Is trophic state or regional location the strongest determinant for Chl-a/TP relationships in lakes?     

Knut L. Seip  Erik Jeppesen  Jens Petter Jensen  Bj?rn Faafeng 《Aquatic Sciences - Research Across Boundaries》2000,62(3):195-204

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Thermal and haline variability over the central Bering Sea shelf: Seasonal and interannual perspectives     

S. Danielson  L. EisnerT. Weingartner  K. Aagaard 《Continental Shelf Research》2011,31(6):539-554

We examine multi-year conductivity-temperature-depth (CTD) data to better understand temperature and salinity variability over the central Bering Sea shelf. Particular consideration is given to observations made annually from 2002 to 2007 between August and October, although other seasons and years are also considered. Vertical and horizontal correlation maps show that near-surface and near-bottom salinity anomalies tend to fluctuate in phase across the central shelf, but that temperature anomalies are vertically coherent only in the weakly or unstratified inner-shelf waters. We formulate heat content (HC) and freshwater content (FWC) budgets based on the CTD observations, direct estimates of external fluxes (surface heat fluxes, ice melt, precipitation (P), evaporation (E) and river discharge), and indirect estimates of advective contributions. Ice melt, P−E, river discharge, and along-isobath advection are sufficient to account for the mean spring-to-fall increase in FWC, while summer surface heat fluxes are primarily responsible for the mean seasonal increase in HC, although interannual variability in the HC at the end of summer appears related to variability in the along-isobath advection during the summer months. On the other hand, FWC anomalies at the end of summer are significantly correlated with the mean wind direction and cross-isobath Ekman transport averaged over the previous winter. Consistent with the latter finding, salinities exhibit a weak but significant inverse correlation between the coastal and mid-shelf waters. The cross-shelf transport likely has significant effect on nutrient fluxes and other processes important to the functioning of the shelf ecosystem. Both the summer and winter advection fields appear to result from the seasonal mean position and strength of the Aleutian Low. We find that interannual thermal and haline variability over the central Bering Sea shelf are largely uncoupled.  相似文献   

Open-system chemical behaviour of Wilcox Group mudstones. How is large scale mass transfer at great burial depth in sedimentary basins possible? A discussion   总被引：1，自引：0，他引：1  

Knut Bjørlykke 《Marine and Petroleum Geology》2011,28(7):1381-1382

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Correlation of late precambrian and early Palaeozoic sequences by eustatic sea-level changes and the selection of the Precambrian-Cambrian boundary     

Knut Bjørlykke 《Precambrian Research》1982,17(2):99-104

The author argues for a pragmatic approach to the problem of defining the Precambrian-Cambrian boundary. A boundary at first occurrence of shelly fauna as recommended by the Precambrian-Cambrian working group will create severe difficulties in worldwide correlation. By placing the boundary stratigraphically somewhat higher (i.e. the Fallotaspis zone) with a more abundant fauna, there is a much better basis for biostratigraphic correlation.Eustatic sea-level changes may be another important tool for late Precambrian-early Palaeozoic correlation, and more detailed facies interpretation of such sequences should be carried out in order to recognise these eustatic sea-level changes.  相似文献   

Ultrarapid chondrite formation by hot chondrule accretion? Evidence from unequilibrated ordinary chondrites     

Knut METZLER 《Meteoritics & planetary science》2012,47(12):2193-2217

Abstract– Unequilibrated ordinary chondrites (UOCs) of all groups (H, L, LL) contain unique chondrite clasts, which are characterized by a close‐fit texture of deformed and indented chondrules. These clasts, termed “cluster chondrites,” occur in 41% of the investigated samples with modal abundances between 5 and 90 vol% and size variations between <1 mm and 10 cm. They show the highest chondrule abundances compared with all chondrite classes (82–92 vol%) and only low amounts of fine‐grained interchondrule matrix and rims (3–9 vol%). The mean degree of chondrule deformation varies between 11% and 17%, compared to 5% in the clastic portions of their host breccias and to values of 3–5% found in UOC literature, respectively. The maximum deformation of individual chondrules is about 50%, a value which seemingly cannot be exceeded due to geometric limitations. Both viscous and brittle chondrule deformation is observed. A model for cluster chondrite formation is proposed where hot and deformable chondrules together with only small amounts of co‐accreting matrix formed a planetesimal or reached the surface of an already existing body within hours to a few days after chondrule formation. They deformed in a hot stage, possibly due to collisional compression by accreting material. Later, the resulting rocks were brecciated by impact processes. Thus, cluster chondrite clasts are interpreted as relicts of primary accretionary rocks of unknown original dimensions. If correct, this places a severe constraint on chondrule‐forming conditions. Cluster chondrites would document local chondrule formation, where chondrule‐forming heating events and the accretion of chondritic bodies were closely linked in time and space.  相似文献   

Late Pleistocene and Holocene terrestrial geomorphodynamics and soil formation in northeastern Germany: a review of geochronological data     

Christoph Kappler  Knut Kaiser  Mathias Küster  Alexander Nicolay  Alexander Fülling  Oliver Bens 《自然地理学》2013,34(5):405-432

ABSTRACT

This study is based on 616 geochronological ages from aeolian and colluvial sediments as well as paleosols, representing the largest database of geochronological data from northeastern Germany available to date. Cumulative probability density functions for radiocarbon data and kernel density estimates for luminescence data were created covering the last 15 ka. The data analysis aimed at the identification of changes and their drivers in geomorphodynamics and soil formation. The ages representing aeolian activity cluster in the Late Glacial, the Early Holocene, and the Late Holocene, where the first two clusters are assumed to result mostly from climatic impact with only a minor share of human impact triggering the mobilization of aeolian sediments. The third cluster is considered to result mainly from human impact. The Late Glacial to Early Holocene activity phase is interrupted by a phase of surface stability around 11.5–12.7 ka, which is indicated by the occurrence of initial soil formations of Finow and Usselo types. Colluvial sedimentation predominantly occurred during the last 7 ka and clearly accelerated since the last 1000 a. According to the ages of specific paleosol types, related soil-forming processes started already in the Late Glacial and were completed in the Holocene.  相似文献   

