Flow and grainsize pattern in a sharply curved river bend     

J. S. L. J. Van Alphen  P. M. Bloks  P. Hoekstra 《地球表面变化过程与地形》1984,9(6):513-522

The main characteristics of river flow and grainsize in a bend of the sand bedded meandering river Dommel, The Netherlands, are presented. Measurements were carried out at a relatively low discharge in a sharply curved bend following a long straight reach. In the studied bend, secondary circulation is restricted to the thalweg area; only in the downstream part of the bend it exists over the entire cross-section. Therefore, on the entire pointbar platform, which comprises the larger part of the bend, the median sedimentation diameter of the bedload material is governed by the distribution of the longitudinal components of the bed shear stress only.  相似文献   

Structures caused by repeated freezing and thawing in various loamy sediments: A comparison of active,fossil and experimental data     

Brigitte Van Vliet-Lanoë  Jean-Pierre Coutard  Albert Pissart 《地球表面变化过程与地形》1984,9(6):553-565

In this paper, the authors present the results of both macroscopic and microscopic investigations on structure development created by repeated ice lensing in various loamy experiments. Experimental data are compared with observations performed on active forms in High Arctic and Alpine Mountain environments. Those observations are also compared with phenomena observed in fossil periglacial formations of Western Europe. Platy and short prismatic structure formation is bonded to the hydraulic and thermal conditions during ice segregation. When a long series of alternating freezing and thawing affects platy structures, the fabric evolves, also being influenced by slope and drainage conditions: cryoturbations, frostcreep, and gelifluction can appear. They are characterized by specific microfabrics which are better developed with an increasing number of cycles: this is clear in experiments where hydraulic and thermal parameters are better controlled. Vesicles are also a prominent characteristic of the surface horizon in experiments and arctic soils. The genesis of vesicles is discussed on the basis of new observations and is related to the mechanical collapse of frost-created aggregates under the mechanical work of soil air escape during soil saturation by water at thaw.  相似文献   

Biogeographic implications of a packrat midden sequence from the Sacramento Mountains, south-central New Mexico     

Thomas R. Van Devender  Julio L. Betancourt  Mark Wimberly 《Quaternary Research》1984,22(3):344-360

Thirteen packrat (Neotoma spp.) and two porcupine (Erethizon dorsatum) middens from 1555 to 1690 m elevation from the Sacramento Mountains, New Mexico, provide an 18,000-yr vegetation record in the northern Chiuahuan Desert. The vegetation sequence is a mesic, Wisconsin fullglacial (18,000–16,000 yr B.P.) pinyon-juniper-oak woodland; a xeric, early Holocene (ca. 11,000–8000 yr B.P.) juniper-oak woodland; a middle Holocene (ca. 8000-4000 yr B.P.) desert-grassland; and a late Holocene (ca. 4000 yr B.P. to present) Chihuahuan desertscrub. The frequency of spring freezes and summer droughts in the late Wisconsin probably set the northern limits of Pinus edulis and Juniperus monosperma at about 34°N, or 6° south of today's limit. Rising summer tempratures in the early Holocene eliminated pinyon and other mesic woodland plants from the desert lowlands and allowed the woodland to move upslope. At this time pinyon-juniper woodland and pine forest dominated by Pinus ponderosa probably began their spectacular Holocene expansions to the north. Continued warming in the middle Holocene led to very warm summers with strong monsoons, relatively dry, cold winters, and widespread desert-grasslands. Desertscrub communities in the northern Chihuahuan Desert did not develop until the late Holocene when the biseasonal rainfall shifted slightly back toward the winter, catastrophic winter freezes decreased, and droughts in all seasons increased. The creosote bush desertscrub corridor across the Continental Divide between the Chihuahuan and Sonoran deserts was probably connected for the first time since the last interglaciation.  相似文献   

