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991.
Tiago A. Silva Stphanie Girardclos Laura Stutenbecker Maarten Bakker Anna Costa Fritz Schlunegger Stuart N. Lane Peter Molnar Jean‐Luc Loizeau 《Sedimentology》2019,66(3):838-858
Deltas are important coastal sediment accumulation zones in both marine and lacustrine settings. However, currents derived from tides, waves or rivers can transfer that sediment into distal, deep environments, connecting terrestrial and deep marine depozones. The sediment transfer system of the Rhone River in Lake Geneva is composed of a sublacustrine delta, a deeply incised canyon and a distal lobe, which resembles, at a smaller scale, deep‐sea fan systems fed by high discharge rivers. From the comparison of two bathymetric datasets, collected in 1891 and 2014, a sediment budget was calculated for eastern Lake Geneva, based on which sediment distribution patterns were defined. During the past 125 years, sediment deposition occurred mostly in three high sedimentation rate areas: the proximal delta front, the canyon‐levée system and the distal lobe. Mean sedimentation rates in these areas vary from 0·0246 m year?1 (distal lobe) to 0·0737 m year?1 (delta front). Although the delta front–levées–distal lobe complex only comprises 17·0% of the analysed area, it stored 74·9% of the total deposited sediment. Results show that 52·5% of the total sediment stored in this complex was transported toward distal locations through the sublacustrine canyon. Namely, the canyon–levée complex stored 15·9% of the total sediment, while 36·6% was deposited in the distal lobe. The results thus show that in deltaic systems where density currents can occur regularly, a significant proportion of riverine sediment input may be transferred to the canyon‐lobe systems leading to important distal sediment accumulation zones. 相似文献
992.
The Role of Electrical Anisotropy in Magnetotelluric Responses: From Modelling and Dimensionality Analysis to Inversion and Interpretation 总被引:2,自引:0,他引:2
Anna Martí 《Surveys in Geophysics》2014,35(1):179-218
The study of electrical anisotropy in the Earth, defined as the electrical conductivity varying with orientation, has experienced important advances in the last years regarding the investigation of its origins, how to identify and model it, and how it can be related to other parameters, such as seismic and mechanical anisotropy. This paper provides a theoretical background and a review of the current state of the art of electrical anisotropy using electromagnetic methods in the frequency domain, focusing mainly on magnetotellurics. The aspects that will be considered are the modelling of the electromagnetic fields with anisotropic structures, the analysis of their responses to identify these structures, and how to properly use these responses in inversion and interpretation. Also, an update on the most recent case studies involving anisotropy is provided. 相似文献
993.
In the process of coal gasification, the phase transition from water to water vapour takes place as a result of high temperature. Thus, the parameters of the fluid flowing through the pores of the elastic skeleton change in a significant way. The goal of this work is to calculate the fluid flow process at a variable temperature using Finite Element Method and to determine the soil consolidation process taking place under its own weight and temperature changes. The mathematical model of thermal consolidation for a Biot body accounts for the phase transition of a liquid. Numerical calculations for a homogeneous and isotropic porous medium, consisting of two conventionally accepted layers, were carried out using the FlexPDE v. 6 software. The obtained results are a first approximation of the actual processes taking place under complex geological conditions. They make it possible to determine, in approximation, the range of the phase transition and the influence of water vapour filtration on soil consolidation. 相似文献
994.
Maria Isabel Arce Daniel von Schiller Rosa Gómez 《Aquatic Sciences - Research Across Boundaries》2014,76(2):295-311
Streams are significant locations for nitrate (NO3 ?) processing within landscapes. This is especially important in dry climates given the limited water availability for biological processes elsewhere. In arid and semiarid regions, many streams are naturally saline. Elevated salinity can constrain the structure and function of aquatic organisms, which is expected to increase worldwide being associated to global warming. We investigated whole-reach NO3 ? uptake and denitrification in nine semiarid streams of variable water salinity (i.e. from freshwater to hyposaline) to test if NO3 ? processing would decrease with increasing salinity. We used pulse additions and Tracer Addition for Spiraling Curve Characterization (TASCC) to measure whole-reach uptake of added NO3 ?, and the acetylene block technique to measure sediment denitrification. TASCC results showed that only five of nine streams were able to retain added NO3 ?. Of these five retentive streams, four were saline; however, salinity did not control significantly the variation in whole-reach NO3 ? uptake observed across streams. Other measured environmental variables such as streambed NH4 + and organic carbon availability were better at explaining this variation. Denitrification was detected in all streams except one and its variation across streams was also independent of salinity. Although denitrification rates tended to be high, their contribution to whole-reach NO3 ? uptake was insignificant (≤2.16 %). Alternative pathways, heterotrophic assimilation and/or dissimilatory NO3 ? reduction to NH4 +, were probably responsible for most whole-reach NO3 ? uptake. Together, our results highlight that the function of streams in controlling external NO3 ? inputs is highly variable and salinity does not apparently constrain this role. 相似文献
995.
