Southern Italy, about 10 m thick. Nevertheless, the hydrogeological behaviour of the epikarst is different from that schematized by several authors. In the test site, the contrast in permeability at the bottom of the epikarst does not cause retention of percolation and storage of water in a perched temporary aquifer within the uppermost portion of the carbonate medium. Because of the high fracture density and good interconnection of openings within the underlying limestone, the percolation is diffuse also below the epikarstic zone, as well as the groundwater flow. The ‘funnelling’ effect into larger shafts does not play an important role on the hydrogeological behaviour of the aquifer. 相似文献
195.
Thi Phuong Quynh Le Nhu Da Le Viet Nga Dao Emma Rochelle-Newall Thi Mai Huong Nguyen Cyril Marchand Thi Thuy Duong Thi Xuan Binh Phung 《Environmental Earth Sciences》2018,77(18):658
Global riverine carbon concentrations and fluxes have been impacted by climate and human-induced changes for many decades. This paper aims to reconstruct the longterm carbon concentrations and carbon fluxes of the Red River, a system under the coupled pressures of environmental change and human activity. Based on (1) the relationships between particulate and dissolved organic carbon (POC, DOC) or dissolved inorganic carbon (DIC), and suspended sediments (TSS) or river water discharge and on (2) the available detailed historical records of river discharge and TSS concentration, the variations of the Red River carbon concentration and flux were estimated for the period 1960–2015. The results show that total carbon flux of the Red River averaged 2555?±?639 kton C year?1. DIC fluxes dominated total carbon fluxes, representing 64% of total, reflecting a strong weathering process from carbonate rocks in the upstream basin. Total carbon fluxes significantly decreased from 2816 kton C year?1 during the 1960s to 1372 kton C year?1 during the 2010s and showed clear seasonal and spatial variations. Organic carbon flux decreased in both quantity and proportion of the total carbon flux from 40.9% in 1960s to 14.9% in 2010s, reflecting the important impact of dam impoundment. DIC flux was also reduced over this period potentially as a consequence of carbonate precipitation in the irrigated, agricultural land and the reduction of the Red River water discharge toward the sea. These decreases in TSS and carbon fluxes are probably partially responsible for different negatives impacts observed in the coastal zone. 相似文献
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198.
Göran Björk Leif G. Anderson Martin Jakobsson Dennis Antony Björn Eriksson Patrick B. Eriksson Benjamin Hell Sofia Hjalmarsson Timothy Janzen Sara Jutterström Johanna Linders Ludvig Löwemark Christian Marcussen K. Anders Olsson Bert Rudels Emma Sellén Morten Sølvsten 《Deep Sea Research Part I: Oceanographic Research Papers》2010,57(4):577-586
The LOMROG 2007 expedition targeted the previously unexplored southern part of the Lomonosov Ridge north of Greenland together with a section from the Morris Jesup Rise to Gakkel Ridge. The oceanographic data show that Canadian Basin Deep Water (CBDW) passes the Lomonosov Ridge in the area of the Intra Basin close to the North Pole and then continues along the ridge towards Greenland and further along its northernmost continental slope. The CBDW is clearly evident as a salinity maximum and oxygen minimum at a depth of about 2000 m. The cross-slope sections at the Amundsen Basin side of the Lomonosov Ridge and further south at the Morris Jesup Rise show a sharp frontal structure higher up in the water column between Makarov Basin water and Amundsen Basin water. The frontal structure continues upward into the Atlantic Water up to a depth of about 300 m. The observed water mass division at levels well above the ridge crest indicates a strong topographic steering of the flow and that different water masses tend to pass the ridge guided by ridge-crossing isobaths at local topographic heights and depressions. A rough scaling analysis shows that the extremely steep and sharply turning bathymetry of the Morris Jesup Rise may force the boundary current to separate and generate deep eddies. 相似文献
199.
Although tourism and recreation can bring economic benefits to an area, the presence of visitors may adversely impact biodiversity, particularly if they make use of sensitive environments. It is anticipated that the effects of global climate change alone may increase the vulnerability of many environments, but these effects may be magnified if warmer and drier weather encourages more visitors, or makes them more likely to participate in ecologically damaging activities. Using case study sites from the UK, this study examines how different types of beach visitors make use of coastal environments. Via a series of visitor surveys, information is elicited on the environmental preferences of a range of visitor types including walkers, bird watchers, and bathers. The use of different habitats by these visitors is also assessed via an analysis of walking routes undertaken in a Geographical Information System. From this, an assessment is made of the likely present day biodiversity impacts arising from different coastal users, and how these may change under a modified climate. This study finds that whilst higher temperatures are expected to increase visitor numbers, warmer weather may encourage greater participation in low impact activities such as bathing. The findings are discussed in the context of coastal management. 相似文献
200.
Emma Newton Howell G M Edwards David Wynn-Williams Julian A Hiscox 《Astronomy& Geophysics》2000,41(5):5.28-5.31
Emma Newton , Howell G M Edwards , David Wynn-Williams and Julian A Hiscox review the combination of biology, geology and physics necessary to search for life off Earth. 相似文献