A Density Correction for Lagrangian Particle Dispersion Models

Current Lagrangian particle dispersion models, used to simulate the dispersion of passive tracers in the turbulent planetary boundary layer (PBL), assume that the density is constant within the PBL. In deep PBLs, where the density at the boundary-layer top may be lower by more than 20% than at the surface, this assumption leads to errors in the tracer concentrations on the order of 10%. In the presence of a vertical wind shear, this also leads to inaccurate calculations of the horizontal tracer transport. To remove this deficiency, a Langevin equation is presented that contains a density correction term. The effect of the density correction is studied using data from a large-scale tracer experiment. It is found that for this experiment, the main effect of the density correction is an increase in the surface tracer concentrations, whereas the horizontal tracer transport patterns remain largely unaffected.     

The Transport Time pdf for a Simple Turbulent Flow

In a turbulent fluid, the time a particle needs to travel from a point source to the observation point, can be considered as a random variable. It is shown that the probability density function (pdf) for this random variable is determined by the Lagrangian particle position pdf. The characteristics of the transport time pdf are discussed for the simple case of a turbulent fluid moving with a constant mean velocity.     

Relative sea level changes during the Holocene in the Sisimiut area,south‐western Greenland

Holocene relative sea level (RSL) changes have been investigated by analysing and dating isolation sequences from five lakes near Sisimiut in south‐western Greenland. The transitions between marine and lacustrine sediments were determined from elemental analyses and analyses of macroscopic plant and animal remains. Radiocarbon dating was used to provide minimum ages for the transitions and to construct a RSL curve. Dating of a shell of the marine bivalve Macoma balthica indicates that deglaciation of the lowlands occurred in the early Holocene, at around 10 900 cal a BP. The RSL curve shows initial rapid regression from the marine limit at around 140 m, implying strong glacio‐isostatic rebound. We suggest that the margin of the Greenland Ice Sheet was located at the shelf break during the Last Glacial Maximum. Frequent remains of the ostracode Sarcypridopsis aculeata, which is a thermophilous brackish water species that is unknown from the extant fauna of Greenland, in one of the basins around 8500 cal a BP may mark the beginning of the Holocene thermal maximum in the region. Copyright © 2011 John Wiley & Sons, Ltd.     

Carbon and oxygen diffusion in calcite: Effects of Mn content and P H2O

The diffusion rates of carbon and oxygen in two calcite crystals of different Mn contents have been studied between 500° and 800° C in a CO₂-H₂O atmosphere (P _{CO 2}=1?5 bars, P _H2_O=0.02?24 bars) labeled with ¹³C and ¹⁸O. Isotope concentration gradients within annealed specimens were measured using a secondary ion microprobe by depth profiling parallel and perpendicular to the c axis. Despite the anisotropic structure of calcite, the diffusion of carbon and oxygen are both very nearly isotropic. Least-squares fitting of the carbon data to an Arrhenius relation gives an activation energy of 87±2 kcal/mole, with D ₀ terms dependent only slightly upon direction: 1 $$D_{\text{0}} {\text{(}}\parallel c{\text{) = }}\left( {9\frac{{ + 12}}{{ - 5}}} \right){\text{x10}}^{\text{2}} cm^2 /s$$ , 2 $$D_{\text{0}} {\text{(}} \bot c{\text{) = }}\left( {5\frac{{ + 6}}{{ - 3}}} \right){\text{x10}}^{\text{2}} cm^2 /s$$ . These results are in close agreement with previous determinations. Results for oxygen diffusion, however, give D values much larger than those previously reported for dry conditions; at 650° to 800° C the D values are two orders of magnitude larger. The diffusion of oxygen, unlike carbon, is strongly dependent on water pressure, as well as Mn content, and does not fit an Arrhenius relation over the entire temperature range. On the basis of these observations and considerations of the defect chemistry of calcite, it is proposed that carbon migrates as a Frenkel pair. The diffusion of oxygen, however, appears to be more complicated and may depend upon several simultaneous mechanisms.     

Mesozoischer Vulkanismus in Nordspanien: Rifting im Keuper und Kreide-Vulkanismus auf Transform-Störungen?