Geomorphological evolution of a Lowland valley system during the Weichselian     

J. Van Huissteden  L. Van Der Valk  J. Vandenberghe 《地球表面变化过程与地形》1986,11(2):207-216

Weichselian geomorphological and fluvial evolution has been recorded in the valley fill of the Reusel, a small tributary of the lower Maas river. It has been studied by means of borehole sections. A first deep incision into the substratum occurred during the Early Weichselian-Lower Pleniglacial. This was followed by aggradation, at first by a braided river, at a later date by a meandering river, dating respectively from the Lower and Middle Pleniglacial. The Upper Pleniglacial aggradation again points to a braided river system. The end of the Pleniglacial is characterized by aeolian infilling of the valley and decreasing carrying capacity of the river.  相似文献   

Biomarker distributions in asphaltenes and kerogens analysed by flash pyrolysis-gas chromatography-mass spectrometry     

Ger Van Graas 《Organic Geochemistry》1986,10(4-6)

Biomarker distributions in a suite of asphaltenes and kerogens have been analysed by flash pyrolysis directly coupled to a GCMS system. Attention has been focussed on biomarkers of the sterane and triterpane types. The sample suite under investigation consists of sediments with different kerogen types and some crude oils. Biomarker distributions in the pyrolysates have been compared with the “free” biomarkers in the corresponding saturated hydrocarbon fractions.The analyses show significant differences between the distributions of the free biomarkers and those in the pyrolysates. The latter have lower amounts of steranes while diasteranes are absent or present at low concentrations only. In the triterpane traces a shift of maximum intensity from C₃₀ (free compounds) to C₂₇/C₂₉ is observed. Furthermore, the pyrolysates contain a set of triterpenes (not present among the free compounds), and there is a selective loss of “non-regular” triterpanes that are present in the saturated hydrocarbon fractions. The observed differences between pyrolysates and free hydrocarbons can be explained partly by the processes occurring during pyrolysis such as bond rupture and subsequent stabilisation of primary pyrolysis products. To a certain extent these differences also show that maturation processes occurring in sediments have effects on free biomarker molecules different from those on molecules that are enclosed in a macromolecular matrix (kerogen or asphaltenes).Differences between biomarker distributions of asphaltene and kerogen pyrolysates are relatively small. A comparison with the pyrolysates from extracted whole sediments suggests that these differences are mainly caused by interactions between the organic material and the mineral matrix during pyrolysis.Oil asphaltenes behave differently from sediment asphaltenes as their pyrolysates are more similar to the corresponding saturated hydrocarbon fractions, i.e. the differences described above are observed to a much smaller extent. This different behaviour appears to be the result of coprecipitation of a part of the maltene fraction with the oil asphaltenes.  相似文献   

A comparison of boundary layer diffusion schemes in unstable conditions over land     

A. A. M. Holtslag  E. Van Meijgaard  W. C. De Rooy 《Boundary-Layer Meteorology》1995,76(1-2):69-95

We compare the results of a local and a nonlocal scheme for vertical diffusion in the atmospheric boundary layer with observations at the 200 m tower at Cabauw. This is done for a 12 h period during daytime on 31 May 1978, which is characterised by strong insolation, clear skies, moderately strong winds and weak advection. The local diffusion scheme uses an eddy diffusivity determined independently at each point along the vertical based on local vertical gradients of wind and virtual potential temperature, similar to the usual approach in atmospheric models. The nonlocal scheme determines an eddy diffusivity profile based on a diagnosed boundary-layer height and a turbulent velocity scale. It also incorporates nonlocal (vertical) transport effects for heat and moisture. The boundary-layer diffusion schemes are forced with the locally observed fluxes for heat and moisture. The outputs of the scheme are compared with the observed mean structure along the Cabauw tower, and the radiosonde profile at a nearby location (De Bilt). Overall, the nonlocal scheme transports moisture away from the surface more rapidly than the local scheme, and deposits the moisture at higher levels. The local scheme tends to saturate the lowest model levels unrealistically in comparison with the observations. We also compare the outputs of the two diffusion schemes with the results of a transilient model simulation. Subsequently, we study the impact on the model behaviour by varying important parameters in both diffusion schemes and we investigate the sensitivity to uncertainty in the environmental conditions. Finally, we study the interaction of the diffusion schemes with a simple surface flux scheme.  相似文献   

