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71.
On the effect of moisture bonding forces in air-dry soils on threshold friction velocity of wind erosion 总被引:5,自引:0,他引:5
SUJITH RAVI TED M. ZOBECK† THOMAS M. OVER‡ GREGORY S. OKIN PAOLO D'ODORICO 《Sedimentology》2006,53(3):597-609
Wind erosion is a dominant geomorphological process in arid and semi-arid regions with major impacts on regional climate and desertification. The erosion process occurs when the wind speed exceeds a certain threshold value, which depends on a number of factors including surface soil moisture. The understanding and modelling of aeolian erosion requires a better understanding of the soil erodibility associated with different moisture conditions. In arid regions during the dry season, the atmospheric humidity plays an important role in determining the surface moisture content and the threshold shear velocity. By a series of wind tunnel tests and theoretical analyses, this dependence of threshold velocity on near surface air humidity is shown for three soils of different textures: sand, sandy loam, and clay loam. The results show that the threshold shear velocity decreases with increasing values of relative humidity for values of relative humidity between about 40% and 65%, while above and below this range the threshold shear velocity increases with air humidity. A theoretical framework is developed to explain these dependencies assuming an equilibrium between the surface soil moisture and the humidity of the overlying atmosphere. The conditions under which soil-atmosphere equilibrium occurs were tested experimentally in the laboratory for different soils in order to determine the effect of grain surface area and texture on the time required to reach equilibrium starting from different initial conditions. 相似文献
72.
GURENKO ANDREY A.; TRUMBULL ROBERT B.; THOMAS RAINER; LINDSAY JAN M. 《Journal of Petrology》2005,46(12):2495-2526
Glass inclusions in plagioclase and orthopyroxene from daciticpumice of the Cabrits Dome, Plat Pays Volcanic Complex in southernDominica reveal a complexity of element behavior and LiBisotope variations in a single volcanic center that would gounnoticed in a whole-rock study. Inclusions and matrix glassesare high-silica rhyolite with compositions consistent with about50% fractional crystallization of the observed phenocrysts.Estimated crystallization conditions are 760880°C,200 MPa and oxygen fugacity of FMQ + 1 to +2 log units (whereFMQ is the fayalitemagnetitequartz buffer). Manyinclusion glasses are volatile-rich (up to 6 wt % H2O and 2900ppm Cl), but contents range down to 1 wt % H2O and 2000 ppmCl as a result of shallow-level degassing. Sulfur contents arelow throughout, with <350 ppm S. The trace element compositionof inclusion glasses shows enrichment in light rare earth elements(LREE; (La/Sm)n = 2·56·6) and elevatedBa, Th and K contents compared with whole rocks and similaror lower Nb and heavy REE (HREE; (Gd/Yb)n = 0·51·0).Lithium and boron concentrations and isotope ratios in meltinclusions are highly variable (2060 ppm Li with 7Li= +4 to +15 ± 2; 60100 ppm B with 11B = +6 to+13 ± 2) and imply trapping of isotopically heterogeneous,hybrid melts. Multiple sources and processes are required toexplain these features. The mid-ocean ridge basalt (MORB)-likeHREE, Nb and Y signature reflects the parental magma(s) derivedfrom the mantle wedge. Positive Ba/Nb, B/Nb and Th/Nb correlationsin inclusion glasses indicate coupled enrichment in stronglyfluid-mobile (Ba, B) and less-mobile (Th, Nb) trace elements,which can be explained by fractional crystallization of plagioclase,orthopyroxene and FeTi oxides. The 7Li and 11B valuesare at the high end of known ranges for other island arc magmas.We attribute the high values to a 11B and 7Li-enriched slabcomponent derived from sea-floor-altered oceanic crust and possiblyfurther enriched in heavy isotopes by dehydration fractionation.The heterogeneity of isotope ratios in the evolved, trappedmelts is attributed to shallow-level assimilation of older volcanicrocks of the Plat Pays Volcanic Complex. KEY WORDS: subduction; volcanic arcs; igneous processes; melt inclusions; SIMS; trace elements; lithium and boron isotopes; diffusion 相似文献
73.
