Mean and eddy motion in the Skagerrak/northern North Sea: insight from a numerical model     

Lars Petter?R?edEmail author  Ingerid?Fossum 《Ocean Dynamics》2004,54(2):197-220

Insight regarding the mean and eddy motion in the Skagerrak/northern North Sea area is gained through an analysis of model-simulated currents, hydrography, kinetic energy and relative vorticity for the 2 years 2000 and 2001. In this a -coordinate ocean model is used. Since the tidal currents are generally strong in the area, care is exercised to distinguish the mesoscale (eddy) motion from higher-frequency motion such as tides, before computing the mean and eddy kinetic energy. The model-simulated response is first compared with available knowledge of the circulation in the area, and when available, also with sea-surface temperature obtained from satellite imagery. It is concluded that the model appears to faithfully reproduce most of what is known, in particularly the upper mixed layer circulation. An analysis of the mean and eddy kinetic energy reveals that many of the mesoscale structures found in the area are recurrent. This is particularly true for the structures off the southern tip of Norway. Also in general, areas of strong mean and eddy kinetic energy are co-located. The exception is the area off the southern tip of Norway, where the eddy kinetic energy is much larger than its mean counterpart. An analysis of the relative vorticity reveals that the variability found is due to the occurrence of recurrent anticyclonic eddies. It is hypothesized that these eddies are generated due to an offshore veering of the Norwegian coastal current (NCC) as it reaches the eastern end of the Norwegian Trench plateau. Here it becomes a free jet, which is then vulnerable to either barotropic instability caused by the horizontal shear in the jet-like structure of the NCC at this point, or a baroclinic (frontal) instability. The latter may come into play when the NCC veers offshore and its relatively fresh water meets the inflowing saline water of Atlantic origin, a frontogenesis that may become strong enough for cyclogenesis to take place. Due to the depth-independent nature of the model-generated eddies, the barotropic instability is the most likely candidate. It remains to resolve the reason for the offshore veering of the NCC. The most likely candidate mechanisms are vortex squeezing or simply that the coastline curvature is large enough for the NCC to separate from the coast in a hydraulic sense.Responsible Editor: Phil Dyke  相似文献   

Nd-Sr-Pb isotopic constraints on metal and fluid sources in W-Sb-Au mineralization at Woxi and Liaojiaping (Western Hunan,China)   总被引：9，自引：0，他引：9  

Bo?Peng  Robert?FreiEmail author 《Mineralium Deposita》2004,39(3):313-327

This paper presents Nd-Sr-Pb isotope data on scheelite, inclusion fluids and residues of gangue quartz, and sulfides from the W-Sb-Au ore deposits at Woxi and Liaojiaping (LJP) in the Xuefeng Uplift Belt (XUB), Western Hunan, China. Sm and Nd concentrations in scheelite from Woxi are much lower than in scheelite from LJP and can be distinguished by their high ¹⁴⁷Sm/¹⁴⁴Nd ratios of ~1.25 from the much lower ratios around 0.26 in scheelites from the LJP. Nd values (compared to values at 200 Ma, which is the average timing of granitoid emplacement during the Indosinian-Yanshanian periods in the XUB) are around –10 for the LJP and compare well with the range of –5 to –11 defined by the granitoids, whereas they are around –27 for scheelite from Woxi. This might indicate that REEs in the mineralizing fluids at LJP originated from granitoids that are concentrated along the southern border of the XUB, whereas in the case of Woxi, the original fluids might have been masked by REEs released during intense high-temperature wall rock alteration of unexposed Precambrian basement rocks at depth. Sr isotopes of scheelite from these two deposits show similar relations to host / nearby rocks, in that ⁸⁷Sr/⁸⁶Sr (T=200 Ma) ratios of ~0.721 for LJP scheelite agree with values ranging between 0.718 – 0.726 for granitoids, whereas these ratios are much higher (i.e. 0.745) for scheelite from Woxi and correspond to the ⁸⁷Sr/⁸⁶Sr (T=200 Ma) ratio range of 0.743 – 0.749 for Precambrian host slates. Crushing experiments to release inclusion fluids from gangue quartz and sulfides deposited during later stages of ore deposition in both deposits failed to provide accurate and geologically meaningful two-point (fluid-residue) tie lines in Rb-Sr isochron diagrams. However, Sr released from fluid inclusions generally reveals lower initial ⁸⁷Sr/⁸⁶Sr ratios than the respective residues and shows affinities to ⁸⁷Sr/⁸⁶Sr (T=200 Ma) values of Indosinian-Yanshanian granitoids, both at Woxi and LJP. Pb stepwise leaching of scheelite and sulfides did not result in sufficient spreads in Pb isotope diagrams and therefore no information regarding exact mineralization ages in the two deposits could be deduced. Overall, ore Pb isotopes reveal upper crustal signatures and are compatible with Pb isotope signatures of the predominant Precambrian slates in the Woxi area. Steep trajectories through late stage quartz-sulfide mineralization in Pb isotope diagrams may hint at mixing scenarios involving Pb from the host rocks and a component with lower ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios relative to ²⁰⁶Pb/²⁰⁴Pb ratios, which cannot be linked to any known reservoir in the XUB mining district. Sr isotopes of four out of seven residual sulfide samples from Woxi plot along a paleomixing line at an age of 199 ± 8 Ma, supporting a mixing scenario for the fluids indicated by the Pb isotopes and pointing to a possible genetic relationship with the emplacement of Indosinian—Yanshanian granitoids. The budgets of REEs, Rb-Sr and Pb in the original fluids were severely affected by contamination of these elements apparently during intense wall rock alteration but, after sealing of the major pathways, the mineralizing fluids tend to have better preserved their original signatures. These attest a genetic relationship between the metallogeny in the XUB W-Sb-Au province and the emplacement of Indosinian-Yanshanian granitoids during Mesozoic intracontinental tectonic uplift and thrusting.Electronic Supplementary Material Supplementary material is available in the online version of this article at Editorial handling: B. Lehmann  相似文献   