Zusammenfassung Während des Keupers und des frühen Jura wurden in Nordspanien petrographisch und chemisch recht einheitliche Vulkanite, die sog. Keuper-Ophite, gefördert. Ihre Haupt- und Spurenelement-Chemie weist sie als Tholeiite aus. Diskriminierungs-Diagramme ordnen sie Ozeanboden-Basalten zu, ihre Gehalte an Leichten Seltenen Erden sind sehr gering. Dieser Befund, der zunächst den geologischen Verhältnissen zu widersprechen schien, ließe sich interpretieren als der fehlgeschlagene Versuch einer Ozean-Bildung. Während des Keupers entwickelte sich in Nordspanien ein Rift-Graben. Eventuell stand dieser Rift-Graben von einem Tripelpunkt ausgehend mit dem später einsetzenden Spreading im westlichen Mittelmeer in Verbindung.Während der Kreide (unteres Cenoman bis Coniac) stiegen Alkali-Basalte auf, die in Gängen erstarrten oder als Pillow-Laven ausflössen. In Diskriminations-Diagrammen fallen sie in die Felder der Intraplatten-Basalte. Der Vulkanismus ist eng verknüft mit der Öffnung der Biskaya. In der östlichen Biskaya kam es zu einer Aufspaltung der Spreading-Achse. Ein nördliches Becken (in der heutigen Biskaya gelegen) und ein südliches Becken (der Basko-Kantabrische Trog) wurden von einer Mikroplatte, dem sog. Biskaya-Hoch, getrennt. Zwischen den beiden Divergenz-Zonen vermittelten vermutlich ozeanische Bruchzonen bzw. undichte Transform-Störungen, die stumpfwinklig zu den Beckenachsen verliefen, und auf denen die Kreide-Vulkanite gefördert wurden.

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During Late Triassic and Early Jurassic the so-called Keuper-Ophite volcanics were extruded in North Spain. These volcanics are petrographically and chemically rather uniform with a main and trace element composition which is typical for tholeiites. Their position in discriminationdiagrams shows that they are related to oceanfloor-basalts. The tholeiites show very low contents of light rare earths. This result, which contradicts geological evidence, could be interpreted as an aborted attempt of ocean-forming. A rift-graben developed in North Spain during Late Triassic. Possibly this rift-graben extended into a triple junction, which was connected with the spreading-zone in the western Mediterranean that became active at a later stage.During the Late Cretaceous (lower Cenomanian to Coniacian) alkali basalts formed dykes or extruded forming pillow-lavas. In discrimination-diagrams these basalts plot in the fields of intra-plate basalts. The volcanism is closely related to the opening of the Bay of Biscay. In the eastern Bay the spreading-axes split. A northern basin (situated in the present Bay of Biscay) and a southern basin (the Basco-Cantabrian Trough) were separated by a microplate, the so-called Biscay-High. Both zones of divergence were probably connected by obtusely striking oceanic fracturezones or leaky transform faults along which the Cretaceous volcanics were extruded.

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Résumé Dans le nord de l'Espagne, des roches volcaniques chimiquement et pétrographiquement uniformes ont été extrudées au cours du Keuper et de l'Eo-jurassique; elles sont connues sous le nom de »Keuper-ophites«. Le chimissme de leurs éléments majeurs et en traces les rattachent aux tholéiites. Leurposition dans les diagrammes de discrimination les rattache aux basaltes de la crocute océanique. Leur teneur en terres rares est très faible. Ces données, qui semblement en contradiction avec la situation géologique, peuvent s'interpréter comme témoignant d'une ouverture océanique avortée. Un rift s'est développé dans le nord de l'Espagne au Triassique supérieur. Il est possible que ce rift, formant une des branches d'une tripöe jonction, ait été en relation avec l'expansion qui a eu lieu plus tard en Méditérrannée occidentale.Au cours du Crétacé (du Cénomannien inférieur au Coniacien), des basaltes alcalins se sont mis en place sous forme de filons et de laves en coussins. Sur les diagrammes de discrimination, ils se situent dans les champs des basaltes intra-plaques. Le volcanisme est étroitement asocié à l'ouverture du golfe de Gascogne. Dans la partie E du golfe, l'axe d'ouverture s'est dichotomisé, donnant naissance à deux bassin méridional (la fosse basquo-cantabrique), séparés par une micro-plaque. Ces deux branches étaient probalement reiées par des zones de fractures océaniques ou par des failles transformates, qui faisaient un angle obtus avec les axes des bassins et le long desquelles ont pu monter les basalter crétacés.

