On local and convective accelerations in steep wave events     

Atle Jensen  Didier Clamond  Morten Huseby  John Grue 《Ocean Engineering》2007,34(3-4):426-435

The local and convective accelerations of steep irregular wave events have been investigated. These properties are measured by a two-camera PIV technique. Furthermore, the experiments are compared with two different theories including a fully nonlinear and a simplified analytical model. An important result is that the convective term is of the same order of magnitude and of opposite sign as the local acceleration. The convective acceleration term can therefore not be neglected in acceleration and force estimates.  相似文献   

The biomass of starry ray (Raja radiata) in the North Sea     

Sparholt  Henrik; Vinther  Morten 《ICES Journal of Marine Science》1991,47(3):295-302

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High-precision U-series measurements of more than 500,000 year old fossil corals     

Morten B. Andersen  Claudine H. Stirling  Emma-Kate Potter  Alex N. Halliday  Steven G. Blake  Malcolm T. McCulloch  Bridget F. Ayling  Michael O'Leary 《Earth and Planetary Science Letters》2008,265(1-2):229-245

Robust, independent age constraints on the absolute timing of climate events based on the U-series dating of fossil coral are sparse before the last glacial cycle. Using multiple-collector inductively coupled plasma mass spectrometry with multiple-Faraday protocols, we are able to date ～ 600 ka samples with an uncertainty of better than ± 15 ka (2σ), representing a three-fold improvement in precision compared with previous techniques. Using these methods, we report U-series measurements for a suite of > 500 thousand year old (ka) corals from Henderson Island, an emergent atoll in the south-central Pacific Ocean. The fossil corals show extraordinarily little diagenetic alteration for their age and the best-preserved sample yields a U-series age of 600 ± 15 ka (2σ), which overlaps with the timing of the warm Marine Isotope Stage (MIS) 15 interglacial. The open-system model of Villemant and Feuillet [Villemant B. and Feuillet N. (2003) Dating open systems by the ²³⁸U–²³⁴U–²³⁰Th method: application to Quaternary reef terraces. Earth and Planetary Science Letters 210(1–2), 105–118.] and the linear regression (or open-system isochron) is clearly limited for such old samples. However, the open-system model developed by Thompson et al. [Thompson W.G., Spiegelman M.W., Goldstein S.L., and Speed R.C. (2003) An open-system model for U-series age determinations of fossil corals. Earth and Planetary Science Letters 210(1–2), 365–381.] appears to reliably correct for open-system effects in roughly half of the corals, giving a MIS 15 origin for these. Thus the data provide evidence that the systematic addition of ²³⁰Th and ²³⁴U through α-recoil is a dominant open-system process occurring in the Henderson Island fossil reef system. Several coral samples yield significantly older Thompson et al. open-system ages between 650 and 750 ka. The uncertainty on these ages (typically ± 30 kyrs) is too great for precise assignment but most data overlap with the MIS 17 interglacial. The reliability of these ages is currently unclear. It is shown that separate aliquots of the same coral can yield different Thompson model ages. Therefore, there appear to be additional diagenetic mechanisms that create further anomalous excursions in the U-series systematics, limiting the reliability of the Thompson et al. open-system model.  相似文献   

Effects of fish farm waste on Posidonia oceanica meadows: synthesis and provision of monitoring and management tools     

Holmer M  Argyrou M  Dalsgaard T  Danovaro R  Diaz-Almela E  Duarte CM  Frederiksen M  Grau A  Karakassis I  Marbà N  Mirto S  Pérez M  Pusceddu A  Tsapakis M 《Marine pollution bulletin》2008,56(9):1618-1629

