Orthopyroxene rim growth between olivine and quartz at low temperatures (750–950°C) and low water concentration     

Ralf Milke  Kornelia Kolzer  Monika Koch-Müller  Bernd Wunder 《Mineralogy and Petrology》2009,97(3-4):223-232

Orthopyroxene reaction rims were synthesized between polished plates of natural olivine or synthetic forsterite and quartz at 1.9 GPa and temperatures of 750–950°C. The experiments were performed in a piston-cylinder apparatus after drying the samples at 600°C. Each experiment comprised 4 or 7 quartz-olivine contacts that were positioned along a temperature gradient. As a monitor for water content in the samples, the water concentration in the two olivines was determined by FTIR before and after the experiments. The orthopyroxene layers show two different structural variants. Type one (normal layers) has very constant thickness at each contact and formed with equal growth rates at both interfaces. Type two (bulging layers) comprises more irregular areas with 3–5 times thicker rims where porosity provides evidence for the local presence of a fluid. In the bulging layers the growth rate at the olivine-orthopyroxene interface exceeds that at the quartz-orthopyroxene interface. The relative growth rates at the interfaces are in accordance with SiO₂-immobile growth of the normal layers and SiO₂-mobile growth of the bulging layers. The natural olivine contains about 60 wt-ppm intracrystalline water before and after experiment and took up about 20 wt-ppm water molecular adsorbed to micro- and nanocracks and -pores during the runs. The synthetic forsterite contains about 7 wt-ppm internally adsorbed molecular water before and after experiment, and during the runs took up hydrogen equivalent to 3 wt-ppm adsorbed water. The IR spectra indicate that large parts of the point defects (possibly tetrahedral) were frozen-in at the conditions of the experiments. In both olivines a new band appeared at 3,355 or 3,357 cm^?1, respectively, equivalent to about 3 wt-ppm water that at the high pressure of the experiments and opx-buffered aSiO₂ of the experiments might already mean water saturation of the olivines. Despite the effective drying before experiment and the absence of porosity, the bulk diffusivity derived from the rim growth rates is perfectly in line with data from water-bearing piston-cylinder experiments at higher temperatures. The bulk diffusivity during rim growth is 4 to 7 orders of magnitude higher than an extrapolation of really dry experiments to the temperature range of this study.  相似文献   

Do swarms of migrating barchan dunes record paleoenvironmental changes? — A case study spanning the middle to late Holocene in the Pampa de Jaguay, southern Peru     

Ralf Hesse   《Geomorphology》2009,104(3-4):185-190

Swarms of barchan dunes are common in arid environments. Unlike immobile or slowly moving dunes whose stratigraphy can be used to reconstruct paleoenvironmental changes, the high migration rate of barchans seems to prohibit their use as geoarchives. In this paper, a possible use of barchan swarms for paleoenvironmental reconstructions is presented. The approach is based on dune migration. In a case study based on 500 dunes in the Pampa de Jaguay aeolian transport corridor in coastal southern Peru, middle to late Holocene changes in the supply of aeolian material are inferred from calculating the time of dune initiation at the coastline. Limitations posed by the assumptions underlying this approach are discussed.  相似文献   

Multiphase flow above explosion sites in debris-filled volcanic vents: Insights from analogue experiments     

Pierre-Simon Ross  James D.L. White  Bernd Zimanowski  Ralf Büttner 《Journal of Volcanology and Geothermal Research》2008

Discrete explosive bursts are known from many volcanic eruptions. In maar–diatreme eruptions, they have occurred in debris-filled volcanic vents when magma interacted with groundwater, implying that material mobilized by such explosions passed through the overlying and enclosing debris to reach the surface. Although other studies have addressed the form and characteristics of craters formed by discrete explosions in unconsolidated material, no details are available regarding the structure of the disturbed debris between the explosion site and the surface. Field studies of diatreme deposits reveal cross-cutting, steep-sided zones of non-bedded volcaniclastic material that have been inferred to result from sedimentation of material transported by “debris jets” driven by explosions. In order to determine the general processes and deposit geometry resulting from discrete, explosive injections of entrained particles through a particulate host, we ran a series of analogue experiments. Specific volumes of compressed (0.5–2.5 MPa) air were released in bursts that drove gas-particle dispersions through a granular host. The air expanded into and entrained coloured particles in a small crucible before moving upward into the host (white particles). Each burst drove into the host an expanding cavity containing air and coloured particles. Total duration of each run, recorded with high-speed video, was approximately 0.5–1 s. The coloured beads sedimented into the transient cavity. This same behaviour was observed even in runs where there was no breaching of the surface, and no coloured beads ejected. A steep-sided body of coloured beads was left that is similar to the cross-cutting pipes observed in deposits filling real volcanic vents, in which cavity collapse can result not only from gas escape through a granular host as in the experiments, but also through condensation of water vapour. A key conclusion from these experiments is that the geometry of cross-cutting volcaniclastic deposits in volcanic vents is not directly informative of the geometry of the “intrusions” that formed them. An additional conclusion is that complex structures can form quickly from discrete events.  相似文献   