Synthesis, crystal structure and crystal chemistry of ferri-clinoholmquistite, ?Li ₂Mg ₃Fe ³⁺ ₂Si ₈O ₂₂(OH) ₂     

G. Iezzi  F. Cámara  R. Oberti  G. Della Ventura  G. Pedrazzi  J-L Robert 《Physics and Chemistry of Minerals》2004,31(6):375-385

This work reports the synthesis of ferri-clinoholmquistite, nominally _Li2(Mg₃Fe³⁺₂)Si₈O₂₂(OH)₂, at varying f_O2 conditions. Amphibole compositions were characterized by X-ray (powder and single-crystal) diffraction, microchemical (EMPA) and spectroscopic (FTIR, Mössbauer and Raman) techniques. Under reducing conditions ( NNO+1, where NNO = Nickel–Nickel oxide buffer), the amphibole yield is very high (>90%), but its composition, and in particular the FeO/Fe₂O₃ ratio, departs significantly from the nominal one. Under oxidizing conditions ( NNO+1.5), the amphibole yield is much lower (<60%, with Li-pyroxene abundant), but its composition is close to the ideal stoichiometry. The exchange vector of relevance for the studied system is ^M2(Mg,Fe²⁺) ^M4(Mg,Fe²⁺) ^M2Fe³⁺_–1 ^M4Li_–1, which is still rather unexplored in natural systems. Amphibole crystals of suitable size for structure refinement were obtained only at 800 °C, 0.4 GPa and NNO conditions (sample 152), and have C2/m symmetry. The X-ray powder patterns for all other samples were indexed in the same symmetry; the amphibole closest to ideal composition has a = 9.428(1) Å, b = 17.878(3) Å, c = 5.282(1) Å, = 102.06(2)°, V = 870.8(3) ų. Mössbauer spectra show that Fe³⁺ is strongly ordered at M2 in all samples, whereas Fe²⁺ is disordered over the B and C sites. FTIR analysis shows that the amount of ^CFe²⁺ increases for increasingly reducing conditions. FTIR data also provide strong evidence for slight but significant amounts of Li at the A sites.  相似文献   

Fish assemblage stability over fifty years in the Lake Pontchartrain estuary; Comparisons among habitats using canonical correspondence analysis     

Martin?T.?O’ConnellEmail author  Robert?C.?Cashner  Christopher?S.?Schieble 《Estuaries and Coasts》2004,27(5):807-817