This paper provides a synthesis of the EU project MedVeg addressing the fate of nutrients released from fish farming in the Mediterranean with particular focus on the endemic seagrass Posidonia oceanica habitat. The objectives were to identify the main drivers of seagrass decline linked to fish farming and to provide sensitive indicators of environmental change, which can be used for monitoring purposes. The sedimentation of waste particles in the farm vicinities emerges as the main driver of benthic deterioration, such as accumulation of organic matter, sediment anoxia as well as seagrass decline. The effects of fish farming on P. oceanica meadows are diverse and complex and detected through various metrics and indicators. A safety distance of 400 m is suggested for management of P. oceanica near fish farms followed by establishment of permanent seagrass plots revisited annually for monitoring the health of the meadows.  相似文献   

Cenozoic sediment distribution and tectonic movements in the Faroe region     

Morten Sparre Andersen  Tove Nielsen  Aage Bach Srensen  Lars Ole Boldreel  Anton Kuijpers 《Global and Planetary Change》2000,24(3-4)

A complex history of Cenozoic vertical movements in the Faroe region has been revealed from interpretation of geophysical and geological data, mainly offshore reflection seismic data, side-scan images, shallow cores, and onshore mapping. The history comprises several phases of tectonic disturbances observed at different scales. On the eastern margin of the Faroe Platform a late Eocene–early Oligocene phase of doming of the Faroe Platform has caused a postdepositional tilting of Eocene strata along the southern margin of the platform; a mid-Miocene phase of compressional tectonics is evidenced on seismic transects as gentle anticlines and associated reverse faults; and possible Pliocene uplift of the Faroe Islands is indicated by a progradational wedge of sediments deposited on the eastern Faroe Platform. At the continental margin/slope north of the Faroe Platform, reflection seismic data imaging the postbasalt sedimentary strata indicate three distinct tectonic events phases in the Eocene–Oligocene, Miocene and Pliocene, respectively. In contrast to the Faroe Platform the Faroe–Shetland Channel was characterised by more or less continuous subsidence dominated throughout the Cenozoic. During the Eocene, sediments deposited in the Faroe–Shetland Channel was mostly derived from a source area on the British shelf.  相似文献   

Accelerating the T‐matrix approach to seismic full‐waveform inversion by domain decomposition          下载免费PDF全文

Morten Jakobsen  Ru‐Shan Wu 《Geophysical Prospecting》2018,66(6):1039-1059

A seismic variant of the distorted Born iterative inversion method, which is commonly used in electromagnetic and acoustic (medical) imaging, has been recently developed on the basis of the T‐matrix approach of multiple scattering theory. The distorted Born iterative method is consistent with the Gauss–Newton method, but its implementation is different, and there are potentially significant computational advantages of using the T‐matrix approach in this context. It has been shown that the computational cost associated with the updating of the background medium Green functions after each iteration can be reduced via the use of various linearisation or quasi‐linearisation techniques. However, these techniques for reducing the computational cost may not work well in the presence of strong contrasts. To deal with this, we have now developed a domain decomposition method, which allows one to decompose the seismic velocity model into an arbitrary number of heterogeneous domains that can be treated separately and in parallel. The new domain decomposition method is based on the concept of a scattering‐path matrix, which is well known in solid‐state physics. If the seismic model consists of different domains that are well separated (e.g., different reservoirs within a sedimentary basin), then the scattering‐path matrix formulation can be used to derive approximations that are sufficiently accurate but far more speedy and much less memory demanding because they ignore the interaction between different domains. However, we show here that one can also use the scattering‐path matrix formulation to calculate the overall T‐matrix for a large model exactly without any approximations at a computational cost that is significantly smaller than the cost associated with an exact formal matrix inversion solution. This is because we have derived exact analytical results for the special case of two interacting domains and combined them with Strassen's formulas for fast recursive matrix inversion. To illustrate the fact that we have accelerated the T‐matrix approach to full‐waveform inversion by domain decomposition, we perform a series of numerical experiments based on synthetic data associated with a complex salt model and a simpler two‐dimensional model that can be naturally decomposed into separate upper and lower domains. If the domain decomposition method is combined with an additional layer of multi‐scale regularisation (based on spatial smoothing of the sensitivity matrix and the data residual vector along the receiver line) beyond standard sequential frequency inversion, then one apparently can also obtain stable inversion results in the absence of ultra‐low frequencies and reduced computation times.  相似文献   