Recent shallow marine ostracods from high latitudes: implications for late Pliocene and Quaternary palaeoclimatology 总被引:2,自引:0,他引:2
ELISABETH M. BROUWERS THOMAS M. CRONIN DAVID J. HORNE ALAN R. LORD 《Boreas: An International Journal of Quaternary Research》2000,29(2):127-142
New information about modern high-latitude marine ostracod faunas and about their occurrence in late Pliocene and Quaternary sequences promotes the application of these microfossils to palaeoenvironmental, palaeobiogeographical and palaeoclimatological problems. Five poorly known species (Cluthia cluthae, Finmarchinella logani, Roundstonia globulifera, Roundstonia macchesneyi and Semicytherura complanata) from North Atlantic, North Pacific and Arctic areas are confirmed as extant and are used to demonstrate the potential of marine ostracods as tools for palaeoclimate studies. The genus Bonnyannella Athersuch, 1982 is shown to be a junior synonym of Roundstonia Neale, 1973. 相似文献
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THOMAS ANDRÉN GREGER LINDEBERG ELINOR ANDRÉN 《Boreas: An International Journal of Quaternary Research》2002,31(3):226-238
A clay-varve chronology based on 14 cross-correlated varve graphs from the Baltic Sea and a mean varve thickness curve has been constructed. This chronology is correlated with the Swedish Time Scale and covers the time span 11530 to 10250 varve years BP. Two cores have been analysed for grain size, chemistry, content of diatoms and changes in colour by digital colour analysis. The final drainage of the Baltic Ice Lake is dated to c . 10800 varve years BP and registered in the cores analysed as a decrease in the content of clay. This event can be correlated with atmospheric Δ14 C content and might have resulted in an increase in these values recorded between 11565 and 11545 years BP. The results of the correlation between the varve chronology from the Baltic Sea, the Greenland GRIP ice core and the atmospheric Δ14 C record indicate that c . 760 years are missing in the Swedish Time Scale in the part younger than c. 10250 varve years BP. A change in colour from a brownish to grey varved glacial clay recorded c . 10770 varve years BP is found to be the result of oxygen deficiency due to an increase in the rate of sedimentation in the early Preboreal. The first brackish influence is recorded c . 10540 varve years BP in the northwestern Baltic Sea and some 90 years later in the eastern Gotland Basin. 相似文献
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79.
ANNE COUDRAIN MICHEL LOUBET THOMAS CONDOM AMAL TALBI PIERRE RIBSTEIN BERNARD POUYAUD 《水文科学杂志》2013,58(2):293-306
Abstract In catchments characterized by spatially varying hydrological processes and responses, the optimal parameter values or regions of attraction in parameter space may differ with location-specific characteristics and dominating processes. This paper evaluates the value of semi-distributed calibration parameters for large-scale streamflow simulation using the spatially distributed LISFLOOD model. We employ the Shuffled Complex Evolution Metropolis (SCEM-UA) global optimization algorithm to infer the calibration parameters using daily discharge observations. The resulting posterior parameter distribution reflects the uncertainty about the model parameters and forms the basis for making probabilistic flow predictions. We assess the value of semi-distributing the calibration parameters by comparing three different calibration strategies. In the first calibration strategy uniform values over the entire area of interest are adopted for the unknown parameters, which are calibrated against discharge observations at the downstream outlet of the catchment. In the second calibration strategy the parameters are also uniformly distributed, but they are calibrated against observed discharges at the catchment outlet and at internal stations. In the third strategy a semi-distributed approach is adopted. Starting from upstream, parameters in each subcatchment are calibrated against the observed discharges at the outlet of the subcatchment. In order not to propagate upstream errors in the calibration process, observed discharges at upstream catchment outlets are used as inflow when calibrating downstream subcatchments. As an illustrative example, we demonstrate the methodology for a part of the Morava catchment, covering an area of approximately 10 000 km2. The calibration results reveal that the additional value of the internal discharge stations is limited when applying a lumped parameter approach. Moving from a lumped to a semi-distributed parameter approach: (i) improves the accuracy of the flow predictions, especially in the upstream subcatchments; and (ii) results in a more correct representation of flow prediction uncertainty. The results show the clear need to distribute the calibration parameters, especially in large catchments characterized by spatially varying hydrological processes and responses. 相似文献
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