Palaeomagnetism and magnetostiatigraphy of Triassic strata in the Sangre de Cristo Mountains and Tucumcari Basin, New Mexico, USA     

Roberto S. Molina-Garza    John W. Geissman    Spencer G. Lucas  Rob Van der  Voo 《Geophysical Journal International》1996,124(3):935-953

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Sr and Ar isotope studies of detrital smectites from the Atlantic Ocean (D.S.D.P., Legs 43, 48 and 50)     

Norbert Clauer  Patrick Giblin  Jacques Lucas 《Chemical Geology》1984,46(2):141-151

Mineralogical, morphological and isotopic (RbSr and KAr) determinations were made on some detrital smectites of Palaeocene and Cenomanian ages from D.S.D.P. drillings in the Atlantic Ocean. These minerals are not inert in their depositional environment; authigenic laths grow on detrital sheets with sharp borders. This authigenesis could occur slightly after deposition in a closed system, for some of these smectites. It has been tentatively quantified by the RbSr and KAr isotopic methods, which seem also well suited to evaluate the chemical extent of this authigenesis. At least, no preferential loss of ⁴⁰Ar vs. ⁸⁷Sr could be detected in the minerals, even in those which are smaller than 0.2 μm.  相似文献   

Attenuation of a Mixed Chromium and Chlorinated Etjeme Ground Wate Plume in Estuarine Influenced Glaciated Sediments     

Lucas A. Hellerich  Matthew A. Poneiera  Nikolaos P. Nikolaidis  Barth F. Smets  Gregory M. Dobbs 《Ground Water Monitoring & Remediation》2003,23(3):74-84

The natural attenuation behavior of a ground water contaminant plume containing chromium and chlorinated ethenes in glaciated sediments was assessed using traditional and nontraditional methods. The mixed waste is transported through and attenuated within an estuarine influenced ground water aquifer of spatially varying redox character and organic carbon content. Contaminant fate and speciation were assessed as a function of geochemical conditions. Total, speciation-based, and sequential chemical extraction analyses were performed to determine contaminant partitioning and the redox capacity of the aquifer. Chromium speciation and partitioning were correlated with the reductive capacity and redox conditions of the aquifer sediments spatially distributed within the aquifer. Reductive dechlorination and partitioning of chlorinated ethenes were correlated with the organic carbon content and redox conditions of the aquifer sediments. The data showed that sharp redox gradients existed within the aquifer. Active reduction and retardation of both chromium and chlorinated ethenes was exhibited. The aqueous hexavalent chromium concentrations decreased to near nondetect levels in the vicinity of the receptor, whereas degradation products of higher-order chlorinated ethenes increased as a fraction of the total chlorinated ethene concentrations along the length of the plume. The potential for competition for reducing power under specific cases within the aquifer was suggested by the data, highlighting the need to include contaminant interactions in natural attenuation assessments.  相似文献   

BASIC: Baltic Sea cyanobacteria. An investigation of the structure and dynamics of water blooms of cyanobacteria in the Baltic Sea—responses to a changing environment     

Lucas J. Stal  Patrizia Albertano  Birgitta Bergman  Klaus von Brckel  John R. Gallon  Paul K. Hayes  Kaarina Sivonen  Anthony E. Walsby 《Continental Shelf Research》2003,23(17-19):1695

The blooms of cyanobacteria that develop each summer in the Baltic Sea are composed of two functional groups, namely the small-sized picocyanobacteria (Synechococcus sp.) and the larger, colony-forming, filamentous N₂-fixing cyanobacteria. The former encompassed both red (phycoerythrin-rich) and blue-green (phycocyanin-rich) species. The majority of the picocyanobacteria measured less than 1 μm and this size fraction comprised as much as 80% of the total cyanobacterial biomass and contributed as much as 50% of the total primary production of a cyanobacterial bloom. The picocyanobacteria are incapable of fixing N₂, do not possess gas vesicles and are not toxic. However, a small filamentous Pseudanabaena sp. that could potentially fix N₂ was isolated from the picocyanobacteria fraction. The larger cyanobacteria may form surface scums because they possess gas vesicles that make them buoyant. Although their biomass was less than the picocyanobacteria, they therefore form the more conspicuous and nuisance-forming part of the bloom. The larger cyanobacteria were composed mainly of three different species: Nodularia spumigena, Aphanizomenon flos-aquae and Anabaena sp. These all belong to the heterocystous, N₂-fixing cyanobacteria. N. spumigena and A. flos-aquae were the dominant species; only N. spumigena was toxic. Although individual Nodularia filaments showed a range of different phenotypes, they all belong to one species as judged from 16S rDNA sequencing. Through determination of the genotypes of many individual Nodularia filaments, it was shown that this population was not clonal and that horizontal exchange of genetic information occurs. N. spumigena and A. flos-aquae were different with respect to their photosynthetic and N₂-fixing potentials. Depending on prevailing environmental conditions, these differences would promote the proliferation of one species over the other and hence would determine overall the toxicity of a bloom. Daily integrals of photon irradiance rather than temperature determined the onset of bloom formation. During a bloom, the diazotrophic cyanobacteria fixed N₂ at a rate that was 10–20% in excess of their own demand for N. Picocyanobacteria assimilated most of this excess N as shown by ¹⁵N incorporation. During bloom conditions, the diazotrophic cyanobacteria met about 50% of the N demand of the total cyanobacterial community. The picocyanobacteria were predominantly N-limited while the diazotrophic cyanobacteria were probably iron limited. These findings allow us to understand the formation of toxic cyanobacterial blooms and also to develop tools to predict bloom formation.  相似文献   

