Surface wave array tomography in SE Tibet from ambient seismic noise and two-station analysis – II. Crustal and upper-mantle structure     

Huajian Yao  Caroline Beghein  Robert D. van der Hilst 《Geophysical Journal International》2008,173(1):205-219

We determine the 3-D shear wave speed variations in the crust and upper mantle in the southeastern borderland of the Tibetan Plateau, SW China, with data from 25 temporary broad-band stations and one permanent station. Interstation Rayleigh wave (phase velocity) dispersion curves were obtained at periods from 10 to 50 s from empirical Green's function (EGF) derived from (ambient noise) interferometry and from 20 to 150 s from traditional two-station (TS) analysis. Here, we use these measurements to construct phase velocity maps (from 10 to 150 s, using the average interstation dispersion from the EGF and TS methods between 20 and 50 s) and estimate from them (with the Neighbourhood Algorithm) the 3-D wave speed variations and their uncertainty. The crust structure, parametrized in three layers, can be well resolved with a horizontal resolution about of 100 km or less. Because of the possible effect of mechanically weak layers on regional deformation, of particular interest is the existence and geometry of low (shear) velocity layers (LVLs). In some regions prominent LVLs occur in the middle crust, in others they may appear in the lower crust. In some cases the lateral transition of shear wave speed coincides with major fault zones. The spatial variation in strength and depth of crustal LVLs suggests that the 3-D geometry of weak layers is complex and that unhindered crustal flow over large regions may not occur. Consideration of such complexity may be the key to a better understanding of relative block motion and patterns of seismicity.  相似文献   

A quantitative reconstruction of organic matter and nutrient diagenesis in Mediterranean Sea sediments over the Holocene   总被引：2，自引：0，他引：2  

Daniel C. Reed  Caroline P. Slomp  Gert J. de Lange 《Geochimica et cosmochimica acta》2011,75(19):5540-5558

A multicomponent diagenetic model was developed and applied to reconstruct the conditions under which the most recent sapropel, S1, was deposited in the eastern Mediterranean Sea. Simulations demonstrate that bottom waters must have been anoxic and sulphidic during the formation of S1 and that organic matter deposition was approximately three times higher than at present. Nevertheless, most present day sediment and pore water profiles — with the exception of pyrite, iron oxyhydroxides, iron-bound phosphorus and phosphate — can be reproduced under a wide range of redox conditions during formation of S1 by varying the depositional flux of organic carbon. As a result, paleoredox indicators (e.g., C_org:S ratio, C_org:P_org ratio, trace metals) are needed when assessing the contribution of oxygen-depletion and enhanced primary production to the formation of organic-rich layers in the geological record. Furthermore, simulations show that the organic carbon concentration in sediments is a direct proxy for export production under anoxic bottom waters.The model is also used to examine the post-depositional alteration of the organic-rich layer focussing on nitrogen, phosphorus, and organic carbon dynamics. After sapropel formation, remineralisation is dominated by aerobic respiration at a rate that is inversely proportional to the time since bottom waters became oxic once again. A sensitivity analysis was undertaken to identify the most pertinent parameters in regulating the oxidation of sapropels, demonstrating that variations in sedimentation rate, depositional flux of organic carbon during sapropel formation, bottom water oxygen concentration, and porosity have the largest impact. Simulations reveal that sedimentary nutrient cycling was markedly different during the formation of S1, as well as after reoxygenation of bottom waters. Accumulation of organic nitrogen in sediments doubled during sapropel deposition, representing a significant nitrogen sink. Following reventilation of deep waters, N₂ production by denitrification was almost 12 times greater than present day values. Phosphorus cycling also exhibits a strong redox sensitivity. The benthic efflux of phosphate was up to 3.5 times higher during the formation of S1 than at present due to elevated depositional fluxes of organic matter coupled with enhanced remineralisation of organic phosphorus. Reoxygenation of bottom waters leads to a large phosphate pulse to the water column that declines rapidly with time due to rapid oxidation of organic material. The oxidation of pyrite at the redox front forms iron oxyhydroxides that bind phosphorus and, thus, attenuate the benthic phosphate efflux. These results underscore the contrasting effects of oxygen-depletion on sedimentary nitrogen and phosphorus cycling. The simulations also confirm that the current conceptual paradigm of sapropel formation and oxidation is valid and quantitatively coherent.  相似文献   