Valeriy Khokhlov Anna Romanova 《Stochastic Environmental Research and Risk Assessment (SERRA)》2014,28(2):369-381
The impact of the North Atlantic Oscillation (NAO) on synoptic conditions in Ukraine can not be considered as fully defined as the territory of Ukraine is located at the transitional boundary of processes associated with the NAO. As a rule, common methods based on the principal component analysis (PCA) can not reveal the impact of the NAO. Moreover, the analysis of synoptic-scale processes based on the data with the one-day (or lesser) discreteness is affected by noise. In this paper, the wavelet analysis is used jointly with the PCA to determine time intervals when a linkage between the NAO and processes over Ukraine was significant. Such an approach allowed revealing the time intervals during the cold seasons, when simultaneous variations of synoptic conditions in North Atlantic and Eastern Europe occurred at quasi-synoptic time scales. Usually, that process is observed not very often, but simultaneous variations can be in progress during a few weeks. The coherent changes of synoptic conditions do not depend on the strength and phase of the NAO and can be both in-phase and anti-phase. The significant wavelet coherence is probably registered when two intensive synoptic processes different from general synoptic situation are observed. 相似文献
996.
Ryan Ekre Paul C. Johnson Bruce E. Rittmann Rosa Krajmalnik‐Brown 《Ground Water Monitoring & Remediation》2014,34(2):60-70
This work focuses on the site‐specific assessment of source zone natural attenuation (SZNA) at chlorinated aliphatic hydrocarbon (CAH)‐impacted sites. The approach is similar in some ways, but different in other ways from recently proposed SZNA assessment paradigms for petroleum‐impacted sites. The similarities lie in the organization of the approach around determining: (1) whether or not SZNA is occurring, (2) the current SZNA rate, and (3) what is the future projection for SZNA rate changes and the final state of the source zone. Differences lie in how those rates are determined, especially with respect to the quantities measured and data reduction. Petroleum‐impacted site SZNA approaches emphasize quantifying fluxes of electron acceptors, while the proposed CAH assessment approach emphasizes quantifying parent and daughter compound fluxes. A paradigm for assessing SZNA at CAH sites is presented and its use is illustrated, for example former dry cleaner site, where the SZNA rate was approximately 3.5 kg/year as tetrachloroethylene (PCE) with about 80% of the mass loss attributed to groundwater flow and 20% attributed to vapor transport. 相似文献
997.
Occurrence of selected estrogens in mangrove sediments 总被引:1,自引:0,他引:1
Froehner S Machado KS Stefan E Bleninger T da Rosa EC Martins Cde C 《Marine pollution bulletin》2012,64(1):75-79
This paper presents results related to the occurrence and distribution of estrogens along the Brazilian coast. Three mangrove areas were chosen to evaluate the presence of estrogens in surface sediments of mangrove forests. The presence of estrogens was observed in all studied sites. 17-α-Ethinylestradiol (EE2), a synthetic estrogen, was the most common and has been found in higher concentration (0.45-129.78 ng/g) compared to 17-β-estradiol (E1) and estrone (E2) (both being natural estrogens). The concentrations of E1 and E2 ranged from 0.02 to 49.27 ng/g and 0.03 to 39.77 ng/g, respectively. Theoretically, under anaerobic conditions EE2 can be reduced to E1 even in environments such as sediments of mangrove forests, which are essentially anaerobic. Even if the concentrations of estrogens seem to be insignificant in some samples, the effects remain uncertain. 相似文献
998.
The drivers (social) and pressures (physical) of marine debris have typically been examined separately. We redress this by using social and beach surveys at nine Tasmanian beaches, across three coastlines and within three categories of urbanisation, to examine whether people acknowledge that their actions contribute to the issue of marine debris, and whether these social drivers are reflected in the amount of marine debris detected on beaches. A large proportion (75%) of survey participants do not litter at beaches; with age, gender, income and residency influencing littering behaviour. Thus, participants recognise that littering at beaches is a problem. This social trend was reflected in the small amounts of debris that were detected. Furthermore, the amount of debris was not statistically influenced by the degree of beach urbanisation, the coastline sampled, or the proximity to beach access points. By linking social and physical aspects of this issue, management outcomes can be improved. 相似文献
999.
