Modelling the formation of residual dune ridges behind barchan dunes in North-east Brazil     

NOAM LEVIN  HAIM TSOAR†  HANS JÜRGEN HERRMANN‡  §  LUIS P. MAIA¶  VANDA CLAUDINO-SALES 《Sedimentology》2009,56(6):1623-1641

Residual dune ridges are often formed by vegetation growing along a line some distance upwind of the lower stoss slope of migrating dunes. This process is common in areas where vegetation germinates along the edge of the water during the rainy period when the water level is higher and interdune areas are flooded. The phenomenon occurs on a large scale in North‐east Brazil, because of the rise and fall in groundwater level at the end of the rainy season. Each residual dune ridge corresponds to the position of the dune during the wet period in each year. Therefore, variations in the distance between these residual dune ridges could be used potentially to monitor climatic fluctuations in rainfall and wind. To examine the potential use of these residual dune ridges for the reconstruction of past climatic fluctuations, a model that simulates them under varying conditions of wind, rainfall and evaporation rates was formulated. The model was tested for sensitivity to climatic variability in North‐east Brazil and validated against residual dune ridge displacements as measured in the field and from high spatial resolution satellite images. Based on the results, it is concluded that residual dune ridges may not form in North‐east Brazil in years which are exceptionally dry, as may happen during El‐Niño events. When this type of event happens, the distance between adjacent residual dune ridges corresponds to more than one year and, therefore, the correlation between dune displacements and wind power becomes weak or even disappears. Additionally, because of biotic, aeolian and hydrological processes, these arcuate residual dune ridges may not preserve their initial shape for long periods. The presence of residual dune ridges testifies to the temporary flooding which may or may not be seasonal. However, the potential for using residual dune ridges to reconstruct the palaeo‐climate of wind regime on a yearly basis or to identify past El‐Niño events seems to be limited.  相似文献   

Ordovician conodonts from the Bulltinden Formation, Motalafjella, central-western Spitsbergen     

HOWARD A. ARMSTRONG  HANS A. NAKREM  YOSHIHIDE OHTA 《Polar research》1986,4(1):17-23

Seven samples from the Limestone and Boulder Conglomerate Members of the Bulltinden Formation have yielded poorly preserved conodonts. On tentative identifications Periodon aculeatus Hadding and Prioniodus cf. P. alobatus Bergstrom at the base of the formation suggest an Ordovician lower Caradoc, or older age. This confirms that the Vcstgotabrcen Formation unconformably beneath underwent high-pressure mctamorphism during the Taconic and/or Finmarkian phases of the Caledonian orogeny. The conodonts show affinity to the North Atlantic Province suggesting a deeper water marine depositional environment. Conodont colour alteration indices of five indicate that the Bulltinden Formation was heated to temperatures in excess of 300°C, probably during the middle-upper Silurian main Caledonian uplift event.  相似文献   

Stability Relations of the Ferruginous Biotite, Annite   总被引：12，自引：0，他引：12  

EUGSTER  HANS P.; WONES  DAVID R. 《Journal of Petrology》1962,3(1):82-125

Annite, KFe₃AISi₃O₁₀(OH)₂ a member of the iron biotites andthe ferrous analogue of phlogopite, has been synthesized andits phase relations have been determined as functions of temperature,fugacity of oxygen (fo₂), and total pressure (P_totalPH₂O+PH₂).A method for controlling fo₂at high total pressures is described,and data for the ‘oxygen buffers’ used are given.Buffers range from quartz+iron+fayalite assemblages (low fo₂)to magnetite-hematite assemblages (high fo₂). Optical propertiesand unit-cell dimensions of synthetic annites depend on theconditions of synthesis. By recalculating published analyses of natural iron-rich biotitesit can be shown that one cannot assume a constant hydrogen contentfor such biotites. Oxidation may have occurred by drying at115?C. Octahedral occupancy therefore cannot be calculated fromsuch data. Phase relations of annite are presented in 2,070 and 1,035 barsections. Depending on fo₂-T values annite was found to decomposeto one of the following assemblages: hematite+ sanidine, magnetite+sanidine,fayalite+leucite+kalsilite, iron+sanidine. All decompositionsare dehydration and redox reactions and are sensitive to changesin fH₂0 and fo₂ (or fH₂0 and fH₂). At 2, 070 bars total pressureannite+magnetite+sanidine can coexist between 425?C and 825?C, depending upon the magnitude of fo₂. In the presence of quartz the stability field of annite is morerestricted. Phase equilibria in the system KAlSiO₄–SiO₂–Fe–O₂–H₂have been summarized schematically. Wherever possible, thermodynamic extrapolations are made totest the internal consistency of the data. Enthalpies of formationare calculated for both annite and phlogopite. Ranges of fo₂values in nature as well as mechanisms for changes in fo₂ areinvestigated. It is useful to distinguish between assemblageswhich are internally buffered with respect to fo₂changes andthose which are not buffered. The applications of individualreactions involving annite to specific geologic problems arediscussed with respect to igneous, metamorphic, and sedimentaryrocks.  相似文献   

