Bathymetry,substrate and fishing areas of Southeast Atlantic high-seas seamounts   总被引：1，自引：0，他引：1  

OA Bergstad  ÅS Høines  R Sarralde  G Campanis  M Gil  F Ramil 《African Journal of Marine Science》2019,41(1):11-28

Most of the Southeast Atlantic Ocean is abyssal, and global bathymetries suggest that only ～3.2% of the areas beyond national jurisdiction (ABNJ; also known as the high seas, as defined in the United Nations Convention on the Law of the Sea [UNCLOS]) are shallower than 2 500 m. This study mapped bathymetry and characterised substrates in selected seamount summit areas, including several that have been or may become fishing areas. The southernmost location, the Schmitt-Ott Seamount, has exposed volcanic bedrock with surrounding flats covered by thin biogenic sediments and/or coral rubble that appears ancient. At Wüst, Vema, Valdivia and Ewing seamounts the basaltic base appears to be overlain by coral caps and other coral substrates (sheets, rubble). Adjacent summit plains have biogenic sediments of varying thickness. Vema has a flat, roughly circular summit, <100 m deep, with the shallowest point being a 22-m-deep summit knoll; the upper slopes have ancient coral framework, but the summit has a mixture of coralline and volcanic rock and coarse sediments, including extensive areas with coralline algae and kelp forests. Valdivia Bank is a 230-m-deep, flat, rocky area (～11 × 5 km), protruding steeply from the extensive multi-summit Valdivia subarea of the Walvis Ridge. The distribution of past fisheries in the Convention Area of the South East Atlantic Fisheries Organisation (SEAFO) was considered in relation to the new information on bathymetry and substrate.  相似文献   

Direct finite element method for nonlinear earthquake analysis of concrete dams: Simplification,modeling, and practical application     

Arnkjell Løkke  Anil K. Chopra 《地震工程与结构动力学》2019,48(7):818-842

A direct finite element (FE) method for nonlinear response history analysis of semi-unbounded dam-water-foundation systems has recently been presented. The analysis procedure employs standard viscous-damper absorbing boundaries to model the semi-unbounded foundation and fluid domains and specifies the seismic input as effective earthquake forces—determined from a control motion defined at the foundation surface—at these boundaries. Presented in this paper are several simplifications to this direct FE method that greatly facilitates its implementation in commercial FE software. Also addressed is the modeling of the principal nonlinear mechanisms for concrete dams, calibration of damping in the numerical model to ensure consistency with values measured at actual dams, and practical procedures for implementation of the direct FE method with a commercial FE program.  相似文献   

High‐resolution bathymetric mapping reveals subaqueous glacial landforms in the Arctic alpine lake Tarfala,Sweden     

Nina Kirchner  Riko Noormets  Jakob Kuttenkeuler  Elias Strandell Erstorp  Erik Schytt Holmlund  Gunhild Rosqvist  Per Holmlund  Marika Wennbom  Torbjrn Karlin 《第四纪科学杂志》2019,34(6):452-462

In Arctic alpine regions, glacio‐lacustrine environments respond sensitively to variations in climate conditions, impacting, for example,glacier extent and rendering former ice‐contact lakes into ice distal lakes and vice versa. Lakefloors may hold morphological records of past glacier extent, but remoteness and long periods of ice cover on such lakes make acquisition of high‐resolution bathymetric datasets challenging. Lake Tarfala and Kebnepakte Glacier, located in the Kebnekaise mountains, northern Sweden, comprise a small, dynamic glacio‐lacustrine system holding a climate archive that is not well studied. Using an autonomous surface vessel, a high‐resolution bathymetric dataset for Lake Tarfala was acquired in 2016, from which previously undiscovered end moraines and a potential grounding line feature were identified. For Kebnepakte Glacier, structure‐from‐motion photogrammetry was used to reconstruct its shape from photographs taken in 1910 and 1945. Combining these methods connects the glacial landform record identified at the lakefloor with the centennial‐scale dynamic behaviour of Kebnepakte Glacier. During its maximum 20^th century extent, attained c. 1910, Kebnepakte Glacier reached far into Lake Tarfala, but had retreated onto land by 1945, at an average of 7.9 m year^–1. Copyright © 2019 John Wiley & Sons, Ltd.  相似文献   