Modelling the water balance of a mesoscale catchment basin using remotely sensed land cover data     

Carsten Montzka  Morton Canty  Ralf Kunkel  Gunter Menz  Harry Vereecken  Frank Wendland 《Journal of Hydrology》2008,353(3-4):322-334

Hydrological modelling of mesoscale catchments is often adversely affected by a lack of adequate information about specific site conditions. In particular, digital land cover data are available from data sets which were acquired on a European or a national scale. These data sets do not only exhibit a restricted spatial resolution but also a differentiation of crops and impervious areas which is not appropriate to the needs of mesoscale hydrological models. In this paper, the impact of remote sensing data on the reliability of a water balance model is investigated and compared to model results determined on the basis of CORINE (Coordination of Information on the Environment) Land Cover as a reference. The aim is to quantify the improved model performance achieved by an enhanced land cover representation and corresponding model modifications. Making use of medium resolution satellite imagery from SPOT, LANDSAT ETM+ and ASTER, detailed information on land cover, especially agricultural crops and impervious surfaces, was extracted over a 5-year period (2000–2004). Crop-specific evapotranspiration coefficients were derived by using remote sensing data to replace grass reference evapotranspiration necessitated by the use of CORINE land cover for rural areas. For regions classified as settlement or industrial areas, degrees of imperviousness were derived. The data were incorporated into the hydrological model GROWA (large-scale water balance model), which uses an empirical approach combining distributed meteorological data with distributed site parameters to calculate the annual runoff components. Using satellite imagery in combination with runoff data from gauging stations for the years 2000–2004, the actual evapotranspiration calculation in GROWA was methodologically extended by including empirical crop coefficients for actual evapotranspiration calculations. While GROWA originally treated agricultural areas as homogeneous, now a consideration and differentiation of the main crops is possible. The accuracy was determined by runoff measurements from gauging stations. Differences in water balances resulting from the use of remote sensing data as opposed to CORINE were analysed in this study using a representative subcatchment. Resulting Nash–Sutcliff model efficiencies improved from 0.372 to 0.775 and indicate that the enhanced model can produce thematically more accurate and spatially more detailed local water balances. However, the proposed model enhancements by satellite imagery have not exhausted the full potential of water balance modelling, for which a higher temporal resolution is required.  相似文献   

Plankton community as an indicator of water quality in tropical shrimp culture ponds   总被引：5，自引：0，他引：5  

Casé M  Leça EE  Leitão SN  Sant Anna EE  Schwamborn R  de Moraes Junior AT 《Marine pollution bulletin》2008,56(7):1343-1352

The plankton was examined as an indicator of water quality in 14 shrimp Litopenaeus vannamei farms in Brazil in 2003. The ponds were categorized by high stocking density (>30 PL m⁻²) of phytoplankton, consisting of 51 species with concentrations ranging from 365,218 ± 416,615 cells mL⁻¹ to 1,961,675 ± 3,160,172 cells mL⁻¹. Diatoms contributed to almost 70% of the species number and high densities resulted from Cyanophyta blooms, mainly Pseudanabaena cf limnetica. Forty zooplankton taxa were registered and were essentially composed of typical marine euryhaline species and suspension-feeders. Copepoda dominated (45%) the make-up, followed by Protozoa (18%), Rotifera (12%), and Mollusca (12%) larvae. Zooplankton varied from 972 ± 209 ind m⁻³ to 4235 ± 2877 ind m⁻³. Enhanced nutrient input affected plankton density and composition. Diatom and Copepoda dominance was replaced by cyanobacteria, protozoan, and rotifers as nutrient concentrations increased with the cultured period, indicating that plankton structure is affected by eutrophic conditions.  相似文献   

What is DMO coverage?     