THE DEPOSITIONAL ENVIRONMENT OF MARINE EVAPORITES: A CASE FOR SHALLOW, CLASTIC ACCUMULATION     

LAWRENCE A. HARDIE  HANS P. EUGSTER 《Sedimentology》1971,16(3-4):187-220

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EDITORIAL     

HANS FÜCHTBAUER 《Sedimentology》1969,12(1-2):7-10

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Pre-Loessic Faulting in the Nanking Hills     

HANS BECKER 《地质学报》1937,(Z1)

During the last year I had ample opportunity for geologicalexcursions in the surroundings of Nanking,both with the students of theDepartment of Geology of National Central University,and in connectionwith fieldwork for the Geological Research Institute of the AcademiaSinica.From casual observations on these trips,I came to some  相似文献   

Statistical downscaling of monthly mean North Atlantic air-pressure to sea level anomalies in the Baltic Sea   总被引：1，自引：0，他引：1  

HAUKE HEYEN  EDUARDO ZORITA  HANS VON STORCH 《地球，A辑:动力气象学与海洋学》1996,48(2):312-323

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An idealized model of the one-dimensional carbon dioxide rectifier effect     

VINCENT E. LARSON  HANS VOLKMER 《Tellus. Series B, Chemical and physical meteorology》2008,60(4):525-536

The net flux of carbon dioxide (CO₂) from the land surface into the atmospheric boundary layer has a diurnal cycle. Drawdown of CO₂ occurs during daytime photosynthesis, and return of CO₂ to the atmosphere occurs during night. Even when the net diurnal-average surface flux vanishes, the diurnal-average profile of atmospheric CO₂ mixing ratio is usually not vertically uniform. This is because of the diurnal rectifier effect, by which atmospheric vertical transport and the surface flux conspire to produce a surplus of CO₂ near the ground and a deficit aloft.
This paper constructs an idealized, 1-D, eddy-diffusivity model of the rectifier effect and provides an analytic series solution. When non-dimensionalized, the intensity of the rectifier effect is related solely to a single 'rectifier parameter'. Given this model's governing equation and boundary conditions, we prove that the existence of the rectifier effect is related to the correlation of CO₂ gradient and transport, and also to the day–night symmetry of transport.
The rectifier-induced near-surface CO₂ surplus ought to be included in inverse calculations that use near-surface CO₂ mixing ratio to infer land–surface sources and sinks of carbon. Such inverse modeling is facilitated by our model's simplicity. To illustrate, we use a 1-D inverse calculation to infer the amplitude of diurnal CO₂ surface flux.  相似文献   

How robust are Holocene treeline simulations? A model–data comparison in the European Arctic treeline region     

KEYAN FANG  JESSE L. MORRIS  J. SAKARI SALONEN  PAUL A. MILLER  HANS RENSSEN  MARTIN T. SYKES  HEIKKI SEPPÄ 《第四纪科学杂志》2013,28(6):595-604

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Distinguishing fluvio‐deltaic facies by bulk geochemistry and heavy minerals: an example from the Miocene of Denmark     

METTE OLIVARIUS  ERIK S. RASMUSSEN  VOLKERT SIERSMA  CHRISTIAN KNUDSEN  GUNVER K. PEDERSEN 《Sedimentology》2011,58(5):1155-1179

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