Zooplankton variability and climatic anomalies from 1994 to 2001 in the Balearic Sea (Western Mediterranean)     

Fernandez de Puelles  M. Luz; Valencia  Joaquin; Vicente  Laura 《ICES Journal of Marine Science》2004,61(4):492-500

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Pathways of clay mineral transport in the coastal zone of the Brazilian continental shelf from Ceará to the mouth of the Amazon River     

J. O. de Morais  M. Tintelnot  G. Irion  L. Souza Pinheiro 《Geo-Marine Letters》2006,26(1):16-22

The transport pathways of fine sediments (fraction <2 μm) along the Brazilian continental shelf from Ceará to the Amazon River mouth were studied by means of clay mineral analyses. On the continental shelf southeast of the Amazon mouth, fluctuations in clay mineral compositions reflect simple mixing between the suspended load of the North Brazil Current and sediment from several smaller rivers. Previously, clay mineral variations west of the Amazon mouth have been explained by variable settling velocities of different grain-size classes within the <2 μm fraction or by selective coagulation of individual clay mineral groups. By contrast, our experiments with river bank samples show that selective coagulation does not occur in Amazon River sediments. A more appropriate explanation for observed variations in clay mineral composition off the Amazon mouth seems to be, similarly to that for the shelf between Ceará and the Amazon mouth, a mixing of Amazon sediments with suspended material of the North Brazil Current. This interpretation is supported by data on clay mineral composition east and south of the Amazon mouth, showing more affinity to sediments of the North Brazil Current than to the suspended load of the Amazon River. Additionally, relatively low sedimentation rates and low concentrations of fine-grained sediments on the shelf suggest that high riverine input by the Amazon River does not overprint the sediments of the North Brazil Current in this region. The strong North Brazil Current shunts the Amazon suspended load in a north-westerly direction along the north-eastern coast of South America. Hence, stronger sedimentation of Amazon sediments would occur only west of the river mouth.  相似文献   

Nutrient budgets and trophic state in a hypersaline coastal lagoon: Lagoa de Araruama, Brazil     

Marcelo F. L. Souza  Bjrn Kjerfve  Bastiaan Knoppers  Weber F. Landim de Souza  Raimundo N. Damasceno 《Estuarine, Coastal and Shelf Science》2003,57(5-6):843-858

Lagoa de Araruama in the state of Rio de Janeiro, Brazil, is a hypersaline lagoon with salinity varying spatially from 45 to 56. We collected water samples during monthly cruises throughout the lagoon, and along the streams feeding the system, from April 1991 to March 1992. Nutrients and other water quality parameters exhibited great spatial and temporal variations. Mass balance calculations indicate large amounts of anthropogenic nutrient inputs. The data indicate that the lagoon currently is oligotrophic but is in a state of transition to become a mesotrophic system. Molar dissolved inorganic nitrogen:dissolved inorganic phosphorus (DIN/DIP) varied between 2.2:1 and 659:1 with a volume-weighted average of 22:1. The high DIN/DIP ratio contrasts with that found in nearby lagoons, suggesting that phytoplankton primary production is limited by phosphorus in Lagoa de Araruama. The major loss of DIP is apparently driven by biological assimilation and diagenic reactions in the sediments. Calculations indicate that the lagoon is slightly net autotrophic at +0.9 mol C m⁻² yr⁻¹. This suggests that the biomass of the primary producers is restricted by phosphorus availability. Phosphorus retention in the sediment and the hypersaline state of the lagoon prevent changes in autotrophic communities and the formation of eutrophic conditions.  相似文献   

The influence of island generated eddies on the carbon dioxide system, south of the Canary Islands     

Melchor Gonzlez-Dvila  J. Magdalena Santana-Casiano  Demetrio de Armas  Jos Escnez  Miguel Suarez-Tangil 《Marine Chemistry》2006,99(1-4):177

Measurements of surface partial pressure of CO₂ and water column alkalinity, pH_T, nutrients, oxygen, fluorescence and hydrography were carried out, south of the Canary Islands during September 1998. Cyclonic and anticyclonic eddies were alternatively observed from the northwestern area to the central area of the Canary Islands. Nutrient pumping and vertical uplifting of the deep chlorophyll maximum by cyclonic eddies were also ascertained by upward displacement of dissolved inorganic carbon. A model was applied to determine the net inorganic carbon balance in the cyclonic eddy. The fluxes were determined considering both the diffusive and convective contributions from the upward pumping and the corresponding horizontal transport of water outside the area. An increase in the total inorganic carbon concentration in the upper layers inside the eddy field of 133 mmol C m^− 2 d^− 1 was determined. The upward flux of inorganic carbon decreased the effect of the increased primary production on the carbon dioxide chemistry. The reduced fCO₂ inside the cyclonic eddy, 15 μatm lower than that observed in non-affected surface water, was explained by thermodynamic aspects, biological activity, eddy upward pumping and diffusion and air–sea water exchange effects.  相似文献   

