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51.
Valley-fill ignimbrites of the Middle Laacher See Tephra (MLST) in the proximal depositional fan south of Laacher See volcano are laterally continuous with fine-grained overbank-facies deposits, 0.5–1.5 m thick, covering higher elevations and interfluves between the paleovalleys. The overbank deposits consist of up to 12 ash layers, each 4–10 cm thick, which show internal structures typical of ash-flow transport, such as poor sorting, reverse size-grading of pumice, local normal grading of coarse ash-sized lithics above a fine-grained basal layer, cross-stratification behind obstacles, and erosional unconformities. Thickness, median grain-size, and number of individual layers decrease systematically with distance from the vent. Overbank ash layers thicken at the valley slopes and form discrete valley-fill ignimbrite flow units in the paleochannels with median grain size increasing from 63 m to 350 m. Toward the center of paleochannels, however, the well-defined overbank facies is obscured by mutual erosion of individual flow units. Overlapping data fields in ternary grain-size variation diagrams indicate the overbank facies to have evolved from ash flows chiefly through depletion of lapilli and coarse ash. Overbank-facies ash layers do not represent dust layers resulting from elutriation clouds of ash flows (co-ignimbrite ash) or surge deposits developed on higher ground due to low concentration of solids. They are similar in some parameters to Taupo-type ignimbrite veneer deposits, but are interpreted differently. The thin, fine-grained, Laacher See ash layers are thought to have been deposited from diluted portions of the flow proceeding directly from the eruption column while the main pyroclastic flows were confined to the paleovalleys radiating away from the eruptive center. The wide distribution of the thin ash layers is attributed to the balance of deposition from various flow parts and turbulent entraining and heating of ambient air that sustained sufficient mobility of the diluted flows to spread across hills and level ground. 相似文献
52.
N. B. Kachel G. L. Hunt Jr. S. A. Salo J. D. Schumacher P. J. Stabeno T. E. Whitledge 《Deep Sea Research Part II: Topical Studies in Oceanography》2002,49(26)
The inner front of the southeastern Bering Sea shows marked spatial variability in frontal characteristics created by regional differences in forcing mechanisms. Differences in forcing mechanisms (sea ice advance/retreat and storm strength and timing) and early spring water properties result in strong interannual variability in biological, chemical, and physical features near the front. We have developed a simple model based on surface heat flux and water-column mixing to explain the existence of cold belts (Cont. Shelf Res. 19(14) (1999) 1833) associated with such fronts. Hydrography, fluorescence and nutrient observations show that pumping of nutrients into the euphotic zone occurs, and this can prolong primary production at the inner front. The effectiveness of this process depends on two factors: the existence of a reservoir of nutrients in the lower layer on the middle shelf and the occurrence of sufficient wind and tidal energy to mix the water column. 相似文献
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Ohne Zusammenfassung 相似文献
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Arnold Schumacher 《Ocean Dynamics》1953,6(4-6):224-226
Summary An official French publication of 1852 is discussed which, by text and accompanying current chart, clearly points out the phenomenon of a Counter Current in the equatorial latitudes of the Western Atlantic.
Le contre-courant équatorial en océan Atlantique Ouest connu depuis cent ans
Résumé L'article précédent traite le chapitre sur les courants généraux de l'Atlantique d'une publication officielle du Dépôt Général de la Marine Française de l'année 1852, dont le texte et la carte signalent le phénomène d'un contre-courant dans les régions équatoriales de l'Atlantique Ouest.相似文献
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Witt Lilli M&#;ller Moritz J. Gr&#;schke Maike Post Vincent E. A. 《Hydrogeology Journal》2021,29(5):1733-1748
Hydrogeology Journal - Aquifer storage and recovery systems using multiple partially penetrating wells (MPPW-ASR) can form a viable solution to the problem of freshwater buoyancy when using... 相似文献
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We use a one-dimensional energy balance global climate model to assess the climatic impacts of various nuclear war scenarios. Our results suggest that mean annual surface temperatures, especially within the war zone, may decrease sharply. Perturbations in the atmospheric circulation system substantially change the distribution and size of the various global heat exchanges. 相似文献