Ralf Ferber 《Geophysical Prospecting》2000,48(6):995-1008

'Coverage' or 'fold' is defined as the multiplicity of common-midpoint (CMP) data. For CMP stacking the coverage is consistent with the number of traces sharing a common reflection point on flat subsurface reflectors. This relationship is not true for dipping reflectors. The deficiencies of CMP stacking with respect to imaging dipping events have long been overcome by the introduction of the dip-moveout (DMO) correction. However, the concept of coverage has not yet satisfactorily been updated to a 'DMO coverage' consistent with DMO stacking. A definition of constant-velocity DMO coverage will be proposed here. A subsurface reflector will be illuminated from a given source and receiver location if the time difference between the reflector zero-offset traveltime and the NMO- and DMO-corrected traveltime of the reflection event is less than half a dominant wavelength. Due to the fact that a subsurface reflector location is determined by its zero-offset traveltime, its strike and its dip, the DMO coverage also depends on these three parameters. For every surface location, the proposed DMO coverage consists of a 3D fold distribution over reflector strike, dip and zero-offset traveltime.  相似文献   

Fluvial erosion and post-erosional processes on Titan     

Ralf Jaumann  Robert H. Brown  Jason W. Barnes  Christophe Sotin  Roger N. Clark  Bonnie J. Buratti  Thomas B. McCord  Kevin H. Baines  Phil D. Nicholson  Mirjam Langhans 《Icarus》2008,197(2):526-538

The surface of Titan has been revealed by Cassini observations in the infrared and radar wavelength ranges as well as locally by the Huygens lander instruments. Sand seas, recently discovered lakes, distinct landscapes and dendritic erosion patterns indicate dynamic surface processes. This study focus on erosional and depositional features that can be used to constrain the amount of liquids involved in the erosional process as well as on the compositional characteristics of depositional areas. Fluvial erosion channels on Titan as identified at the Huygens landing site and in RADAR and Visible and Infrared Mapping Spectrometer (VIMS) observations have been compared to analogous channel widths on Earth yielding average discharges of up to 1600 m³/s for short recurrence intervals that are sufficient to move centimeter-sized sediment and significantly higher discharges for long intervals. With respect to the associated drainage areas, this roughly translates to 1-150 cm/day runoff production rates with 10 years recurrence intervals and by assuming precipitation this implies 0.6-60 mm/h rainfall rates. Thus the observed surface erosion fits with the methane convective storm models as well as with the rates needed to transport sediment. During Cassini's T20 fly-by, the VIMS observed an extremely eroded area at 30° W, 7° S with resolutions of up to 500 m/pixel that extends over thousands of square kilometers. The spectral characteristics of this area change systematically, reflecting continuous compositional and/or particle size variations indicative of transported sediment settling out while flow capacities cease. To account for the estimated runoff production and widespread alluvial deposits of fine-grained material, release of area-dependent large fluid volumes are required. Only frequent storms with heavy rainfall or cryovolcanic induced melting can explain these erosional features.  相似文献   

The star formation efficiency and its relation to variations in the initial mass function     

Paul C. Clark  Ian A. Bonnell  Ralf S. Klessen 《Monthly notices of the Royal Astronomical Society》2008,386(1):3-10

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Climate change impact on North Sea wave conditions: a consistent analysis of ten projections   总被引：1，自引：1，他引：0  

Iris Grabemann  Nikolaus Groll  Jens Möller  Ralf Weisse 《Ocean Dynamics》2015,65(2):255-267

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Pressure effects on regional mean sea level trends in the German Bight in the twenty-first century     

Frauke Albrecht  Ralf Weisse 《Ocean Dynamics》2014,64(5):633-642

The effect of large-scale atmospheric pressure changes on regional mean sea level projections in the German Bight in the twenty-first century are considered. A developed statistical model is applied to climate model data of sea level pressure for the twenty-first century to assess the potential contribution of large-scale atmospheric changes to future sea level changes in the German Bight. Using 78 experiments, an ensemble mean of 1.4-cm rise in regional mean sea level is estimated until the end of the twenty-first century. Changes are somewhat higher for realisations of the special report on emission scenarios (SRES) A1B and A2, but generally do not exceed a few centimeters. This is considerably smaller than the changes expected from steric and self-gravitational effects. Large-scale changes in sea level pressure are thus not expected to provide a substantial contribution to twenty-first century sea level changes in the German Bight.  相似文献