Introduction     

Gualtiero Badin  Jörn Behrens  Christian Franzke  Jens Rademacher 《地球物理与天体物理流体动力学》2019,113(5-6):425-427

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A multi‐storage groundwater concept for the SWAT model to emphasize nonlinear groundwater dynamics in lowland catchments          下载免费PDF全文

Matthias Pfannerstill  Björn Guse  Nicola Fohrer 《水文研究》2014,28(22):5599-5612

Hydrological models are useful tools to analyze present and future conditions of water quantity and quality. The integrated modelling of water and nutrients needs an adequate representation of the different discharge components. In common with many lowlands, groundwater contribution to the discharge in the North German lowlands is a key factor for a reasonable representation of the water balance, especially in low flow periods. Several studies revealed that the widely used Soil and Water Assessment Tool (SWAT) model performs poorly for low flow periods. This paper deals with the extension of the groundwater module of the SWAT model to enhance low flow representation. The current two‐storage concept of SWAT was further developed to a three‐storage concept. This was realized due to modification of the groundwater module by splitting the active groundwater storage into a fast and a slow contributing aquifer. The results of this study show that the groundwater module with three storages leads to a good prediction of the overall discharge especially for the recession limbs and the low flow periods. The improved performance is reflected in the signature measures for the mid‐segment (percent bias ?2.4% vs ?15.9%) and the low segment (percent bias 14.8% vs 46.8%) of the flow duration curve. The three‐storage groundwater module is more process oriented than the original version due to the introduction of a fast and a slow groundwater flow component. The three‐storage version includes a modular approach, because groundwater storages can be activated or deactivated independently for subbasin and hydrological response unit level. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

High‐resolution reconstruction of a coastal barrier system: impact of Holocene sea‐level change          下载免费PDF全文

Mikkel Fruergaard  Thorbjørn J. Andersen  Lars H. Nielsen  Peter N. Johannessen  Troels Aagaard  Morten Pejrup 《Sedimentology》2015,62(3):928-969

This study presents a detailed reconstruction of the sedimentary effects of Holocene sea‐level rise on a modern coastal barrier system. Increasing concern over the evolution of coastal barrier systems due to future accelerated rates of sea‐level rise calls for a better understanding of coastal barrier response to sea‐level changes. The complex evolution and sequence stratigraphic framework of the investigated coastal barrier system is reconstructed using facies analysis, high‐resolution optically stimulated luminescence and radiocarbon dating. During the formation of the coastal barrier system starting 8 to 7 ka rapid relative sea‐level rise outpaced sediment accumulation. Not before rates of relative sea‐level rise had decreased to ca 2 mm yr^?1 did sediment accumulation outpace sea‐level rise. From ca 5·5 ka, rates of regionally averaged sediment accumulation increased to 4·3 mm yr^?1 and the back‐barrier basin was filled in. This increase in sediment accumulation resulted from retreat of the barrier island and probably also due to formation of a tidal inlet close to the study area. Continued transgression and shoreface retreat created a distinct hiatus and wave ravinement surface in the seaward part of the coastal barrier system before the barrier shoreline stabilized between 5·0 ka and 4·5 ka. Back‐barrier shoreline erosion due to sediment starvation in the back‐barrier basin was pronounced from 4·5 to 2·5 ka but, in the last 2·5 kyr, barrier sedimentation has kept up with and outpaced sea‐level. In the last 0·4 kyr the coastal barrier system has been prograding episodically. Sediment accumulation shows considerable variation, with periods of rapid sediment deposition and periods of non‐deposition or erosion resulting in a highly punctuated sediment record. The study demonstrates how core‐based facies interpretations supported by a high‐resolution chronology and a well‐documented sea‐level history allow identification of depositional environments, erosion surfaces and hiatuses within a very homogeneous stratigraphy, and allow a detailed temporal reconstruction of a coastal barrier system in relation to sea‐level rise and sediment supply.  相似文献   

Scattering enabled retrieval of Green's functions from remotely incident wave packets using cross correlations     

Maarten V. de Hoop  Josselin Garnier  Sean F. Holman  Knut Sølna 《Comptes Rendus Geoscience》2011,343(8-9):526-532