Stratigraphic framework of gas accumulations in the Ría de Pontevedra (NW Spain)     

Ruth Durán  Soledad García-Gil  Raquel Diez  Federico Vilas 《Geo-Marine Letters》2007,27(2-4):77-88

Knowledge of marine geological environments in which shallow gas is accumulating is becoming increasingly important in global studies of climate change because a measurable proportion of the total methane source comes from continental margins. Previous studies have revealed that coastal environments represent important geological environments where microbial methane is being generated, is accumulating, and is being released. In the Ría de Pontevedra, at least 4.5 km² of seafloor in the innermost part of the ría is underlain by sediments containing natural gas. Seismic interpretation contributes new findings for the definition of periods and geological environments in which the gas could have been generated, and is accumulating and released in the Ría de Pontevedra. Groundtruthing the seismic data (facies, environments) makes it possible to identify favourable geological environments for gas generation in the sedimentary infill of the Ría de Pontevedra. Sequence stratigraphy based on high-resolution seismic profiles and post-Last Glacial Maximum sea-level records makes it possible to establish the stratigraphic architecture of the ría and to define the periods in which gas could have been generated. The results of this study show that the sedimentary infill is composed of a fifth-order sequence developed since the Last Glacial Maximum. Within this sequence, gas appears to have accumulated in the Holocene deposits associated with the latest transgressive and highstand system tracts. Seismic analysis shows that gas could have been generated in different geological environments in the Ría de Pontevedra. If coastal environments at times of lower sea level were similar to those of the present, organic-rich mud deposits (deposited mainly in lakes, estuaries and floodplains) could have survived transgression and remained buried as potential gas sources in the inner part of the ría.  相似文献   

Diversity and community structure of epibenthic invertebrates and fish in the North Sea   总被引：5，自引：3，他引：5  

Callaway  R.; Alsvag  J.; de Boois  I.; Cotter  J.; Ford  A.; Hinz  H.; Jennings  S.; Kroncke  I.; Lancaster  J.; Piet  G.; Prince  P.; Ehrich  S. 《ICES Journal of Marine Science》2002,59(6):1199-1214

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The Bayesian detection of discontinuities in a polynomial regression and its application to the cycle-slip problem   总被引：4，自引：1，他引：3  

Maria Clara de Lacy  Mirko Reguzzoni  Fernando Sansò  Giovanna Venuti 《Journal of Geodesy》2008,82(9):527-542

This paper deals with the problem of detecting and correcting cycle-slips in Global Navigation Satellite System (GNSS) phase data by exploiting the Bayesian theory. The method is here applied to undifferenced observations, because repairing cycle-slips already at this stage could be a useful pre-processing tool, especially for a network of permanent GNSS stations. If a dual frequency receiver is available, the cycle-slips can be easily detected by combining two phase observations or phase and range observations from a single satellite to a single receiver. These combinations, expressed in a distance unit form, are completely free from the geometry and depend only on the ionospheric effect, on the electronic biases and on the initial integer ambiguities; since these terms are expected to be smooth in time, at least in a short period, a cycle-slip in one or both the two carriers can be modelled as a discontinuity in a polynomial regression. The proposed method consists in applying the Bayesian theory to compute the marginal posterior distribution of the discontinuity epoch and to detect it as a maximum a posteriori (MAP) in a very accurate way. Concerning the cycle-slip correction, a couple of simultaneous integer slips in the two carriers is chosen by maximazing the conditional posterior distribution of the discontinuity amplitude given the detected epoch. Numerical experiments on simulated and real data show that the discontinuities with an amplitude 2 or 3 times larger than the noise standard deviation are successfully identified. This means that the Bayesian approach is able to detect and correct cycle-slips using undifferenced GNSS observations even if the slip occurs by one cycle. A comparison with the scientific software BERNESE 5.0 confirms the good performance of the proposed method, especially when data sampled at high frequency (e.g. every 1 s or every 5 s) are available.  相似文献   

