Stability,morphology and surface grain size patterns of channel bifurcation in gravel–cobble bedded anabranching rivers     

Leif M. Burge 《地球表面变化过程与地形》2006,31(10):1211-1226

This study presents the first detailed field‐based analysis of the morphology of bifurcations within anabranching cobble–gravel rivers. Bifurcations divide the flow of water and sediment into downstream anabranches, thereby influencing the characteristics of the anabranches and the longevity of river islands. The history, morphology, bed grain size, and flow vectors at five bifurcations on the Renous River, New Brunswick, Canada, were studied in detail. The angles of bifurcations within five anabranching rivers in the Miramichi basin were investigated. The average bifurcation angle was 47°, within the range of values cited for braided river bifurcations. Bifurcation angle decreased when anabranches were of similar length. Shields stresses in channels upstream of bifurcations were lower than reported values for braided rivers. Stable bifurcations displayed lower Shields stresses than unstable bifurcations, contrary to experimental results from braided river bifurcations. Bifurcations in anabranching rivers are stabilized by vegetation that slows channel migration and helps to maintain a uniform upstream flow field. The morphology of stable bifurcations enhances their stability. A large bar, shaped like a shallow ramp that increases in elevation to floodplain level, forms at stable bifurcations. Floodplains at stable bifurcations accrete upstream at rates between 0·9 and 2·5 m a^?1. Bars may also form within the entrance of an anabranch downstream of the bifurcation node. These bars are associated with bifurcation instability, forming after a period of stability or an avulsion. Channel abandonment occurs when a bar completely blocks the entrance to one anabranch. The stability of channels upstream of bifurcations and the location of bars at bifurcations influence bifurcation stability and the maintenance of river anabranching in the long term. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

Benthic trophic status and nutrient fluxes in shallow-water sediments     

Anna Engelsen  Stefan Hulth  Leif Pihl  Kristina Sundbck 《Estuarine, Coastal and Shelf Science》2008,78(4):783-795

Proliferation of fast-growing ephemeral macroalgae in shallow-water embayments constitutes a large-scale environmental change of coastal marine ecosystems. Since inorganic nutrients essential for the initiation and maintenance of macroalgal growth may be supplied from the underlying sediment, we investigated the coupling between benthic inorganic nutrient (mainly N and P) fluxes and sediment properties in 6 bays representing a wide gradient of sediment characteristics (grain size, organic matter content, solid phase C and N). The initial characterization of bays was made in June and also included measurements of oxygen flux and microphytobenthic and macrofaunal biomass. In September, still within the growth season of the macroalgae, complementary experiments with sediment-water incubations for benthic flux measurements of oxygen and nutrients focused on trophic status (balance between auto- and heterotrophy) as a controlling factor for rates of measured benthic nutrient fluxes. Generally, sediments rendered autotrophic by microphytobenthic photosynthesis removed nutrients from the overlying water, while heterotrophic sediments supplied nutrients to the overlying bottom water. Estimations of the green-algal nutrient demand suggested that late in the growth season, net heterotrophic sediments could cover 20% of the N-demand and 70% of the P demand. As the benthic trophic status is a functional variable more closely coupled to nutrient fluxes than the comparably conservative structural parameter organic matter content, we suggest that the trophic status is a more viable parameter to classify sediments and predict benthic nutrient fluxes in shallow-water environments.  相似文献   

Configuration,history and impact of the Norwegian Channel Ice Stream     

Hans Petter Sejrup  Eiliv Larsen  Haflidi Haflidason  Ida M. Berstad  Berit O. Hjelstuen  Hafdis E. Jonsdottir  Edward L. King  Jon Landvik  Oddvar Longva  Atle Nygrd  Dag Ottesen  Stle Raunholm  Leif Rise  Knut Stalsberg 《Boreas: An International Journal of Quaternary Research》2003,32(1):18-36

The Norwegian Channel between Skagerrak, in the southeast, and the continental margin of the northern North Sea, in the northwest, is the result of processes related to repeated ice stream activity through the last 1.1 m yr. In such periods the Skagerrak Trough (700 m deep) has acted as a confluence area for glacial ice from southeastern Norway, southern Sweden and parts of the Baltic. Possibly related to the threshold in the Norwegian Channel off Jæren (250 m deep), the ice stream, on a number of occasions over the last 400 ka, inundated the coastal lowlands and left an imprint of NW‐oriented ice directional features (drumlins, stone orientations in tills and striations). Marine interstadial sediments found up to 200 m a.s.l. on Jæren have been suggested to reflect glacial isostasy related to the Norwegian Channel Ice Stream (NCIS). In the channel itself, the ice stream activity is evidenced by mega‐scale glacial lineations on till surfaces. As a result of subsidence, the most complete sedimentary records of early phases of the NCIS are preserved close to the continental margin in the North Sea Fan region. The strongest evidence for ice stream erosion during the last glacial phase is found in the Skagerrak. On the continental slope the ice stream activity is evidenced by the large North Sea Fan, which is mainly a result of deposition of glacial‐fed debris flows. Northwards of the North Sea Fan, rapid deposition of meltwater plume deposits, possibly related to the NCIS, is detected as far north as the Vøring Plateau. The NCIS system offers a unique possibility to study ice stream related processes and the impact the ice stream development had on open ocean sedimentation and circulation.  相似文献   

Onshore climate change during the Late Triassic marine inundation of the Central European Basin     