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Experimental study of the carbonation of partially serpentinized and weathered peridotites   总被引：1，自引：0，他引：1  

Jörn Hövelmann  Håkon Austrheim  Andreas Beinlich  Ingrid Anne Munz 《Geochimica et cosmochimica acta》2011,75(22):6760-6779

Carbonation of partially serpentinized and weathered peridotites was studied experimentally under hydrothermal conditions (T: 200 °C, P_CO2: 130-180 bars). Experiments were performed in a closed system using whole rock drill core samples (height: 1 cm, diameter: 1 cm) as starting material. The initial samples were composed mainly of meshwork serpentine, relicts of primary olivine and an olivine weathering product (deweylite assemblage). Two types of solutions, each with a total salt content corresponding to that of average seawater (35 g/L dissolved salts), were used: (1) a Na-Ca-Cl solution (12.5 g/L CaCl₂ + 22.5 g/L NaCl) and (2) a NaCl solution (35 g/L NaCl). After 15-25 days of experimental treatment, the samples were partly covered with carbonates. In addition, noticeable carbonation reactions had occurred below the sample surfaces within zones with thicknesses up to 250 μm. In the Na-Ca-Cl solution, both the olivine relicts and the deweylite assemblage were partly replaced by calcite along the surrounding serpentine veins. However, the extent of calcitization was found to be considerably larger for the deweylite assemblage than for the olivine. Bulk fluid analyses show an increase in the Mg and Si concentrations with reaction time. In the NaCl solution, the deweylite assemblage was partly dissolved resulting in large voids within the reaction zone. In contrast, the olivine was replaced by magnesite. Under the conditions of our experiments, the meshwork serpentine was not reactive, but aided fluid infiltration into the rock samples. The experimentally produced microtextures closely resemble those found in natural examples. Our study elucidates the mechanisms by which carbonates form in ultramafic rocks under relatively high P_CO2-T conditions and particularly in the presence of Ca-bearing aqueous solutions. The existence of a serpentine meshtexture and the presence of weathering products formed from primary Mg-silicates may have significant beneficial effects on in situ CO₂ mineral sequestration in ultramafic rocks.  相似文献   

Conservation planning, boundary-making and border terrains: The desire for forest and order in the Gran Sabana, Venezuela     

Bjørn Sletto 《Geoforum》2011,42(2):197-210

Conflicts surrounding protected area management often emerge from contested processes of boundary-making. Such productions of bounded conservation spaces are contingent in part on processes of identity formation, where some social groups are legitimized as belonging to conservation units, while others are constructed as out-of-place. This article draws on the literature in postmodern geopolitics and the political ecology of fire to interrogate processes of boundary-making and identity formation in the savanna landscape of Canaima National Park, Venezuela. The institutional culture of the environmental management agency EDELCA is in part premised on narratives of history and indigeneity coupled with a desire for an imagined, forested landscape. Because of the social constructions shaping this institutional culture, the agency maintains an approach to fire management that emphasizes fire suppression. However, an ecological field study suggests that fire suppression is leading to increased fuel loads, especially in ecologically significant boundaries between grasslands and forest. Although these boundary zones are the focus of indigenous practices of prescribed burning, they fall in-between the state management categories of forest and ecotone. As a result, these interstitial spaces become theaters for performances of domination and resistance, leading to contradictory and inconsistent approaches to fire management that place gallery forests at risk.  相似文献   

First- and second-order adaptation to natural hazards and extreme events in the context of climate change   总被引：3，自引：1，他引：2  

Jörn Birkmann 《Natural Hazards》2011,58(2):811-840

This paper deals with coping and adaptation processes to natural hazards and (potential) climate change impacts. Selected response measures to natural hazards are critically reviewed with respect to the actual and potential consequences that these measures might imply for additional adaptation needs and changes within social-ecological systems. In this context, the author introduces the concept of first- and second-order adaptation processes in order to underline the cascade process that adaptation implies, the potentials for failure, and the limits of adaptation. Important aspects of first- and second-order adaptation processes as well as the issue of the limits of adaptation are illustrated based on observations and empirical work conducted in Vietnam. In its final part, the paper discusses the findings in light of selected theoretical approaches that might help to understand how stabilization and destabilization processes are influenced or even triggered by adaptation measures.  相似文献