A one-year (2005) comparison of seawater temperature series between in situ and modelling data: Application to the Strait of Bonifacio (South Corsica)     

Olivia Gerigny  Bernard Di Martino  Jean-Claude Romano  Caroline Ulses 《Comptes Rendus Geoscience》2011,343(4):278-283

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Unlearning the colonial cultures of planning – By Libby Porter     

Caroline Miller 《New Zealand geographer》2011,67(1):67-68

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Transport of mecoprop from agricultural soils to an adjacent salt marsh     

Fletcher CA  Scrimshaw MD  Lester JN 《Marine pollution bulletin》2004,48(3-4):313-320

Salt marshes are important ecological areas and play a significant role in coastal flood defence schemes. In many areas of the UK they are adjacent to agricultural areas utilised for the growth of cereal crops, for which mecoprop is used as a selective herbicide in the control of broad-leaf weeds. This study measured concentrations of mecoprop in soils, drainage ditch waters and sediments and salt marsh sediments over a period of 138 days following spring application. Soil concentrations of up to 1827 microg/g were recorded after application, which demonstrated a half life for mecoprop of from 9 to 12 days, with first order kinetics. However, a major rainfall event 9 days after application resulted in significant transport of herbicide to the salt marsh via subsurface field drains, drainage ditches and discharge sluice. Mecoprop concentrations of up to 386 microg/l observed in water samples were above UK guidelines.  相似文献   

Characterization of juvenile pyroclasts from the Kos Plateau Tuff (Aegean Arc): insights into the eruptive dynamics of a large rhyolitic eruption     

Caroline Bouvet de Maisonneuve  Olivier Bachmann  Alain Burgisser 《Bulletin of Volcanology》2009,71(6):643-658

Silicic pumices formed during explosive volcanic eruptions are faithful recorders of the state of the magma in the conduit, close to or at the fragmentation level. We have characterized four types of pumices from the non-welded rhyolitic Kos Plateau Tuff, which erupted 161,000 years ago in the East Aegean Arc, Greece. The dominant type of pumice (>90 vol.%) shows highly elongated tubular vesicles. These tube pumices occur throughout the eruption. Less common pumice types include: (1) “frothy” pumice (highly porous with large, sub-rounded vesicles), which form 5–10 vol.% of the coarsest pyroclastic flow deposits, (2) dominantly “microvesicular” and systematically crystal-poor pumices, which are found in early erupted, fine-grained pyroclastic flow units, and are characterized by many small (<50 μm in diameter) vesicles and few mm-sized, irregular voids, (3) grey or banded pumices, indicating the interaction between the rhyolite and a more mafic magma, which are found throughout the eruption sequence and display highly irregular bubble shapes. Except for the grey-banded pumices, all three other types are compositionally identical and were generated synchronously as they are found in the same pyroclastic units. They, therefore, record different conditions in the volcanic conduit leading to variable bubble nucleation, growth and coalescence. A total of 74 pumice samples have been characterized using thin section observation, SEM imagery, porosimetry, and permeametry. We show that the four pumice types have distinct total and connected porosity, tortuosity and permeability. Grey-banded pumices show large variations in petrophysical characteristics as a response to mingling of two different magmas. The microvesicular, crystal-poor, pumices have a bimodal bubble size distribution, interpreted as reflecting an early heterogeneous bubble nucleation event followed by homogeneous bubble nucleation close to fragmentation. Finally, the significant differences in porosity, tortuosity and permeability in compositionally identical tube and frothy pumices are the result of variable shear rates in different parts of the conduit. Differential shear rates may be the result of either: (1) pure shear, inducing a vertical progression from frothy to tube and implying a relatively thick fragmentation zone to produce both types of pumices at the same time or (2) localized simple shear, inducing strongly tubular vesicles along the wall and near-spherical bubbles in the centre of the conduit and not necessarily requiring a thick fragmentation zone.  相似文献   

Flow and nutrient dynamics in a subterranean estuary (Waquoit Bay, MA, USA): Field data and reactive transport modeling   总被引：3，自引：0，他引：3  

Claudette Spiteri  Caroline P. Slomp 《Geochimica et cosmochimica acta》2008,72(14):3398-3412