Spin rate estimation of sounding rockets using GPS wind-up   总被引：2，自引：1，他引：1  

M. García-Fernández  M. Markgraf  O. Montenbruck 《GPS Solutions》2008,12(3):155-161

Carrier phase wind-up is a well-known effect that arises from the relative rotation between a transmitting and receiving antenna. In GPS measurements at L1 frequency, this effect translates into an error of 19.029 cm per full relative rotation of antennas. Since this effect is independent of the satellite elevation for pure rotation about the antenna boresight axis, it is usually absorbed by the clock estimation in navigation algorithms. Therefore, the impact of wind-up is usually neglected for applications that do not require accuracies to the cm level like RTK. However, in receiving platforms with high rotation rate, the accumulated wind-up value can be important and actually be larger than receiver noise or even ionospheric variations. Therefore, in such scenarios, the wind-up contribution can be isolated and used as a source of information to compute the spin rate of such platforms using an appropriate combination of GPS observables. This work shows some results of a coarse, yet simple, approach to monitor the rotation angle and spin-rate of spin stabilized sounding rockets flown by DLR.  相似文献   

Modelling Positional Uncertainty of Line Features by Accounting for Stochastic Deviations from Straight Line Segments   总被引：1，自引：0，他引：1  

Sytze de Bruin 《Transactions in GIS》2008,12(2):165-177

The assessment of positional uncertainty in line and area features is often based on uncertainty in the coordinates of their elementary vertices which are assumed to be connected by straight lines. Such an approach disregards uncertainty caused by sampling and approximation of a curvilinear feature by a sequence of straight line segments. In this article, a method is proposed that also allows for the latter type of uncertainty by modelling random rectangular deviations from the conventional straight line segments. Using the model on a dense network of sub‐vertices, the contribution of uncertainty due to approximation is emphasised; the sampling effect can be assessed by applying it on a small set of randomly inserted sub‐vertices. A case study demonstrates a feasible way of parameterisation based on assumptions of joint normal distributions for positional errors of the vertices and the rectangular deviations and a uniform distribution of missed sub‐vertices along line segments. Depending on the magnitudes of the different sources of uncertainty, not accounting for potential deviations from straight line segments may drastically underestimate the positional uncertainty of line features.  相似文献   

Simulations of 3D DC borehole resistivity measurements with a goal-oriented <Emphasis Type="Italic">hp</Emphasis> finite-element method. Part II: through-casing resistivity instruments     

D. Pardo  C. Torres-Verdín  M. Paszynski 《Computational Geosciences》2008,12(1):83-89

We simulate direct current (DC) borehole resistivity measurements acquired in steel-cased deviated wells for the assessment of rock formation properties. The assumed data acquisition configuration considers one current (emitter) and three voltage (collector) electrodes that are utilized to measure the second difference of the electric potential along the well trajectory. We assume a homogeneous, 1.27-cm-thick steel casing with resistivity equal to 10^− 5 Ω· m. Simulations are performed with two different numerical methodologies. The first one is based on transferring two-dimensional (2D) axisymmetric optimal grids to a three-dimensional (3D) simulation software. The second one automatically produces optimal 3D grids yielded by a 3D self-adaptive goal-oriented algorithm. Both methodologies utilize high-order finite elements (FE) that are specially well-suited for problems with high-contrast coefficients and rapid spatial variations of the electric field, as it occurs in simulations that involve steel-cased wells. The method based on transferring 2D-optimal grids is efficient in terms of CPU time (few seconds per logging position). Unfortunately, it may produce inaccurate 3D simulations in deviated wells, even though the error remains below 1% for the axisymmetric (vertical) well. The method based on optimal 3D grids, although less efficient in terms of CPU time (few hours per logging position), produces more accurate results that are validated by a built-in a posteriori error estimator. This paper provides the first existing simulations of through-casing resistivity measurements in deviated wells. Simulated resistivity measurements indicate that, for a 30° deviated well, measurements in conductive layers 0.01 Ω· m) are similar to those obtained in vertical wells. However, in resistive layers (10,000 Ω· m), we observe 100% larger readings in the 30° deviated well. This difference becomes 3,000% for the case of a 60° deviated well. For this highly-deviated well, readings corresponding to the conductive formation layer are about 30% smaller in magnitude than those in a vertical well. Shoulder effects significantly vary in deviated wells.  相似文献