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91.
Maarten Wynants Aloyce Patrick Linus Munishi Kelvin Mtei Samuel Bodé Alex Taylor Geoffrey Millward Neil Roberts David Gilvear Patrick Ndakidemi Pascal Boeckx William H. Blake 《地球表面变化过程与地形》2021,46(15):3112-3126
Soil resources in parts of Tanzania are rapidly being depleted by increased rates of soil erosion and downstream sediment transport, threatening ecosystem health, water and livelihood security in the region. However, incomplete understanding to what effect the dynamics of soil erosion and sediment transport are responding to land-use changes and climatic variability are hindering the actions needed to future-proof Tanzanian land-use practices. Complementary environmental diagnostic tools were applied to reconstruct the rates and sources of sedimentation over time in three Tanzanian river systems that have experienced changing land use and climatic conditions. Detailed historical analysis of sediment deposits revealed drastic changes in sediment yield and source contributions. Quantitative sedimentation reconstruction using radionuclide dating showed a 20-fold increase in sediment yield over the past 120 years. The observed dramatic increase in sediment yield is most likely driven by increasing land-use pressures. Deforestation, cropland expansion and increasing grazing pressures resulted into accelerating rates of sheet erosion. A regime shift after years of progressive soil degradation and convergence of surface flows resulted into a highly incised landscape, where high amounts of eroded soil from throughout the catchment are rapidly transported downstream by strongly connected ephemeral drainage networks. By integrating complementary spatial and temporal evidence bases, this study demonstrated links between land-use change, increased soil erosion and downstream sedimentation. Such evidence can guide stakeholders and policy makers in the design of targeted management interventions to safeguard future soil health and water quality. 相似文献
92.
Paulina Lopez Pascal Sirguey Yves Arnaud Bernard Pouyaud Pierre Chevallier 《Global and Planetary Change》2008,61(3-4):103-116
The snow cover of the Northern Patagonia Icefield (NPI) was monitored after applying the Normalized Difference Snow Index (NDSI) and the Red/NIR band ratio to 134 Moderate Resolution Imaging Spectroradiometer (MODIS) images captured between 2000 and 2006. The final results show that the snow cover extent of the NPI fluctuates a lot in winter, in addition to its seasonal behaviour. The minimum snow cover extent of the period (3600 km2) was observed in March 2000 and the maximum (11,623 km2) in August 2001. We found that temperature accounts for approximately 76% of the variation of the snow cover extent over the entire icefield. We also show two different regimes of winter snow cover fluctuations corresponding to the eastern and the western sides of the icefield. The seasonality of the snow cover on the western side was determined by temperature rather than precipitation, while on the east side the seasonality of the snow cover was influenced by the seasonal behaviour of both temperature and precipitation. This difference can be explained by the two distinct climates: coastal and continental. The fluctuations in the winter snow cover extent were more pronounced and less controlled by temperature on the western side than on the eastern side of the icefield. Snow cover extent was correlated with temperature R2 = 0.75 and R2 = 0.74 for the western and eastern sides, respectively. Since limited meteorological data are available in this region, our investigation confirmed that the change in snow cover is an interesting climatic indicator over the NPI providing important insights in mass balance comprehension. Since snow and ice were distinguished snow cover fluctuations can be associated to fluctuations in the snow accumulation area of the NPI. In addition, days with minimum snow covers of summer season can be associated to the period in which Equilibrium Line Altitude (ELA) is the highest. 相似文献
93.
Jason W. Barnes Robert H. Brown Christophe Sotin Sebastien Rodriguez Ross A. Beyer Karly Pitman Roger Clark 《Icarus》2008,195(1):400-414
Fine-resolution (500 m/pixel) Cassini Visual and Infrared Mapping Spectrometer (VIMS) T20 observations of Titan resolve that moon's sand dunes. The spectral variability in some dune regions shows that there are sand-free interdune areas, wherein VIMS spectra reveal the exposed dune substrate. The interdunes from T20 are, variously, materials that correspond to the equatorial bright, 5-μm-bright, and dark blue spectral units. Our observations show that an enigmatic “dark red” spectral unit seen in T5 in fact represents a macroscopic mixture with 5-μm-bright material and dunes as its spectral endmembers. Looking more broadly, similar mixtures of varying amounts of dune and interdune units of varying composition can explain the spectral and albedo variability within the dark brown dune global spectral unit that is associated with dunes. The presence of interdunes indicates that Titan's dunefields are both mature and recently active. The spectrum of the dune endmember reveals the sand to be composed of less water ice than the rest of Titan; various organics are consistent with the dunes' measured reflectivity. We measure a mean dune spacing of 2.1 km, and find that the dunes are oriented on the average in an east-west direction, but angling up to 10° from parallel to the equator in specific cases. Where no interdunes are present, we determine the height of one set of dunes photoclinometrically to be between 30 and 70 m. These results pave the way for future exploration and interpretation of Titan's sand dunes. 相似文献
94.