Resolving frontal structures: on the payoff using a less diffusive but computationally more expensive advection scheme     

Barthel  Knut  Daewel  Ute  Pushpadas  Dhanya  Schrum  Corinna  &#;rthun  Marius  Wehde  Henning 《Ocean Dynamics》2012,62(10):1457-1470

This article presents some advantages using a shape-preserving total variation diminishing (TVD) advection scheme in an ecosystem model. The superbee flux-limiter has been used to the second-order Lax–Wendroff advection scheme to make it TVD. We performed simulations for three shelf sea regions with different characteristic time scales, namely, the North Sea, the Barents Sea, and the Baltic Sea. To explore the advantages, we also performed reference runs with the much simpler and computationally cheaper upwind advection scheme. Frontal structures are much better resolved with the TVD scheme. The bottom salinity in the Baltic Sea stays at realistic values throughout the 10-year simulation with the TVD scheme, while with the upwind scheme, it drifts towards lower values and the permanent haline stratification in the Baltic is almost completely eroded within one seasonal cycle. Only when applying TVD for both the vertical and horizontal advections the model succeeded to preserve haline stratification in the decadal simulation. Lower trophic level patterns are far better reproduced with the TVD scheme, and for the estimated cod larval survival, the advantages seem to be even stronger. Simulations using the TVD-derived prey fields identified distinct regions such as Dogger Bank to favor potential larvae survival (PLS), which did not appear as particularly favorable in the upstream simulations. The TVD scheme needs about 25 % more time on the central processing unit (CPU) in case of a pure hydrodynamic setup with only two scalar state variables (Barents Sea application). The additional CPU time cost increases for a coupled physical–biological model application with a large number of state variables. However, this is more than compensated by all the advantages found, and, hence, we conclude that it is worthwhile to use the TVD scheme in our ecosystem model.

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A reindeer herder's perspective on caribou, weather and socio-economic change on the Seward Peninsula, Alaska     

Kumi Rattenbury  Knut Kielland  Greg Finstad  & William Schneider 《Polar research》2009,28(1):71-88

Non-climate variables shape vulnerability and adaptive capacity to climate change. Here, we describe how recent environmental and socio-economic developments have transformed reindeer herding and perceptions of weather on the Seward Peninsula, Alaska. The reindeer industry has shrunk considerably since the early 1990s, when the winter range of the Western Arctic Caribou Herd expanded, and over 17 000 reindeer mixed with migrating caribou and left the region. Socio-economic and environmental repercussions make the continuation of herding tenuous, and erode the ability of herders to cope with weather variability, among other perturbations. We present a case study of one herder's annual cycle, and juxtapose physical drivers of herding activities, including weather-station and herder observations of local weather variability, with socio-economic factors. There is an increased urgency to access and monitor reindeer with caribou present, but herding plans are constrained by lower economic returns and the need to spend more time in non-herding jobs. Although weather is a greater concern now for immediate herd access, standard weather data are largely irrelevant to the mechanics of herding, whereas variables pertaining to the timing of biotic events (e.g., synchrony of spring break-up and calving) and visibility are attributed to lost herding opportunities. Short-term responses to weather conditions stem from more long-term vulnerability associated with caribou presence, reduced herd size, difficulties affording snowmobile maintenance or crew assistance, and dwindling market opportunities. We emphasize the environmental and socio-economic interactions that affect vulnerability and adaptive capacity for modern herding.  相似文献   

Generating Random Earthquake Events for Probabilistic Tsunami Hazard Assessment     

Randall J. LeVeque  Knut Waagan  Frank I. González  Donsub Rim  Guang Lin 《Pure and Applied Geophysics》2016,173(12):3671-3692

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Observations of offshore bar decay: Sediment budgets and the role of lower shoreface processes     

Troels Aagaard  Aart Kroon  Brian Greenwood  Michael G. Hughes 《Continental Shelf Research》2010

Long-term, net offshore bar migration is a common occurrence on many multiple-barred beaches. The first stage of the process involves the generation of a longshore bar close to the shoreline that oscillates about a mean position for some time, followed by a stage of net offshore migration across the upper shoreface, and finally a stage of decaying bar form through loss of sediment volume at the outer boundary of the upper shoreface. The phenomenon has been previously documented in the Netherlands, the USA, the Canadian Great Lakes, and in New Zealand, but our present understanding of the morphodynamic processes and sediment transport pathways involved in bar decay is limited. In this paper, long-term, net offshore bar migration is investigated at Vejers Beach, located on the North Sea coast of Denmark where offshore bar migration rates are of the order of 45–55 m a⁻¹. A wave height transformation model confirmed that the decay of the outer bar results in increased wave heights and undertow speeds at the more landward bar potentially causing this bar to speed up its offshore migration. The causes for outer bar decay were investigated through field measurements of sediment transport at the decaying bar and at a position further seaward on the lower shoreface. The measurements showed that a cross-shore transport convergence exists between the bar and the lower shoreface and that the loss of sediment involved in bar decay is associated with a longshore directed transport by non-surf zone processes. At Vejers, and possibly elsewhere, the net offshore migration of bars and the subsequent loss of sand during bar decay is an important part of the beach and shoreface sediment budget.  相似文献