Maris Gullying: canonical analysis of erosion and sedimentation sequences (Baronnies - France)     

Michel Lecompte  Jan Chodzko  René Lhénaff  Alain Marre 《Geodinamica Acta》2013,26(3):115-124

Abstract

The gullying action on Jurassic and Cretaceous marls in the Baronnies is observed, warm season after cold season, thanks to a network of measuring stakes arranged in 12 gullies. The climate, characterized by summer heath, rather cold but luminous winters, and quite heavy rain, can be classified as mountainous-mediterranean. Seasonal ablation/sedimentation measures have been carried out upon 119 stakes five times within two years (768 days): two in automn, two in early spring (april), and one at end-of-spring.

Ablation and sedimentation sequences are described among the stakes population by Correspondance Factor Analysis (CFA), and their covariations in space and time are explained by Canonical Correspondance Analysis (CCA) according to biophysical environment variables.

Axis 1 of the CFA opposes ground level variations according to their amplitude, high or moderate, regardless of the direction of variation and of measurement date. Axis 2 presents a temporal variation, opposing, on one side, sedimentation sequences at post-winter terms and ablation sequences at postsummer terms, to their converse on the other side.

The CCA indicates that talwegs in their medium and lower parts are more likely to undergo hard gullying or strong accumulation, especially on steep, or conversely flat, vegetationfree slopes of oxfordian marls; on the contrary, interfluve ridges and Gonson moderate gentle to moderate slopes on cenomanian marls, possibly with some vegetation are less dynamic (canonical axis 1). This result allows to extend former observations on rythmic, contrasted variations of gullies bottoms, well different of interfluves and gonson, progressive, regular, ones.

Owing to their varied reactions to seasonal erosive agents (cryoclasty, concentrated run-off), upper parts of talwegs and lower parts of gully slopes and interfluves (canonical axis 2) originate seasonal contrasts revealed by CFA’s axis 2. However, to insure this point of view, more sampling is needed.  相似文献   

Tectonic evolution of a restricted ocean basin: the Powell Basin (Northeastern Antarctic Peninsula)     

José Rodriguez-Fernandez  Juan-Carlos Balanya  Jesús Galindo-Zaldivar  Andrés Maldonado 《Geodinamica Acta》2013,26(4):159-174

Abstract

The Powell Basin is one of the few present-day examples of a small isolated ocean basin largely surrounded by blocks of continental crust. The continental blocks in this basin result from the fragmentation of the northern Antarctic Peninsula. This basin was created by the eastward motion of the South Orkney microcontinent relative to the Antarctic Peninsula. The axial rift, identified by multichannel seismic profiles obtained during the HESANT 92/93 cruise, and the gravimetric anomalies of the basin plain, together with the transcurrent faults along the northern and southern margins, indicate a predominant WSW-ENE trend of basin extension. The South Orkney microcontinent was incorporated into the Antarctic Plate during the Miocene as a consequence of the end of basin spreading. The eastern and western margins are conjugate and have an intermediate crust in the region of transition to the basin plain. The differences in the basement structure and the architecture of the depositional units suggest that the extensional process was asymmetrical. The southern transtensive margin and the northern transcurrent margin are rectilinear and steep, without any intermediate crust in the narrow fault zone between the base of the continentalblocks slope and the oceanic crust. The multichannel seismic profiles across the central sector of the basin reveal a spreading axis with a double ridge and a central depression filled with sediments. The geometry of the reflectors in this depression indicates that the ponded deposits belong to the early stages of oceaniccrust accretion. This structure is similar to the overlapping spreading centres observed in fast-spreading oceanic axes, where the spreading axis has relay and overlapping segments.

