Circulation on the north central Chukchi Sea shelf   总被引：8，自引：0，他引：8  

Thomas Weingartner  Knut Aagaard  Rebecca Woodgate  Seth Danielson  Yasunori Sasaki  Donald Cavalieri 《Deep Sea Research Part II: Topical Studies in Oceanography》2005,52(24-26):3150

Mooring and shipboard data collected between 1992 and 1995 delineate the circulation over the north central Chukchi shelf. Previous studies indicated that Pacific waters crossed the Chukchi shelf through Herald Valley (in the west) and Barrow Canyon (in the east). We find a third branch (through the Central Channel) onto the outer shelf. The Central Channel transport varies seasonally in phase with Bering Strait transport, and is 0.2 Sv on average, although some of this might include water entrained from the outflow through Herald Valley. A portion of the Central Channel outflow moves eastward and converges with the Alaskan Coastal Current at the head of Barrow Canyon. The remainder appears to continue northeastward over the central outer shelf toward the shelfbreak, joined by outflow from Herald Valley. The mean flow opposes the prevailing winds and is primarily forced by the sea-level slope between the Pacific and Arctic oceans. Current variations are mainly wind forced, but baroclinic forcing, associated with upstream dense-water formation in coastal polynyas might occasionally be important.Winter water-mass modification depends crucially on the fall and winter winds, which control seasonal ice development. An extensive fall ice cover delays cooling, limits new ice formation, and results in little salinization. In such years, Bering shelf waters cross the Chukchi shelf with little modification. In contrast, extensive open water in fall leads to early and rapid cooling, and if accompanied by vigorous ice production within coastal polynyas, results in the production of high-salinity (>33) shelf waters. Such interannual variability likely affects slope processes and the transport of Pacific waters into the Arctic Ocean interior.  相似文献   

A survey-based assessment of the Northeast Arctic cod stock     

Korsbrekke  Knut; Mehl  Sigbjorn; Nakken  Odd; Pennington  Michael 《ICES Journal of Marine Science》2001,58(4):763-769

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Nutrient-chlorophyll trajectories across trophic gradients     

Knut L. Seip  Hein Sas  Steven Vermij 《Aquatic Sciences - Research Across Boundaries》1992,54(1):58-76

We estimate the response of chl-a (mg · m^–3) to changes in concentrations of total phosphorus (TP) by calculating the slopeS = chl-a/TP in chl-a =f(TP) graphs. Results show that in years where algae are P-limited oligotrophic lakes respond less (median slope 0.21) to changes in nutrient concentrations than eutrophic lakes, (median slope 0.31) and these again less than hypereutrophic lakes, (median slope 1.02). We find no saturation value for the slope within the TP range considered (6–480 mg · m^–3). Chl-a in eutrophic lakes responds more frequently to non-nutrient factors than oligotrophic and hypereutrophic lakes. Results obtained by replacing TP with a new nutrient parameter, TP = 0.056 · TP · IN^0.226, in which inorganic nitrogen, IN, is factored in, suggest that nitrogen has an influence on chl-a in oligotrophic lakes. Blue-green algae respond less to changes in TP than other algal species, e.g., diatoms.  相似文献   

Phosphorus and nitrogen limitation of algal biomass across trophic gradients   总被引：1，自引：0，他引：1  

Knut L. Seip 《Aquatic Sciences - Research Across Boundaries》1994,56(1):16-28

Regression results based on data from 46 northern temperate lakes show that total phosphorus (TP) is the best predictor for phytoplankton (as chl-a) at lower trophic levels, TP < 200 mg · m^–3. A regression including both TP and TN as regressors is the best predictor for lakes with TP > 200 mg · m^–3. However, the good correlation is probably due to a high correlation between lake average chl-a (all years observed) and lake average TP and TN. Within single hypereutrophic lakes, TN alone is the best predictor. It was not possible to identify a medium trophic domain where TN and TP in combination was the best predictor for chl-a. The ratio TN:TP in the water decreases from about 40 to about 5 with increasing trophic level. Optimum TN:TP ratio for algal species with high abundance during late summer and autumn reflects this decreasing ratio, but within a lesser range, i.e., 20 to 5. In contrast, TN:TP ratios for species abundant during the early vernal period showed no, or an inverse, relation to the TN:TP ratio of the water.  相似文献   

Analysis of underground openings with a distinct shear zone     

Knut Langhagen 《Rock Mechanics and Rock Engineering》1977,9(1):1-8

SummaryAnalysis of Underground Openings with a Distinct Shear Zone A procedure for the analysis of deformations and stresses in the surrounding of an underground opening is demonstrated. For duplication of rock discontinuities in the finite element analysis the joint element of Goodman, Taylor and Brekke is used. A special technique was developed in order to consider the limited tensile strength, the reduction of shearing strength after reaching the peak strength and the limited closure of the joint. A Coulomb failure criterion is used for determining shear fracture. Results of two examples of tunnels with one distinct joint are shown.