High latitude river runoff in a doubled CO2 climate     

Scott C. Van Blarcum  James R. Miller  Gary L. Russell 《Climatic change》1995,30(1):7-26

A global atmospheric model is used to calculate the monthly river flow for nine of the world's major high latitude rivers for the present climate and for a doubled CO₂ climate. The model has a horizontal resolution of 4° × 5°, but the model's runoff from each grid box is quartered and added to the appropriate river drainage basin on a 2° × 2.5° resolution. A routing scheme is used to move runoff from a grid box to its neighboring downstream grid box and ultimately to the mouth of the river. In a model simulation in which atmospheric carbon dioxide is doubled, mean annual precipitation and river flow increase for all of these rivers, increased outflow at the river mouths begins earlier in the spring, and the maximum outflow occurs approximately one month sooner due to an earlier snow melt season. In the doubled CO₂ climate, snow mass decreases for the Yukon and Mackenzie rivers in North America and for rivers in northwestern Asia, but snow mass increases for rivers in northeastern Asia.  相似文献   

Seismically active fracture zones and distribution of thermal springs in Ecuador     

Václav Hanuš  Jiří Vaněk  Gaston Sandoval  Reviewer J. Zahradník 《Studia Geophysica et Geodaetica》1987,31(3):258-265

Summary The analysis of the geometry of distribution of earthquake foci occurring in the continental South American plate above the subducted Nazca plate reveals that in the region of Ecuador these earthquakes accumulate in 8 well-separated fracture zones. These zones are interpreted as a system of deep seismically active fractures induced in the continental plate by the process of subduction. It is shown that the position of known thermal springs and of observed hydrothermal alterations in Ecuador correlates with the outcrops of the fracture zones.

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Ice sheets and the CO2 problem     

C. J. Van Der Veen 《Surveys in Geophysics》1987,9(1):1-42

In this review, the carbon dioxide problem is discussed, with special reference to the possible effects of a global warming on the ice sheets of Greenland and Antarctica. Instead of detailed projections of future climate and the consequences, the basic mechanisms are explained and illustrated with results described in the literature.It is concluded that a doubling of the atmospheric CO₂ content (most likely to occur somewhere in the second half of the next century) will result in a globally-averaged warming of 2–4°C, and an intensification of the hydrological cycle. In the polar regions, this warming will be a few degrees larger and as a consequence the Greenland Ice Sheet will decrease in size. Antarctica, on the other hand, is expected to grow because of the increased snowfall. The instability of the West Antarctic Ice Sheet is also discussed and, although no conclusive prediction to its long-term response can be made, it is argued that on a short time scale (less than about 100 y) nothing dramatically wil happen to this part of Antarctica.  相似文献   

Deep seismically active fracture zones in ecuador and northern peru     

Václav Hanuš  Jiří Vaněk  Reviewer J. Zahradník 《Studia Geophysica et Geodaetica》1987,31(1):8-25

Summary A system of 8 seismically active fracture zones was delineated on the basis of the distribution of earthquake foci in the continental lithosphere of Ecuador. The position and width of the outcrop, thickness, dip and maximum depth of the individual fracture zones were estimated and correlated with surface geological and tectonic phenomena, volcanism and hydrothermal manifestations. The existence and strike of the fracture zones was independently confirmed by the occurrence of historical disastrous earthquakes.

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Visiting professors at Instituto Geofísico and Facultad de Geología, Minas y Petróleos, Escuela Politécnica Nacional, Quito (Ecuador).  相似文献