The depositional units of the margins and basin plain have been grouped into four depositional sequences, comprising the classic stages in the formation of an ocean basin: pre-rift (S1), syn-rift (S2), syn-drift (S3), and post-drift (S4). The pre-rift sequence has deformed reflectors and is observed in the southern and eastern margins. The syn-rift sequence, tectonically disrupted, fills depressions bounded by faults and is well-developed in the eastern margin where it is truncated by an erosive surface identified as the break-up unconformity. The syn-drift sequence is wedge-shaped in the basin, thickening towards the margins and having onlap relations on the flanks of the spreading ridge. The post-drift sequence is the thickest unit and is characterised by a cyclic pattern of alternating packages of high-amplitude reflectors, very continuous, and low-amplitude reflectors. Towards the western and eastern margins, the same sequence has channel-levee complexes and channelised, wedged bodies attributed to turbiditic deposits of submarine fans derived from canyons located in the slope and outer shelf. The cyclic nature of this sequence is probably related to advancing and receding grounded ice sheets in the continental shelf since the latest Miocene.  相似文献   

Formation of drying crack patterns in soils: a deterministic approach     

Hervé Peron  Lyesse Laloui  Liang-Bo Hu  Tomasz Hueckel 《Acta Geotechnica》2013,8(2):215-221

Soils, as well as most of deformable multiphase porous materials, are likely to suffer from desiccation cracking, leading to the formation of regular crack patterns affecting their permeability. The ensuing crack spacing has often been related to a concept sometimes called “sequential infilling”: it is assumed that desiccation cracks are formed by successive generations. However, such a concept does not consider the pattern of a simultaneous crack formation at a given moment. Using our desiccation cracking test results and their numerical simulation, we propose a consistent explanation for the formation of desiccation crack patterns in soils. We show that the “sequential infilling” concept is suitable only when the position of the crack(s) clearly stems from the stress field. To derive an estimate of the desiccation crack spacing, the overall energy of the system needs to be considered. Statistical variability should be superimposed on the mean deterministic conditions discussed here.  相似文献   

On-line fluorometry of multiple reactive and conservative tracers in streams   总被引：1，自引：0，他引：1  

Dennis Lemke  Pierre-André Schnegg  Marc Schwientek  Karsten Osenbrück  Olaf A. Cirpka 《Environmental Earth Sciences》2013,69(2):349-358

Performing tracer tests using artificial tracer compounds is a common practice to characterize natural streams regarding their (reactive) transport properties. Recently, the fluorescent compound resazurin was introduced as a reactive stream tracer to quantify hyporheic exchange and metabolic activity of streams. This tracer, together with its reaction product resorufin and a conservative tracer (in our case fluorescein), provides additional information about transport properties of the stream and its hyporheic zone and can therefore overcome restrictions that are commonly affiliated with the use of conservative tracers alone. However, all previously published studies using this tracer system were based on manual sampling of the water. This usually limits the number of measurements and thus the achievable temporal resolution, and potentially endangers data quality due to inadequate handling of samples. In this paper, a modified version of the GGUN-FL30 on-line fluorometer is presented in which the optics have been modified to allow measuring the concentrations of all three tracers simultaneously at intervals of 10 s. Experiments under controlled and natural conditions showed that the performance of the on-line fluorometer regarding tracer separation efficiency and practical detection limits is comparable to a high-performance laboratory spectrofluorometer. Furthermore, suggestions are given on how to correct tracer signal fluctuations caused by temporal changes in temperature and pH that might occur during a field tracer test.  相似文献   

Assessing benthic health under multiple human pressures in Bohai Bay (China), using density and biomass in calculating AMBI and M‐AMBI     

Wenqian Cai  Ángel Borja  Lusan Liu  Wei Meng  Iñigo Muxika  José Germán Rodríguez 《Marine Ecology》2014,35(2):180-192

In recent decades, China has undergone an impressive development, which has produced increasing human pressures including coastal degradation. Bohai Bay, located in the west of the Bohai Sea (north of China), has been affected by various human pressures, including waste water discharges, land reclamation and oil exploitation. To monitor the effects of these pressures on this ecosystem, benthic macroinvertebrates and environmental variables (dissolved oxygen, nutrients, metals, etc.) were sampled in 2009 and 2011. To assess the ecological status of the benthic communities, the AZTI Marine Biotic Index (AMBI) and multivariate‐AMBI (M‐AMBI) were used, including both density and biomass in the calculation. This is the first research in which M‐AMBI based on biomass (M‐bAMBI) is applied. Both methods showed that the nearshore areas, especially close to the Haihe and Jiyun estuaries, were affected by human pressures, with a clear gradient of decreasing impacts offshore. The ecological status as assessed using density and biomass was quite similar. Significant relationships were also found between both methods and environmental variables. The use of AMBI and M‐AMBI, based upon density and biomass, in assessing the ecological status of water bodies in China seems to be promising.  相似文献   