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ZusammenfassungBerechnung von Felshohlräumen mit einer ausgeprägten Scherzone Ein Verfahren zur Berechnung der Verformungen und Spannungen in dem einen Hohlraum umschließenden Gebirge wird erläutert. Zur Nachbildung von Diskontinuitäten wird das von Goodman, Taylor und Brekke entwickelte Kluftelement verwendet. Für dieses linienförmige Element wurde eine spezielle Rechentechnik entwickelt, die die begrenzte Zugfestigkeit der Kluft, den Abfall der Scherfestigkeit nach Erreichen der Spitzenfestigkeit auf die Restscherfestigkeit und die begrenzte Zusammendrückbarkeit erfaßt. Zur Bestimmung des Schubbruches wird das Coulombsche Bruchkriterium benutzt. Berechnungsergebnisse von zwei Beispielen aus dem Tunnelbau mit einer diskret nachgebildeten Großkluft werden gezeigt.

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RésuméAnalyse numérique des excavations en roche parcourue par un joint On montre une méthode pour calculer les déformations et les contraintes autour d'un tunnel. Dans la méthode des Eléments Finis on prend un élément développé par Goodman, Taylor et Brekke pour copier des joints. Une méthode de calcul a été développée pour tenir compte de la résistance de traction limitée, la réduction de l'effort de cisaillement après avoir exhausté la résistance de pointe et la fermeture limitée des joints. Pour déterminer la rupture de cisaillement le critère de Coulomb est utilisé. A deux exemples montrés la roche est parcourue par un grand joint.

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Abstract of a short report delivered at the International Symposium on Numerical Methods in Soil Mechanics and Rock Mechanics, Karlsruhe, 1975.

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With 8 Figures  相似文献   

Charcoal and fossil wood from palaeosols,sediments and artificial structures indicating Late Holocene woodland decline in southern Tibet (China)     

Knut Kaiser  Lars Opgenoorth  Werner H. Schoch  Georg Miehe 《Quaternary Science Reviews》2009,28(15-16):1539-1554

Charcoal and fossil wood taken from palaeosols, sediments and artificial structures were analysed in order to evaluate the regional pedoanthracological potential and to obtain information on Holocene environmental changes, particularly on possible past tree occurrences in southern Tibet. This research was initiated by the question to what extent this area is influenced by past human impact. Even recent evaluations have perceived the present treeless desertic environment of southern Tibet as natural, and the previous Holocene palaeoenvironmental changes detected were predominantly interpreted to be climate-determined. The material analysed – comprising a total of 53 botanical spectra and 55 radiocarbon datings from 46 sampling sites (c. 3500–4700 m a.s.l.) – represents the largest systematically obtained data set of charcoal available from Tibet so far. 27 taxa were determined comprising trees, (dwarf-) shrubs and herbs as well as grasses. The predominant tree taxa were Juniperus, Hippophae, Salix and Betula. According to their present-day occurrence in the region, the genera Juniperus and Hippophae can be explicitly attributed to tree species. Further, less frequently detected tree taxa were Populus, Pinus, Quercus, Taxus and Pseudotsuga. Charcoal of Juniperus mainly occurred on southern exposures, whereas Betula was associated with northern exposures. In contrast, the (partly) phreatophytic taxa Hippophae and Salix showed no prevalent orientation. The distribution of radiocarbon ages on charcoal revealed a discontinuous record of burning events cumulating in the Late Holocene (c. 5700–0 cal BP). For southern Tibet, these results indicated a Late Holocene vegetation change from woodlands to the present desertic pastures. As agrarian economies in southern and south-eastern Tibet date back to c. 3700 and 5700 cal BP, respectively, and the present-day climate is suitable for tree growth up to c. 4600 m a.s.l., we concluded that the Late Holocene loss or thinning out of woodlands had been primarily caused by humans.  相似文献   

Structure and evolution of single WR stars     

Knut Jørgen Røed Ødegaard 《Astrophysics and Space Science》1996,238(1):107-111

New computations of massive stars follow the evolution up to advanced stages and include:

-A large and flexible nuclear network consisting of 174 nuclear species that are linked by 1742 nuclear reactions.

-Semiconvection, overshooting and mass loss.

-Modern rates for both strong and weak interaction processes as well as the latest rates for the neutrino processes.

-Improved grid distribution and a large number of grid points.