We assessed fish assemblage stability over the last half century in Lake Pontchartrain, an environmentally degraded oligohaline estuary in southeastern Louisiana. Because assemblage instability over time has been consistently associated with severe habitat degradation, we attempted to determine whether fish assemblages in demersal, nearshore, and pelagic habitats exhibited change that was unrelated to natural fluctuations in environmental variables (e.g., assemblage changes between wet and dry periods). Collection data from three gear types (trawl, beach seine, and gill nets) and monthly environmental data (salinity, temperature, and Secchi depth) were compared for four collecting periods: 1954 (dry period), 1978 (wet period), 1996–1998 (wet period), and 1998–2000 (dry period). Canonical correspondence analysis (CCA) revealed that although the three environmental variables were significantly associated with the distribution and abundance patterns of fish assemblages in all habitats (with the exception of Secchi depth for pelagic samples), most fish assemblage change occurred among sampling periods (i.e., along a temporal gradient unrelated to changing environmental variables). Assemblage instability was the most pronounced for fishes collected by trawls from demersal habitats. A marked lack of cyclicity in the trawl data CCA diagram indicated a shift away from a baseline demersal assemblage of 50 yr ago. Centroid positions for the five most collected species indicated that three benthic fishes, Atlantic croaker (Micropogonias undulatus), spot (Leiostomus xanthurus), and hardhead catfish (Arius felis), were more dominant in past demersal assemblages (1954 and 1978). A different situation was shown for planktivorous species collected by trawls with bay anchovy (Anchoa mitchilli) becoming more dominant in recent assemblage and Gulf menhaden (Brevoortia patromus) remaining equally represented in assemblages over time. Changes in fish assemblages from nearshore (beach seine) and pelagic (gill net) habitats were more closely related to environmental fluctuations, though the CCA for beach seine data also indicated a decrease in the dominance ofM. undulatus and an increase in the proportion ofA. mitchilli over time. The reduced assemblage role of benthic fishes and the marked assemblage change indicated by trawl data suggest that over the last half century demersal habitats in Lake Pontchartrain have been impacted more by multiple anthropogenic stressors than nearshore or pelagic habitats.  相似文献   

Meteor44 Video Meteor Photometry     

Wesley R. Swift  Robert M. Suggs  William J. Cooke 《Earth, Moon, and Planets》2004,95(1-4):533-540

Meteor44 is a software system developed at MSFC for the calibration and analysis of video meteor data. The photometric range of the (8 bit) video data is extended from a visual magnitude range of from 8 to 3 to from 8 to −8 for both meteors and stellar images using saturation compensation. Camera and lens specific saturation compensation coefficients are derived from artificial variable star laboratory measurements. Saturation compensation significantly increases the number of meteors with measured intensity and improves the estimation of meteoroid mass distribution. Astrometry is automated to determine each image's plate coefficient using appropriate star catalogs. The images are simultaneously intensity calibrated from the contained stars to determine the photon sensitivity and the saturation level referenced above the atmosphere. The camera's spectral response is used to compensate for stellar color index and typical meteor spectra in order to report meteor light curves in traditional visual magnitude units. Recent efforts include improved camera calibration procedures and long focal length "streak" meteor photometry. Meteor44 has been used to analyze data from the 2001, 2002 and 2003 MSFC Leonid observational campaigns as well as several lesser showers.  相似文献   

Processes affecting deposition of sediment in a small, morphologically complex lake     

Robert Gilbert  Scott Lamoureux 《Journal of Paleolimnology》2004,31(1):37-48

Devil Lake is morphologically complex as a result of Pleistocene glacial erosion of the Frontenac Axis of the Canadian Shield. In order to assess the processes causing highly variable sedimentation in the lake, we monitored currents, suspended sediment and temperature in the lake before and during autumn overturn in 2002. Strong summer thermal stratification (stability number to 0.11 s^–1 declining with the approach of overturn) was insufficient to prevent a dynamic response in the hypolimnion to wind forcing. Superimposed on a gradual increase in suspended sediment concentration in the last weeks of stratification from less than 2 g/l to about 30 g/l were shorter-term rises lasting up to several days. Associated with these events was an increase in particle size of the sediment from a mode of 40–50 to 150–200 m ascribed to flocculation from primary particles. These events culminated in rapid (<1 h) clearing of the water associated with strong, sustained winds over the lake, especially from the southwest. After overturn, the events were more frequent, and flocculation was unable to develop as well in the more vigorous circulation. However, currents in the hypolimnion occurred throughout the period before, during and after overturn with speed related to wind speed, but direction largely independent of wind direction. The results represent an approach to understanding the nature of sedimentary processes and thus to strengthening the use of sedimentary records as proxy in environmental and paleoenvironmental assessment.  相似文献   