High‐resolution study of Icelandic tephras in the Kangerlussuaq Trough,southeast Greenland,during the last deglaciation     

Anne E. Jennings  Karl Grnvold  Rachael Hilberman  Mikie Smith  Morten Hald 《第四纪科学杂志》2002,17(8):747-757

Tephra abundance data and geochemistry in Late‐glacial and Holocene sediments on the East Greenland shelf are presented. Two well‐known tephras were identified from electron microprobe analysis of tephra shards picked from ash peaks in the cores. These are the Vedde Ash and Saksunarvatn Ash, which probably were deposited on the shelf after transport on drifting ice. The radiocarbon dates (marine reservoir corrected by −550 yr) that constrain the timing of deposition of the tephra layers compare well with the terrestrial and ice‐core ages of the tephras without requiring additional reservoir correction to align them with the known tephra ages. Several prominent tephra layers with a composition of Ash Zone 2 tephra punctuate the deglacial sediments. These tephra peaks coincide with significant light stable isotope events (signifying glacial meltwater) and fine‐grained sediments poor in ice‐rafted detritus. We interpret the Ash Zone 2 tephra peaks as sediment released from the Greenland Ice Sheet during strong melting pulses of the deglaciation. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

Role of landscape parameters in riverine run-off,and sediment and organic matter yield on Disko Island,West Greenland     

Morten Rasch  Niels Nielsen  Christian Christiansen  Thomas Balstrøm  Robert Gilbert  Joseph Desloges 《Geografisk tidskrift / udgivet af Bestyrelsen for Det Kongelige danske geografiske selskab》2013,113(2):1-11

Measurements of discharge and suspended sediment and organic matter yield from nine different drainage basins on the island Disko in central West Greenland were carried out in the period 19–30 July 1997. A series of landscape properties (glacier cover, altitude, slope, aspect and vegetation cover) were measured for each drainage basin from a digital elevation model and a satellite image. Principal Component Analysis and regression statistics have been used to examine which landscape parameters dominate the measured discharges and yields. It is concluded that differences in suspended sediment and organic matter yield between the drainage basins can be explained by the measured morphometric properties while differences in water run-off can not. Glacier cover percentage and percent area above 800 m a.s.l. were the most important parameters influencing suspended sediment and organic matter yield in the drainage basins on Disko during the study period.  相似文献   

From groundwater abstraction to vegetative response in fen ecosystems          下载免费PDF全文

Ole Munch Johansen  Jacob Birk Jensen  Morten Lauge Pedersen 《水文研究》2014,28(4):2396-2410

Hydrological effects of groundwater abstraction near a Danish river valley have been assessed by integrated hydrological modelling. The study site contains groundwater‐dependent terrestrial ecosystems in terms of fen and spring habitats that are highly dependent on regional and local scale hydrology. Fens are rare and threatened worldwide due to pressures from agriculture, to lack of appropriate management and to altered catchment hydrology. A solid foundation for hydrological modelling was established based on intensive monitoring at the site, combined with full‐scale pumping tests in the area. A regional groundwater model was used to describe the dynamics in groundwater recharge and the large‐scale discharge to streams. A local grid refinement approach was then applied in a detailed assessment of damage in order to balance the computational effort and the need for a high spatial resolution. A considerable flow reduction in the natural spring was monitored during a full‐scale pumping test while no significant effects on the water table in the fen habitats were observed. A modelled abstraction scenario predicted a lowering of 2–3 cm in the centre of the main fen area during summer periods. The predicted change in water table conditions in the fen habitat is compared to the variability found in 35 Danish fens, and the ecological response is discussed based on statistical water‐level vegetation relations. The results provide a rare quantitative foundation for decision making in relation to management of groundwater‐dependent terrestrial ecosystems. Copyright © 2013 John Wiley & Sons, Ltd.  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