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A versatile flow channel for laboratory experiments with running water macroinvertebrates

Experiments with running water organisms should be conducted under running water conditions. A flow channel which has proved useful for long-term rearing of running water macroinvertebrates is described. Through its arena-shaped design and water jet drive, damage to drifting animals is practically excluded.     

STABILITY OF GRAVEL BED RIVERS BASED ON SIEVE ANALYSIS DATA

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On wavelet analysis of nonstationary turbulence

Wavelets are new tools for turbulence analysis that are yielding important insights into boundary-layer processes. Wavelet analysis, however, has some as yet undiscussed limitations: failure to recognize these can lead to misinterpretation of wavelet analysis results. Here we discuss some limitations of wavelet analysis when applied to nonstationary turbulence. Our main point is that the analysis wavelet must be carefully matched to the phenomenon of interest, because wavelet coefficients obscure significant information in the signal being analyzed. For example, a wavelet that is a second-difference operator can provide no information on the linear trend in a turbulence signal. Wavelet analysis also yields no meaningful information about nonlinear behavior in a signal — contrary to claims in the literature — because, at any instant, a wavelet is a single-scale operator, while nonlinearity involves instantaneous interactions among many scales.     

Hydrometric and hydrochemicai evidence for fast flowpaths at La Cuenca, Western Amazonia

A hydrological reconnaissance study in a first-order tropical rainforest catchment in western Amazonia implicated overland flow as an important hydrological pathway. A complementary hydrometric and hydrochemical approach that involved the recording of overland flow hydrographs and the determination of streamflow, overland flow, groundwater, soil water, and throughfall chemical signatures, was essential to establish unambiguously the importance of this pathway. Largely uncontrolled by topography, overland flow does occur in any season, regardless of antecedent moisture conditions, which only influence the volumes generated. The latter effect is also reflected in a close approximation of stormflow and overland flow chemical signatures, as expressed in the K/SiO ratio. We conclude that, despite its greater logistical demands, a complementary hydrometric/hydrochemical approach is essential to understand a catchment's hydrological behaviour, especially where fast pathways are at work; such pathways are apparently common in more forest ecosystems than has been previously assumed.     

Characterization of and correction for cultural noise

Surveys of time varying electromagnetic fields result in time series consisting of signals and noise, the latter defined as that part of the data which cannot be explained by a theory. Man-made contributions to noise can be subdivided into active and passive sources and are complex in character. As Szarka has treated this topic extensively in a recent review paper (Szarka, 1988), only a few further examples are presented here. Following discussion of noise correction in transient electromagnetic investigations which consists mainly of sophisticated stacking and filter procedures, several aspects of its correction in magnetotelluric and geomagnetic depth sounding data are considered. These include:

1. The methods of treatment of single time series in the presence of visible noise—its detection, removal and sometimes replacement by data predicted from undisturbed intervals.
2. The investigation of time series interrelations. This is mainly coherence based and—if possible—takes advantage of remote reference techniques.
3. The examination of the statistical properties of the time series by regression analysis. This leads to the weighting of time segments of data in order to achieve unbiased and minimum variance estimates based on identically and independently Gaussian distributed residuals.
4. The application of constraints. These can further improve the estimates' quality.
5. The use of simultaneously recorded multistation data. This can contribute remarkably to noise suppression as well as to the treatment of non-uniform source fields.
6. Leveraging and confidence limits. Problems relating to the former have not yet been solved satisfactorily while the Jacknife method seems to be an easy way of determining the latter.

Thanks to the modern processing techniques reviewed in this paper it should be possible to obtain a rather dense net of high quality data in spite of the world-wide increasing noise level. As most processing codes are widely accessible current problems are more related to availability of instruments, carrying out the measurements and reserving enough time for thorough data processing.