Boron isotopes as an artificial tracer     

Quast KW  Lansey K  Arnold R  Bassett RL  Rincon M 《Ground water》2006,44(3):453-466

A field study was conducted using a combination of intrinsic and artificial tracers to estimate travel times and dilution during transport of infiltrate from a reclaimed water infiltration basin to nearby monitoring wells. A major study objective was to validate boric acid enriched in (10)B as an artificial tracer. Basin 10E at the Rio Hondo Spreading Grounds in Whittier, California, was the site of the test. The basin normally receives a mixture of treated municipal waste water, purchased State Project water, and local runoff from the San Gabriel River. Approximately 3.5 kg of (10)B-enriched boric acid was dispersed among 2.05 x 10(5) m(3) of basin water to initiate the experiment. The resultant median delta(11)B in the infiltration basin was -71 per thousand. Prior to tracer addition, the basin water had an intrinsic delta(11)B of +2 per thousand. Local monitoring wells that were used to assess travel times had delta(11)B values of +5 per thousand and +8 per thousand at the time of tracer addition. Analytic results supported an assumption that boron is conserved during ground water transport and that boron enriched in (10)B is a useful artificial tracer. Several intrinsic tracers were used to reinforce the boric acid tracer findings. These included stable isotopes of oxygen (delta(18)O) and hydrogen (deltaD), sulfate concentration, and the boron to chloride ratio. Xenon isotopes, (136)Xe and (124)Xe, also supported boron isotope results. Xenon isotopes were added to the recharge basin as dissolved gases by investigators from the Lawrence Livermore National Laboratory.  相似文献   

Archean greenstone-tonalite duality: Thermochemical mantle convection models or plate tectonics in the early Earth global dynamics?   总被引：2，自引：0，他引：2  

Robert Kerrich  Ali Polat   《Tectonophysics》2006,415(1-4):141-165

Mantle convection and plate tectonics are one system, because oceanic plates are cold upper thermal boundary layers of the convection cells. As a corollary, Phanerozoic-style of plate tectonics or more likely a different version of it (i.e. a larger number of slowly moving plates, or similar number of faster plates) is expected to have operated in the hotter, vigorously convecting early Earth. Despite the recent advances in understanding the origin of Archean greenstone–granitoid terranes, the question regarding the operation of plate tectonics in the early Earth remains still controversial. Numerical model outputs for the Archean Earth range from predominantly shallow to flat subduction between 4.0 and 2.5 Ga and well-established steep subduction since 2.5 Ga [Abbott, D., Drury, R., Smith, W.H.F., 1994. Flat to steep transition in subduction style. Geology 22, 937–940], to no plate tectonics but rather foundering of 1000 km sectors of basaltic crust, then “resurfaced” by upper asthenospheric mantle basaltic melts that generate the observed duality of basalts and tonalities [van Thienen, P., van den Berg, A.P., Vlaar, N.J., 2004a. Production and recycling of oceanic crust in the early earth. Tectonophysics 386, 41–65; van Thienen, P., Van den Berg, A.P., Vlaar, N.J., 2004b. On the formation of continental silicic melts in thermochemical mantle convection models: implications for early Earth. Tectonophysics 394, 111–124]. These model outputs can be tested against the geological record. Greenstone belt volcanics are composites of komatiite–basalt plateau sequences erupted from deep mantle plumes and bimodal basalt–dacite sequences having the geochemical signatures of convergent margins; i.e. horizontally imbricated plateau and island arc crust. Greenstone belts from 3.8 to 2.5 Ga include volcanic types reported from Cenozoic convergent margins including: boninites; arc picrites; and the association of adakites–Mg andesites- and Nb-enriched basalts.Archean cratons were intruded by voluminous norites from the Neoarchean through Proterozoic; norites are accounted for by melting of subduction metasomatized Archean continental lithospheric mantle (CLM). Deep CLM defines Archean cratons; it extends to  350 km, includes the diamond facies, and xenoliths signify a composition of the buoyant, refractory, residue of plume melting, a natural consequence of imbricated plateau-arc crust. Voluminous tonalites of Archean greenstone–granitoid terranes show a secular trend of increasing Mg#, Cr, Ni consistent with slab melts hybridizing with thicker mantle wedge as subduction angle steepens. Strike-slip faults of 1000 km scale; diachronous accretion of distinct tectonostratigraphic terranes; and broad Cordilleran-type orogens featuring multiple sutures, and oceanward migration of arcs, in the Archean Superior and Yilgarn cratons, are in common with the Altaid and Phanerozoic Cordilleran orogens. There is increasing geological evidence of the supercontinent cycle operating back to  2.7 Ga: Kenorland or Ur  2.7–2.4 Ga; Columbia  1.6–1.4 Ga; Rodinia  1100–750 Ma; and Pangea  230 Ma. High-resolution seismic reflection profiling of Archean terranes reveals a prevalence of low angle structures, and evidence for paleo-subduction zones. Collectively, the geological–geochemical–seismic records endorse the operation of plate tectonics since the early Archean.  相似文献   