Dual-frequency ADCPs measuring turbidity     

Frédéric Jourdin  Caroline Tessier  Pierre Le Hir  Romaric Verney  Michel Lunven  Sophie Loyer  André Lusven  Jean-François Filipot  Jérémy Lepesqueur 《Geo-Marine Letters》2014,34(4):381-397

A pair of self-contained acoustic Doppler current profilers (SC-ADCPs) operating with different frequencies were moored on a muddy sea bottom at about 20 m depth in the Bay of Vilaine off the French Atlantic coast. With their acoustic beams oriented upwards, the SC-ADCPs ensonified most of the water column. The results of several months of in situ recorded echo intensity data spanning 2 years (2003 to 2004) from the dual-frequency ADCPs are presented in this paper. The aim was to estimate suspended particle mass concentration and mean size. A concentration index CI is proposed for the estimation of particle concentration. Based on theory the CI—unlike the volume backscatter strength—does not depend on particle size. Compared with in situ optical data, the CI shows reasonable precision but not increased with respect to that of the highest-frequency backscatter strength. Concerning the mean particle size, despite a lack of quantitative validation with optical particle-size measurements, the method yielded a qualitative discrimination of mineral (small) and organic (large) particles. This supports the potential of dual-frequency ADCPs to quantitatively determine particle size. A cross-calibration of the transducers of each ADCP shows that a specific component of the precision of the backscatter strength measured by ADCP depends on the acoustic frequency, the cell thickness and the ensemble integration time. Based on these results, the use of two ADCPs operating with distinctly different frequencies (two octaves apart) or a single dual-frequency ADCP is recommended.  相似文献   

Numerical study on the effect of submerged depth on the horizontal plate wave energy converter   总被引：2，自引：0，他引：2  

Flávio Medeiros SEIBT  ;Eduardo Costa COUTO  ;Elizaldo Domingues dos SANTOS  ;Liércio André ISOLDI  ;Luiz Alberto Oliveira ROCHA 《中国海洋工程》2014,28(5):687-700

The growing search for clean and renewable energy sources has given rise to the studies of exploring sea wave energy. This paper is concerned with the numerical evaluation of the main operational principle of a submerged plate employed for the conversion of wave energy into electrical one. The numerical model used to solve the conservation equations of mass, momentum and transport of volume fraction is based on the finite volume method （FVM）. In order to tackle with the flow of mixture of air-water and its interaction with the device, the multiphase model volume of fluid （VOF） is employed. The purpose of this study is the evaluation of a numerical model for improvement of the knowledge about the submerged plate wave energy converter, as well as the investigation of the effect of the distance from the plate to the bottom of the sea （HP） on the performance of the converter. The simulations for several distances of the plate from the seabed show that the optimal efficiency is 64%, which is obtained for HP=0.53 m （88% of the depth）. This efficiency is 17% larger than that found in the worst case （HP=0.46 m, 77% of the depth）.  相似文献   

Evaluating conservation effectiveness of nature reserves established for surrogate species: Case of a giant panda nature reserve in Qinling Mountains, China   总被引：1，自引：0，他引：1  

XU Weihua  Andrés VIA  QI Zengxiang  OUYANG Zhiyun  LIU Jianguo  LIU Wei  WAN Hui 《中国地理科学(英文版)》2014,24(1):60-70