The nuclear network and the diffusion equation are solved for each time step during the whole evolution. In this way the accuracy of nuclear yields and chemical abundances are mainly limited by uncertainties in the diffusion coefficient found from the convection theories. Several instability mechanisms may affect the mass loss rates of massive stars and thereby the structure and abundances of WR stars. Due to heavy mass loss at the LBV and WR stages, the masses at the pre-SN stage may be less than 5M _⊙. Yields and abundances throughout the stars are discussed together with the amount of all elements expelled.  相似文献   

Formation of accretionary dust mantles in the solar nebula: Evidence from preirradiated olivines in CM chondrites     

Knut Metzler 《Meteoritics & planetary science》2004,39(8):1307-1319

Abstract— CM chondrites are regolith breccias consisting of lithic clasts embedded in a fine‐grained clastic matrix. The majority of these lithic clasts belongs to a texturally well‐defined rock type (primary rock) that can be described as an agglomerate of chondrules and other coarse‐grained components, most of which are surrounded by fine‐grained rims (dust mantles). Metzler et al. (1992) explain these textures as the result of accretionary processes in the solar nebula, while an alternative model explains them to be the result of regolith processes on the parent body (Sears et al. 1993). The main intention of the present study is to discern between both models by investigating the occurrence, frequency, spatial distribution, and textural setting of preirradiated (track‐rich) olivines in CM chondrites. Track‐rich olivines were studied in situ in six polished thin sections from 4 different CM chondrites (Cold Bokkeveld, Mighei, Murchison, Nogoya) by optical and scanning electron microscopy (SEM). It was found that their occurrence is restricted to the clastic matrix of these meteorites. The primary rock seems to have formed in an environment shielded from cosmic radiation, since fragments of this rock are free of track‐rich grains and solar noble gases. This finding supports the solar nebula model for the formation of dust mantles around chondrules and other coarse‐grained components, and points against a regolith origin. In Cold Bokkeveld, a small breccia‐in‐breccia clast was found, which has been irradiated as an entity within the uppermost millimeters to meters of its parent body for at least about 3 Ma. This clast seems to represent a compacted subsurface layer that was later excavated by impact and admixed to the host breccia. Furthermore, the results of this study may affect the interpretation of compaction ages obtained by fission track methods, since these ages may be mixtures of different contact ages between finegrained, U‐rich dust and U‐poor olivines. In some cases, they may date the formation of dust mantles in the solar nebula, while in other cases the lithification of the host breccias may be dated.  相似文献   

Luminescence-dated aeolian deposits of late Quaternary age in the southern Tibetan Plateau and their implications for landscape history   总被引：5，自引：0，他引：5  

ZhongPing Lai  Knut Kaiser  Helmut Brückner   《Quaternary Research》2009,72(3):421-430

Aeolian deposits are widely distributed in the interior of the Tibetan Plateau, and their chronology is poorly known. It is not yet clear whether they accumulated only after the last deglaciation, or over a longer time. We applied quartz OSL dating to aeolian samples from the Lhasa area with OSL ages ranging from 2.9 ± 0.2 to at least 118 ± 11 ka. The probability density frequency (PDF) distribution of 24 ages reveals age clusters at about 3, 8, 16–21, 33, and 79–83 ka, indicating enhanced sediment accumulation then. The results show that aeolian deposition occurred throughout most of the last 100 ka. This implies that: 1) an ice sheet covering the whole Tibetan Plateau during the last glacial maximum (LGM) could not have existed; and 2) erosion during the last deglaciation was not as strong as previously proposed, such that not all pre-Holocene loess was removed. The age distribution shown in the PDF indicates that aeolian accumulation is episodic. Sand-formation events revealed by age clusters at 3, 8, and 16–21 ka imply roughly synchronous environmental responses to corresponding global-scale arid events.  相似文献   

Meltwater production in Antarctic blue-ice areas: sensitivity to changes in atmospheric forcing     

Glen E. Liston  Oddbjørn Bruland  Jan-Gunnar Winther  Hallgeir Elvehøy  Knut Sand 《Polar research》1999,18(2):283-290

In the near coastal regions of Dronning Maud Land, Antarctica, below-surface ice-melt in blue-ice areas has been observed. The low scattering coefficients of the large-grained blue-ice allow penetration of solar radiation, thus providing an energy source below the ice surface. The sub-surface meltwater is significant enough to show up on remote-sensing imagery in the form of ice-covered lakes. Adjacent snow-accumulation areas have much higher scattering coefficients and consequently limit solar radiation penetration in these regions. These snow and ice surfaces are generally below freezing, and little surface melting occurs. To assess the response of these melt features to changes in atmospheric forcings such as cloudiness, air temperature, and snow accumulation, a physically-based model of the coupled atmosphere, radiation, snow, and blue-ice system has been developed. The model consists of a heat transfer equation with a spectrally-dependent solar-radiation source term. The penetration of radiation into the snow and blue-ice depends on the surface albedo, and the snow and blue-ice grain size and density. Model simulations show that ice melt occurring in this area is sensitive to potential variations in atmospheric forcing. Under certain conditions more traditional surface melting occurs and, under other conditions, the existing melt processes can be shut down completely. In light of the sensitivity of this system to variations in atmospheric forcing, and the ability to view melt-related features using remote sensing, a tool exists to efficiently monitor variations in Antarctic coastal climate.  相似文献