Deep‐water dunes on drowned isolated carbonate terraces (Mozambique Channel,south‐west Indian Ocean)
Elda Miramontes Stephan J. Jorry Gwenael Jouet John W. Counts Simon Courgeon Pascal Le Roy Charline Guerin F. Javier Hernndez‐Molina 《Sedimentology》2019,66(4):1222-1242
Subaqueous sand dunes are common bedforms on continental shelves dominated by tidal and geostrophic currents. However, much less is known about sand dunes in deep‐marine settings that are affected by strong bottom currents. In this study, dune fields were identified on drowned isolated carbonate platforms in the Mozambique Channel (south‐west Indian Ocean). The acquired data include multibeam bathymetry, multi‐channel high‐resolution seismic reflection data, sea floor imagery, a sediment sample and current measurements from a moored current meter and hull‐mounted acoustic Doppler current profiler. The dunes are located at water depths ranging from 200 to 600 m on the slope terraces of a modern atoll (Bassas da India Atoll) and within small depressions formed during tectonic deformation of drowned carbonate platforms (Sakalaves Seamount and Jaguar Bank). Dunes are composed of bioclastic medium size sand, and are large to very large, with wavelengths of 40 to 350 m and heights of 0·9 to 9·0 m. Dune migration seems to be unidirectional in each dune field, suggesting a continuous import and export of bioclastic sand, with little sand being recycled. Oceanic currents are very intense in the Mozambique Channel and may be able to erode submerged carbonates, generating carbonate sand at great depths. A mooring located at 463 m water depth on the Hall Bank (30 km west of the Jaguar Bank) showed vigorous bottom currents, with mean speeds of 14 cm sec?1 and maximum speeds of 57 cm sec?1, compatible with sand dune formation. The intensity of currents is highly variable and is related to tidal processes (high‐frequency variability) and to anticyclonic eddies near the seamounts (low‐frequency variability). This study contributes to a better understanding of the formation of dunes in deep‐marine settings and provides valuable information about carbonate preservation after drowning, and the impact of bottom currents on sediment distribution and sea floor morphology. 相似文献
95.
The ability of discrete element models to describe quantitatively (and not only qualitatively) the constitutive behaviour of a dense sand is assessed in this paper. Two kinds of 3D discrete models are considered. Both consider spheres as elementary particles. Nevertheless, the first model implements a contact law with rolling resistance whereas the second takes into account clumps made of two spheres. The discrete models are calibrated and validated from mechanical tests performed on a dense Hostun sand with a true triaxial apparatus. The calibration is carried out from axisymmetric drained compression tests, while the validation is discussed from monotonic and cyclic stress proportional loading paths and from a circular stress path in the deviatoric stress plane. The quality of the predictions of the discrete models are evaluated by comparison with the predictions given with advanced phenomenological constitutive relations, mainly an incrementally non-linear relation. Predictions given by the discrete models are remarkable, particularly when it is put in perspective with respect to the very few number of mechanical tests required for their calibration. However, these results and conclusions were reached in enabling conditions, and some limitations of such discrete models should be kept in mind. 相似文献
96.