Anders Ahlberg  Leif Arndorff  & Dorothy Guy-Ohlson 《地学学报》2002,14(4):241-248

Sedimentological, pedological and palynological evidence reveal radically increasing onshore humidity during the Rhaetian marine invasion of central Europe along the north-eastern margin of the Central European Basin (southern Sweden). Pre-Rhaetian aridity favoured the formation of Carnian redbeds with calcrete, which were succeeded by Norian hematite-cemented conglomerates, arkoses, arkosic wackestones, and smectititic mudstones deposited on braidplains and in lakes. Superimposed autochthonous coals and gleysols indicate the Rhaetian onset of year-round humidity. Chemically mature sandstones, kaolinitic mudstones and luvisols also formed at this time, influenced by a permanent vegetation cover which lowered soil pH and strongly intensified chemical weathering. The Rhaetian deposits accumulated in floodplain lakes repeatedly subjected to sediment infill, plant colonization and palaeosol development. The humidity shift resembles that contemporaneously recorded in the North Sea region.  相似文献   

Quantized Redshifts of Galaxies: Stimulated Raman Scattering in Cold Intergalactic Rydberg Matter     

Leif Holmlid 《Astrophysics and Space Science》2004,291(2):99-111

That the redshifts for galaxies in the local supercluster are quantizedwas recently confirmedby Guthrie and Napier(A&Amp;Amp;A310 (1996) 353). These redshifts are here proposed to be due to stimulatedStokes Raman processes in intergalactic matter in the form of Rydberg Matter (RM). Rydberg Matteris an electronically excited material, as demonstrated by its use as laser medium in a thermally excitedultra-broadband tunable IR laser (Chem. Phys. Lett. 376 (2003) 812). Its existence in interstellar andintergalactic space is demonstrated by several observational results, notably the unidentified IR bands,that agree well with the emission from Rydberg Matter. A stimulated Raman process will allow theH I 21 cm radiation to proceed without deflection, in agreement with observation. Such redshiftswill be additive during the passage through space. The process in Rydberg Matter here proposed togive rise to the Stokes Raman process is excitation of electronic translational modes in the planarclusters forming the matter. The specific cluster sizes found in laboratory experiments give rise toa few differently sized redshift quanta, which is in good agreement with the observed quanta. Anexcitation level (principal quantum number) of Rydberg Matter in intergalactic space between 175and 200 gives the correct size of the redshift quanta.  相似文献   

Flow of Canadian basin deep water in the Western Eurasian Basin of the Arctic Ocean     

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   

The recession of the Inland Ice margin during the Holocene climatic optimum in the Jakobshavn Isfjord area of West Greenland     

Anker Weidick  Hans Oerter  Niels Reeh  Henrik Hjmark Thomsen  Leif Thorning 《Global and Planetary Change》1990,2(3-4)

Recent subsurface mapping of parts of the Greenland Inland Ice margin in the region of Jakobshavn Isbræ indicates that the fjord system in the period of at least 2700–4700 calendar yr B.P. was more ice free than at present, and that the front of the glacier was at least 15 km behind the present position. The ¹⁴C-dating of subfossils brought to the present ice margin fit with the climatic records from ice cores and confirm the favourable conditions for Greenland's first settlers, the Sarqaq people, who arrived in the region about 400 yr ago to find hunting grounds 10–20% larger than the present.  相似文献   

Coronal magnetic structure at a solar sector boundary     

Wilcox  John M.  Svalgaard  Leif 《Solar physics》1974,34(2):461-470

The persistent large-scale coronal magnetic structure associated with a sector boundary appears to consist of a magnetic arcade loop structure extending from one solar polar region to the other in approximately the north-south direction. This structure was inferred from computed coronal magnetic field maps for days on which a stable magnetic sector boundary was near central meridian, based on an interplanetary sector boundary observed to recur during much of 1968 and 1969.  相似文献   

Evaluation of map precision by means of the area of influence method     

Ingmar Lundström  Leif Björk 《Mathematical Geology》1979,11(6):701-715

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Ecological tree line history and palaeoclimate – review of megafossil evidence from the Swedish Scandes     

Leif Kullman 《Boreas: An International Journal of Quaternary Research》2013,42(3):555-567

The postglacial tree line and climate history in the Swedish Scandes have been inferred from megafossil tree remains. Investigated species are mountain birch (Betula pubescens ssp. czerepanovii), Scots pine (Pinus sylvestris) and grey alder (Alnus incana). Betula and Pinus first appeared on early deglaciated nunataks during the Lateglacial. Their tree lines peaked between 9600 and 9000 cal. a BP, almost 600 m higher than present‐day elevations. This implies (adjusted for land uplift) that early Holocene summer temperatures may have been 2.3°C above modern ones. Elevational tree line retreat characterized the Holocene tree line evolution. For short periods, excursions from this trend have occurred. Between c. 12 000 and 10 000 cal. a BP, a pine‐dominated subalpine belt prevailed. A first major episode of descent occurred c. 8200 cal. a BP, possibly forced by cooling and an associated shift to a deeper and more persistent snow pack. Thereafter, the subalpine birch forest belt gradually evolved at the expense of the prior pine‐dominated tree line ecotone. A second episode of pine descent took place c. 4800 cal. a BP. Historical tree line positions are viewed in relation to early 21st century equivalents, and indicate that tree line elevations attained during the past century and in association with modern climate warming are highly unusual, but not unique, phenomena from the perspective of the past 4800 years. Prior to that, the pine tree line (and summer temperatures) was consistently higher than present, as it was also during the Roman and Medieval periods, c. 1900 and 1000 cal. a BP, respectively.  相似文献