Many nature reserves are established to protect the habitat needs of particular endangered species of interest but their effec-tiveness for protecting other species is questionable.In this study,this effectiveness was evaluated in a nature reserve network located in the Qinling Mountains,Shaanxi Province,China.The network of reserves was established mainly for the conservation of the giant panda,a species considered as a surrogate for the conservation of many other endangered species in the region.The habitat suitability of nine protected species,including the giant panda,was modeled by using Maximum Entropy（MAXENT） and their spatial congruence was analyzed.Habitat suitability of these species was also overlapped with nature reserve boundaries and their management zones（i.e.,core,buffer and experimental zones）.Results show that in general the habitat of the giant panda constitutes a reasonable surrogate of the habitat of other protected species,and giant panda reserves protect a relatively high proportion of the habitat of other protected species.Therefore,giant panda habitat conservation also allows the conservation of the habitat of other protected species in the region.However,a large area of suitable habitat was excluded from the nature reserve network.In addition,four species exhibited a low proportion of highly suitable habitat inside the core zones of nature reserves.It suggests that a high proportion of suitable habitat of protected species not targeted for conservation is located in the experimental and buffer zones,thus,is being affected by human activities.To increase their conservation effectiveness,nature reserves and their management zones need to be re-examined in order to include suitable habitat of more endangered species.The procedures described in this study can be easily implemented for the conservation of many endangered species not only in China but in many other parts of the world.  相似文献   

The surface radiation budget over South America in a set of regional climate models from the CLARIS-LPB project   总被引：1，自引：0，他引：1  

Natalia L. Pessacg  Silvina A. Solman  Patrick Samuelsson  Enrique Sanchez  José Marengo  Laurent Li  Armelle Reca C. Remedio  Rosmeri P. da Rocha  Caroline Mourão  Daniela Jacob 《Climate Dynamics》2014,43(5-6):1221-1239

The performance of seven regional climate models in simulating the radiation and heat fluxes at the surface over South America (SA) is evaluated. Sources of uncertainty and errors are identified. All simulations have been performed in the context of the CLARIS-LPB Project for the period 1990–2008 and are compared with the GEWEX-SRB, CRU, and GLDAS2 dataset and NCEP-NOAA reanalysis. Results showed that most of the models overestimate the net surface short-wave radiation over tropical SA and La Plata Basin and underestimate it over oceanic regions. Errors in the short-wave radiation are mainly associated with uncertainties in the representation of surface albedo and cloud fraction. For the net surface long-wave radiation, model biases are diverse. However, the ensemble mean showed a good agreement with the GEWEX-SRB dataset due to the compensation of individual model biases. Errors in the net surface long-wave radiation can be explained, in a large proportion, by errors in cloud fraction. For some particular models, errors in temperature also contribute to errors in the net long-wave radiation. Analysis of the annual cycle of each component of the energy budget indicates that the RCMs reproduce generally well the main characteristics of the short- and long-wave radiations in terms of timing and amplitude. However, a large spread among models over tropical SA is apparent. The annual cycle of the sensible heat flux showed a strong overestimation in comparison with the reanalysis and GLDAS2 dataset. For the latent heat flux, strong differences between the reanalysis and GLDAS2 are calculated particularly over tropical SA.  相似文献   

Changes in European greenhouse gas and air pollutant emissions 1960–2010: decomposition of determining factors     

Peter Rafaj  Markus Amann  José Siri  Henning Wuester 《Climatic change》2014,124(3):477-504

This paper analyses factors that contributed to the evolution of SO₂, NO_x and CO₂ emissions in Europe from 1960 to 2010. Historical energy balances, along with population and economic growth data, are used to quantify the impacts of major determinants of changing emission levels, including energy intensity, conversion efficiency, fuel mix, and pollution control. Time series of emission levels are compared for countries in Western and Eastern Europe, throwing light on differences in the importance of particular emission-driving forces. Three quarters of the decline in SO₂ emissions in Western Europe resulted from a combination of reduced energy intensity and improved fuel mix, while dedicated end-of-pipe abatement measures played a dominant role in the reduction of NO_x emissions. The increase in atmospheric emissions in Eastern Europe through the mid-1990s was associated with the growth of energy-intensive industries, which off-setted the positive impact of better fuel quality and changes in fuel mix. A continuous decrease in energy intensity and higher conversion efficiencies have been the main factors responsible for the moderate rate of growth of European CO₂ emissions.  相似文献