Carole Decker Marie Morineaux Saskia Van Gaever Jean‐Claude Caprais Anna Lichtschlag Olivier Gauthier Ann C. Andersen Karine Olu 《Marine Ecology》2012,33(2):205-230
Håkon Mosby mud volcano (HMMV) is one of the most active and most studied seep sites in European waters. Many authors have described its thermal activity, dynamic of mud flows, and geochemical and microbial processes. It is characterised by a concentric zonation of successive biogenic habitats related to an activity and geochemical gradient from its centre to its periphery. Around the central area covered by mud flows, white and grey microbial mats occur among areas of bare sediment, whereas siboglinid tubeworm fields of Sclerolinum contortum and/or Oligobrachia haakonmosbiensis colonise the peripheral areas. The meiofaunal community is known to be structured among habitats, but the macrofauna has rarely been investigated and has never been sampled in situ. As part of the European project HERMES, using the ROVs Victor 6000 and Quest 4000, we sampled quantitatively the different habitats of the volcano for macrofauna sensus lato, retained on a 250‐ or 500‐μm sieve. We also sampled a newly discovered pockmark on Storegga slide (cne 5.6) and two pockmarks (G11, G12) in the Nyegga area. Macrofauna was identified and counted from phylum to family level. Our results on HMMV showed a gradient of increasing density and diversity from the volcano centre (1–3 taxa; 260 ind·m?2) to the peripheral siboglinid fields (8–14 taxa, 93,000 ind·m?2), with an intermediate situation for microbial mats. For macrofauna ≥500 μm, non‐siboglinid polychaetes dominated the communities of the central mud volcano area, white mats and S. contortum fields (83, 89 and 37% of the total, respectively), whereas gastropods dominated grey mats and O. haakonmosbiensis fields (89 and 44% of the total, respectively). Polychaete families followed the same pattern of diversity according to habitats within HMMV. Of 23 polychaete families identified, only one occurred in the centre, and three in the microbial mats. Capitellidae and Dorvilleidae (typical of organically and sulphide‐enriched areas) occurred at remarkably high densities in white microbial mats and in O. haakonmosbiensis fields. The S. contortum fields were the most diverse habitat with 12 polychaete families. The 250‐μm fraction showed similar taxa dominating the habitats, but taking meiofauna into account, nematodes became the major taxon in white mats and in S. contortum fields, where they were particularly large in size, whereas copepods dominated in other habitats. Meiofauna and macrofauna did not show the same patterns of density according to habitats. Using principal components analysis the habitats at HMMV were clearly distinct, and clustered according to dominant species of siboglinids and type of microbial mats. Pockmarks at Nyegga showed a similar concentric pattern of habitats around fluid sources as on the volcano, which seemed similarly to influence macrofauna composition, but at a much smaller scale. Total taxa and polychaete diversity are high in the S. contortum fields in these pockmarks as well. Regional‐scale comparisons including HMMV and Storegga suggested a higher influence of habitat‐type than seep‐site on the community structure. 相似文献
1000.
Changes in the water table level result in variable water saturation and variable hydrological fluxes at the interface between the unsaturated and saturated zone. This may influence the transport and fate of contaminants in the subsurface. The objective of this study was to examine the impact of a decreasing and an increasing water table on solute transport. We conducted tracer experiments at downward flow conditions in laboratory columns filled with two different uniform porous media under static and transient flow conditions either increasing or decreasing the water table. Tracer breakthrough curves were simulated using a mobile–immobile transport model. The resulting transport parameters were compared to identify dominant transport processes. Changes in the water table level affected dispersivities and mobile water fractions depending on the direction of water table movement and the grain size of the porous media. In fine glass beads, the water flow velocity was similar to the decline rate of the water table, and the mobile water fraction was decreased compared with steady‐state saturated conditions. However, immobile water was negligible. In coarse glass beads, water flow was faster because of fingered flow in the unsaturated part, and the mobile water fraction was smaller than in the fine material. Here, a rising water table led to an even smaller mobile water fraction and increased solute spreading because of diffusive interaction with immobile water. We conclude that changes of the water table need to be considered to correctly simulate transport in the subsurface at the transition of the unsaturated–saturated zone. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献