Abstract— The distribution of Li, Be, and B was studied by ion microprobe mass spectrometry in 36 chondrules from the Semarkona, Bishunpur, Allende, Clovis #1, and Hedjaz meteorites. Within a single chondrule, Li-Be-B concentrations can vary up to one order of magnitude. For example, in a chondrule from Hedjaz, concentrations range from 0.3 to 2.4 ppm for Li, from <0.001 to 0.17 ppm for Be, and from 0.4 to 5.5 ppm for B. Among chondrules from Semarkona and Bishunpur, clear crystal-mesostasis partitioning was observed in nine chondrules for Li, in nine chondrules for Be, and in three chondrules for B. The heterogeneities in the distribution of Li, Be, and B in chondrules from Semarkona and Bishunpur appear to be primary features that were inherited from the chondrules' precursors and not totally obscured during the chondrules' formation. A redistribution of B was nevertheless observed at the whole-rock scale for Allende (B-Al2O3 correlation) and Hedjaz (B–SiO2 correlation). At the scale of bulk chondrules, a robust correlation is observed for all studied meteorites between the B/Be and the B/Li ratios, which indicates that Li and Be are much less heterogeneously distributed in chondrites than B. Mean Li, Be, and B concentrations of chondrules ([Li] ? 0.83+0.86 ppm; [Be] ? 0.0430.027 ppm; [B] ? 0.89+3.71-0.72 ppm) are consistent with those of Orgueil ([Li] ? 1.49 ppm; [Be] ? 0.0249 ppm; [B] ? 0.87 ppm), but the mean Li/Be ratio of chondrules (24.5+6.5-9.1) is a factor of ~4 depleted relative to Orgueil (Li/Be ratio of ~78). Such a depletion is puzzling because no correlation between Li and Na or B has been found as would be expected to result from volatilization processes during chondrule melting and cooling. As a consequence, the exact abundance of solar system Li remains an open question. 相似文献
97.
应用三种化学示踪剂,对桂林岩溶水文地质试验场“31号泉系统”的补给边界进行了研究。试验在雨期进行,示踪剂分别是降雨后投入落水洞随表层岩溶水带入地下;或在降雨前投入无水的落水洞中,待降雨后由地表产流带入地下。 试验表明,氯化锌、钼酸铵是较理想的示踪剂,地下水速度为4~250米/小时,速度差可能是由于包气带岩溶含水介质渗透性和水力坡度不同所致;泉口历时浓度曲线常呈多峰型,多是由于降雨脉冲影响所致;峰丛区高程不等的洼地多通道补给山边泉,峰丛区含水介质结构具有叠置性,即水平方向多通道,垂直方向多层次,故水均衡计算不可忽视;“31号泉系统”的补给边界,通过圈定有水力连系之洼地的地表分水岭来确定。 相似文献
98.
Pierre Bosser Olivier Bock Christian Thom Jacques Pelon Pascal Willis 《Journal of Geodesy》2010,84(4):251-265
This paper investigates the impact of rapid small-scale water vapor fluctuations on GPS height determination. Water vapor
measurements from a Raman lidar are used for documenting the water vapor heterogeneities and correcting GPS signal propagation
delays in clear sky conditions. We use data from four short observing sessions (6 h) during the VAPIC experiment (15 May–15
June 2004). The retrieval of wet delays from our Raman lidar is shown to agree well with radiosonde retrievals (bias and standard
deviation (SD) were smaller than 1 and 2.8 mm, respectively) and microwave radiometers (from two different instruments, bias
was 6.0/−6.6 mm and SD 1.3/3.8 mm). A standard GPS data analysis is shown to fail in accurately reproducing fast zenith wet
delay (ZWD) variations. The ZWD estimates could be improved when mean post-fit phase residuals were removed. Several methodologies
for integrating zenith lidar observations into the GPS data processing are also presented. The final method consists in using
lidar wet delays for correcting a priori the GPS phase observations and estimating a scale factor for the lidar wet delays
jointly with the GPS station position. The estimation of this scale factor allows correcting for a mis-calibration in the
lidar data and provides in the same way an estimate of the Raman lidar instrument constant. The agreement of this constant
with an independent determination using radiosonde data is at the level of 1–4%. The lidar wet delays were derived by ray-tracing
from zenith pointing measurements: further improvement in GPS positioning is expected from slant path lidar measurements that
would properly account for water vapor anisotropy. 相似文献
99.
100.
Drouzy Marion Douillet Pascal Fernandez Jean-Michel Pinazo Christel 《Ocean Dynamics》2019,69(8):967-987
Ocean Dynamics - Identifying zones of stagnation and deposition of terrigenous matter or contaminants induced by human activity is a key issue in coastal areas. In this paper, circulation processes... 相似文献