Automatic Surface Reconstruction Based on the Fusion of Fuzzy Logic and Robust Estimation Techniques     

Farhad Samadzadegan  Ali Azizi  Caro Lucas  & Michael Hahn 《The Photogrammetric Record》2002,17(100):651-674

Automatic surface reconstruction by means of digital image matching essentially involves computation in both image and object spaces. In image space, image matching is performed according to certain radiometric and geometric similarity criteria for conjugate point determination. The matching operations may therefore be regarded as decision–making processes. In practice, however, the similarity criteria for conjugate point determination demonstrate non–deterministic behaviour. Thus, the nature of these decision–making processes is more compatible with fuzzy reasoning concepts. In object space, on the other hand, surface reconstruction procedures, which operate on the basis of previously determined conjugate points, are essentially deterministic in nature and hence demand rigorous geometric modelling. In this paper, a new approach for automatic surface modelling is proposed, based on two different mathematical treatments: a fuzzy logic reasoning method for the conjugate point determination in image space; and a robust finite elements approach for the surface modelling in object space. Tests carried out on real data demonstrate the high potential of the strategy proposed.  相似文献   

Characterisation of a Pleistocene debris-avalanche deposit in the Tenteniguada Basin,Gran Canaria Island,Spain     

Alejandro Lomoschitz  Javier Hervás  Jorge Yepes  Joaquín Meco 《Landslides》2008,5(2):227-234

We studied a large debris-avalanche deposit of Pleistocene age in the Tenteniguada Basin, Gran Canaria Island, Spain. This deposit, which is well preserved because it is mostly covered by basanite lava flows, has distinctive matrix and block facies, hummocky topography and internal structures typical of debris avalanches. However, neither syneruptive lavas nor some characteristic features of volcanic debris-avalanche deposits, such as a stratovolcano edifice or a horseshoe-shaped crater, are present. The occurrence of internal features characteristic of volcanic avalanche deposits could be attributed to the volcanic materials involved in the movement rather than to the triggering of the avalanche during a volcanic eruption. The conditioning factors are shown to be associated with specific structural and hydrological conditions, such as the presence of old volcanic domes, strength reduction of the rocks, effective stress decrease, active gully erosion and water table rise during Pleistocene humid episodes. We finally suggest that the possible triggering factor of the avalanche was a neighbouring volcanic or tectonic earthquake.  相似文献   

Plant macrofossil assemblages from surface sediment represent contemporary species and growth forms of aquatic vegetation in a shallow Mediterranean lake     

Jorge García-Girón  Camino Fernández-Aláez  Alejandro Nistal-García  Margarita Fernández-Aláez 《Journal of Paleolimnology》2018,60(4):495-509

Macrofossils are known as a useful tool in reconstructing their original plant communities. However, most studies have been focused on comparing the composition and distribution of living plant communities and their remains in temperate lakes. Mediterranean shallow lakes have been historically far less studied and little is known about the relationships between Mediterranean macrophyte communities and their remains. The aim of our study is to assess how contemporary aquatic macrophyte communities are represented by their sedimentary remains in terms of composition, distribution and concordance between the contemporary and the subfossil assemblages in a procrustean superimposition space, and to determine which surface sediment cores, collected along a depth gradient, may represent best the whole-lake macrofossil assemblage. These analyses were carried out for both species and macrophyte growth forms (submerged hydrophytes, floating-leaved hydrophytes, helophytes and charophytes) in order to check which of the two (species and growth forms) were represented best by their macro-remains. The most abundant present-day species (Myriophyllum alterniflorum DC. and Potamogeton trichoides L.) were under-represented while Characeae and some floating-leaved hydrophytes (Polygonum amphibium L. and Ranunculus peltatus Schrank) were over-represented in sedimentary samples. Additionally, macro-remains of submerged hydrophytes and helophytes were generally found in the central areas and in close proximity to contemporary vegetation, whereas floating-leaved hydrophytes distributed close to the near-shore. Notwithstanding some disparities between contemporary vegetation and their macrofossil assemblages, we found a good agreement between present-day and sedimentary datasets for both species and macrophyte growth forms. Furthermore, our study suggests that sediment cores from deep areas are more likely to represent best the whole-lake macrofossil assemblage because of their high diversity, equitability and heterogeneity. We conclude that aquatic macrophyte subfossils from the central areas of the basin can be a very useful tool in tracking the species composition and structure of the original macrophyte communities in shallow Mediterranean lakes. Additionally, when considering the use of macro-remains to reconstruct the composition and structure of macrophyte growth forms, we recommend a multicore approach that uses transects running from the shore to the lake center.  相似文献   