Survival under chronic stress from sediment load: spatial patterns of hard coral communities in the southern islands of Singapore     

Dikou A  van Woesik R 《Marine pollution bulletin》2006,52(1):7-21

Six reef sites were chosen along the west coasts of Singapore's southern islands, to: (1) quantitatively assess and compare coral community composition and structure, and recruitment rates, (2) assess the relationship between the aforementioned patterns and the environmental conditions, and (3) provide insights on potential processes that incorporate history at these study sites. Chronic exposure to high sediment load was the most obvious form of anthropogenic stress. Recruitment rates on ceramic tiles were low (1.4+/-1.0-20+/-14.7 recruits m(-2)year(-1)) but decreased towards the main island of Singapore as did hard coral cover and coral density. Coral fauna consisted of genera generally found in deeper waters (e.g., fungiids, foliose Oxypora, Leptoseris, and Echinopora) or those well adapted to turbid waters (e.g., Porites, Pectinia, Leptastrea, Montipora). Light extinction coefficient (K) and % live coral cover (%LCC) showed a strong and inverse curvilinear relationship (%LCC=13.60 *K(-3.40)). Similarly, the rate of sediment deposition (DFSPM) (Recruitment rate, RR=1.51-0.17 *DFSPM) and water clarity (RR=3.56-2.92 *K) exhibited strong and inverse relationships with recruitment rates. Although measured levels of the down-ward flux of suspended particulate matter and suspended solids were well within "normal" levels recorded in the literature, it was the proportion of benthic space, generic coral composition, and site history that offered compelling evidence of chronic exposure to increased sediment load. Clearly a reduction in both water clarity and live-coral cover has taken place since monitoring efforts began in the early 1970s, in fact coral cover has more than halved at all sites examined since the 1980s and benthic space was predominantly occupied by dead corals covered with sediment and filamentous algae.  相似文献   

Use of biliary PAH metabolites as a biomarker of pollution in fish from the Baltic Sea   总被引：2，自引：0，他引：2  

Vuorinen PJ  Keinänen M  Vuontisjärvi H  Barsiene J  Broeg K  Förlin L  Gercken J  Kopecka J  Köhler A  Parkkonen J  Pempkowiak J  Schiedek D 《Marine pollution bulletin》2006,53(8-9):479-487

During field campaigns of the BEEP project (Biological Effects of Environmental Pollution in Marine Coastal Ecosystems) in 2001-2002, metabolites of polycyclic aromatic hydrocarbons (PAHs) were determined in bile samples from three fish species, flounder (Platichthys flesus), perch (Perca fluviatilis) and eelpout (Zoarces viviparus), from four separate areas in the Baltic Sea. Two determination methods were applied: fixed wavelength fluorescence (FF) for pyrene-type metabolites and high-performance liquid chromatography with fluorescence detection (HPLC). There was a good correlation between the FF method and 1-OH pyrene determined by HPLC. Normalisation of the FF data for absorbance at 380 nm or bile protein concentrations greatly increased variance in one third and decreased it in two thirds of the cases and resulted in a loss of significant differences (protein normalisation) between the sampling stations, but normalisation of the HPLC data had little effect on the results. The biliary PAH metabolite content was usually higher in males than in females. In perch and eelpout the biliary PAH contents were at similar levels, whereas in flounder the levels were lower. The sampling areas arranged in decreasing order of biliary PAH contents were: Wismar Bay > Gulf of Gdansk > Lithuanian coast > Kvadofj?rden (reference area). It is concluded that FF with un-normalised data is a reliable and simple method for monitoring purposes and only one sex of a selected species should be used.  相似文献