A two-dimensional (2D) reactive transport model is used to investigate the controls on nutrient (, , PO₄) dynamics in a coastal aquifer. The model couples density-dependent flow to a reaction network which includes oxic degradation of organic matter, denitrification, iron oxide reduction, nitrification, Fe²⁺ oxidation and sorption of PO₄ onto iron oxides. Porewater measurements from a well transect at Waquoit Bay, MA, USA indicate the presence of a reducing plume with high Fe²⁺, , DOC (dissolved organic carbon) and PO₄ concentrations overlying a more oxidizing -rich plume. These two plumes travel nearly conservatively until they start to overlap in the intertidal coastal sediments prior to discharge into the bay. In this zone, the aeration of the surface beach sediments drives nitrification and allows the precipitation of iron oxide, which leads to the removal of PO₄ through sorption. Model simulations suggest that removal of through denitrification is inhibited by the limited overlap between the two freshwater plumes, as well as by the refractory nature of terrestrial DOC. Submarine groundwater discharge is a significant source of to the bay.  相似文献   

PORTRAITS OF PLACELESSNESS: BRITISH NINETEENTH-CENTURY IMAGES OF THE AFRICAN LANDSCAPE     

Rebecca Caroline Lammas 《Singapore journal of tropical geography》1997,17(1):40-51

Landscape art provides a window on the world of subjective interpretation and shared perceptions of place. Portraits of places are therefore simultaneously portraits of a society, its preferences and prejudices, and the meaning it imparts to the world. In the nineteenth century, settlers in Africa felt alienated, uncomfortable, abandoned and detached, and their landscape art—both paintings and poetry—was testimony to their emotional and psychological responses to the foreign environment. Both the imported artistic techniques for portraying nat?re and the symbols populating the landscape images revealed the temperament of the pioneers’ attitudes to place.  相似文献   

Contextualizing vulnerability assessment: a support to geo-risk management in central Africa     

Michellier  Caroline  Pigeon  Patrick  Kervyn  Francois  Wolff  Eleonore 《Natural Hazards》2016,81(1):27-43

European Union directives as well as national legislation are placing great emphasis on the inclusion of stakeholder perspectives in the governance of risks from natural hazards. This should help decision makers formulate better policies. However, to date, there is little information on stakeholders’ perspectives with respect to landslide risk governance. This paper addresses the gap by reporting on research in Nocera Inferiore, Italy. The research is based on a documentary analysis, 43 semi-structured interviews and a survey submitted to 373 residents. The political instability, the unfairness of national funding allocation across municipalities and the residents’ lack of knowledge about risk assessment and emergency planning are some of the main barriers to effective risk governance. Moreover, there are divergent, sometimes even opposite, stakeholders’ views on several issues, such as the relevance of illegal development in risky areas. The results highlight the importance of addressing these divergent views and including the plurality of voices as a prerequisite for inclusive risk governance. The research provided essential background information for a participatory process, which was designed to support decisions on landslide risk mitigation measures in Nocera Inferiore (Linnerooth-Bayer et al. this issue). The methodology will be of more general interest to researchers and policymakers intent upon including stakeholder perspectives in natural risk governance.  相似文献   

Sources and sinks of sediment to the Amazon margin: The Amapa coast     

M. A. Allison  C. A. Nittrouer  L. E. C. Faria Jr.  O. M. Silveira  A. C. Mendes 《Geo-Marine Letters》1996,16(1):36-40

Modern and Holocene muddy strata were studied along the shoreline adjacent to the Amazon river mouth using sedimentological, radiochemical, physical, and seismic methods. The present paper is a synthesis of the results, collected during the AmasSeds project, that is used to outline a regional shoreline sediment budget. Erosion of relict Amazon muds in southern Amapa supplies 10⁶ tons yr^–1 to the Amazon advective mud stream. Local rivers are sediment-poor (total suspended discharge ~ 1 × 10⁶ tons yr^–1), but form depositional sandflats on the shoreface downdrift of the river mouths. Mudflat accumulation in northern Amapa sequesters 10⁶–10⁷ tons yr^–1 by tidal-flat aggradation, alongshore mudcape accretion, and sediment trapping by mangroves. The processes temporarily store 1.5 × 10⁸ tons of Amazon mud in January–June.  相似文献