A very cool brown dwarf in UKIDSS DR1     

S. J. Warren  D. J. Mortlock  S. K. Leggett  D. J. Pinfield  D. Homeier  S. Dye  R. F. Jameson  N. Lodieu  P. W. Lucas  A. J. Adamson  F. Allard  D. Barrado y Navascués  M. Casali  K. Chiu  N. C. Hambly  P. C. Hewett  P. Hirst  M. J. Irwin  A. Lawrence  M. C. Liu †  E. L. Martín  R. L. Smart  L. Valdivielso  B. P. Venemans 《Monthly notices of the Royal Astronomical Society》2007,381(4):1400-1412

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Spatial and temporal variation of picoplanktic cyanobacteria population in a density stratified estuary,and the introduction of a cellular gradient number     

Alejandro Carrillo  Pablo Huq  Mari C. Pérez  José M. Redondo 《Estuarine, Coastal and Shelf Science》2008

Spatial and temporal variations in the distribution of the marine picoplanktic cyanobacteria population and mixing conditions were found in the Ebro River estuary outflow to the Mediterranean Sea in Spain. Six sampling surveys were undertaken between July 1999 and February 2000 for distances up to 15 km from the river mouth. Measurements were taken of flow velocity, salinity, temperature, depth and picocyanobacteria (PCB) abundances. Gradient Richardson (Rig) and Reynolds (Re) numbers were determined to evaluate hydrodynamics. In summer, large values of Rig arise from the small flow rates, and small values of velocity shear between the surface fresh water layer and the bottom saline layer; conversely, in winter the large flow rates and attendant large velocity shears between the layers give rise to small values of Rig. Flow conditions in the fall are an intermediate case between the summer and winter cases. Vertical abundance distributions were resolved through the river water, interfacial region, and the bottom salt wedge; longitudinal gradients of PCB abundances were also resolved. Seasonal differences in the PCB abundance values were observed. Analysis of cell numbers (C) showed that the variable dC/dS, the dependence of cell number upon salinity gradient (dS) was critical. A non-dimensional number; the cellular gradient number (Cg) is introduced. Cg has useful biological interpretations that can potentially be included in ecological modeling. For example, Cg = 1 pertains to perfect adaptability of the organism to adjust to changing environmental conditions, whereas Cg = 0 describes total mortality. For a system with strong advection there is insufficient time for cells to adapt to the changing environment, and so those cell counts are unchanged. This is the case for the Ebro estuary in winter as advection of salinity (and hence PCB abundance) dominates the other loss processes for large flow rates.  相似文献   

Total mercury and methylmercury in sediments near offshore drilling sites in the Gulf of Mexico     

John H. Trefry  Robert P. Trocine  Michelle L. McElvaine  Robert D. Rember  Lucas T. Hawkins 《Environmental Geology》2007,53(2):375-385

Concentrations of total Hg in sediments near six drilling sites in the Gulf of Mexico were elevated well above average background values of 40–80 ng/g. The excess Hg was associated with barite from discharged drilling mud. In contrast with total Hg, concentrations of methylmercury (MeHg) in these sediments did not vary significantly at nearfield (<100 m) versus farfield (>3 km) distances from the drilling sites. Observed variability in concentrations of MeHg were related to local differences in redox state in the top 10 cm of sediment. Low to non-detectable concentrations of MeHg were found in nearfield sediments that were anoxic, highly reducing and contained abundant H₂S. At most drilling sites, nearfield samples with high concentrations of total Hg (>200 ng/g) had similar or lower amounts of MeHg than found at background (farfield) stations. Higher values of MeHg were found in a few nearfield sediments at one site where concentrations of TOC were higher and where sediments were anoxic and moderately reducing. Overall, results from this study support the conclusion that elevated concentrations of MeHg in sediments around drilling sites are not a common phenomenon in the Gulf of Mexico.  相似文献