Methanogenesis in the sediment of the acidic Lake Caviahue in Argentina     

Matthias Koschorreck  Katrin Wendt-Potthoff  Burkhard Scharf  Hans H. Richnow 《Journal of Volcanology and Geothermal Research》2008

The biogeochemistry of methane in the sediments of Lake Caviahue was examined by geochemical analysis, microbial activity assays and isotopic analysis. The pH in the water column was 2.6 and increased up to a pH of 6 in the deeper sediment pore waters. The carbon isotope composition of CH₄ was between − 65 and − 70‰ which is indicative for the biological origin of the methane. The enrichment factor ε increased from − 46‰ in the upper sediment column to more than − 80 in the deeper sediment section suggesting a transition from acetoclastic methanogenesis to CO₂ reduction with depth. In the most acidic surface layer of the sediment (pH < 4) methanogenesis is inhibited as suggested by a linear CH₄ concentration profile, activity assays and MPN analysis. The CH₄ activity assays and the CH₄ profile indicate that methanogenesis in the sediment of Lake Caviahue was active below 40 cm depth. At that depth the pH was above 4 and sulfate reduction was sulfate limited. Methane was diffusing with a flux of 0.9 mmol m^− 2 d^− 1 to the sediment surface where it was probably oxidized. Methanogenesis contributed little to the sediments carbon budget and had no significant impact on lake water quality. The high biomass content of the sediment, which was probably caused by the last eruption of Copahue Volcano, supported high rates of sulfate reduction which probably raised the pH and created favorable conditions for methanogens in deeper sediment layers.  相似文献   

A laboratory study of Equotip surface hardness measurements on a range of sandstones: What influences the values and what do they mean?     

Julie Desarnaud  Kyoko Kiriyama  Beril Bicer Simsir  Katrin Wilhelm  Heather Viles 《地球表面变化过程与地形》2019,44(7):1419-1429

The Equotip rebound tester is a simple, non-destructive technique to measure the surface hardness of materials. Having a low impact energy gives the Equotip advantages over the commonly used Schmidt Hammer on weathered rock and stone. In this study we have investigated the influence of different parameters (sample size, moisture content and surface roughness) on the surface hardness values obtained from freshly cut blocks of four types of sandstone. In a series of laboratory experiments both Single Impacts (SIM) and Repeat Impacts (RIM) methods have been used with C and D probes (which have different impact energies). Our results show that whilst sample size is of great importance we find that smaller samples can be reliably evaluated than previously reported. Moisture contents are also found to exert a more important influence on both SIM and RIM results than previously thought, with up to 26% lower hardness values recorded on saturated vs dry sandstone. Conversely, we find that surface roughness (over Sz values of 100 to 800 microns) does not have a significant impact on SIM measurements collected using the D probe. Both SIM and RIM data are found to be good proxies for compressive strength and open porosity, with SIM data collected with the C probe showing the best fits. Data collected using 3D microscopy helps visualize and quantify the small impact marks created by the Equotip and confirms that these are much reduced when using the C vs D probe. The results highlight the benefits of the Equotip to studies of the nature and deterioration of sandstone, the need for careful evaluation of any confounding factors which might influence the values obtained, and illustrate the different advantages of C and D probes. © 2019 John Wiley & Sons, Ltd.  相似文献   

Bottom-up and top-down control of seagrass overgrazing by the sea urchin Tripneustes gratilla     

Agustín Moreira-Saporiti  Dieuwke Hoeijmakers  Hauke Reuter  Flower E. Msuya  Katrin Gese  Mirta Teichberg 《Marine Ecology》2023,44(1):e12734

The lack of top-down control on Tripneustes gratilla, a sea urchin commonly known to graze on seagrass, and the bottom-up control of its feeding preference, led to the overgrazing of seagrass meadows of the species Thalassodendron ciliatum in Changuu Island (Zanzibar Archipelago). The impact of overgrazing on seagrasses was assessed by mapping the presence of grazed versus non-grazed seagrass patches in the study site, while the top-down control on T. gratilla was assessed by measuring the abundance of its fish predators. The feeding preference and distribution of T. gratilla were characterized by calculating the electivity indexes for each seagrass species and measuring sea urchin density, respectively. Approximately half of the patches of T. ciliatum were overgrazed, while predatory fishes of T. gratilla were absent from the site. The Vanderploeg and Scavia's Relativized Electivity Index indicated that T. gratilla had a feeding preference for T. ciliatum, which was also supported by higher urchin densities within T. ciliatum dominated patches. Bottom-up control of grazing activity was observed by quantifying and analyzing morphological, nutritional, and the chemical defense traits of the seagrass in relation to feeding preference and urchin density. Feeding was positively correlated to the seagrass tissue C:P ratio (ρ = 0.9), whereas urchin density showed no correlations. The bottom-up control of the feeding preference and agglomeration of T. gratilla in T. ciliatum meadows, together with the lack of evidence of substantial top-down control and the long recovery time of T. ciliatum led to the overgrazing of this species at this site. Overgrazing, therefore, was shown to be the result of multiple factors ranging from the traits of the seagrass and feeding preference of T. gratilla, to the abundance of predators in this area.  相似文献   

Influence of boundary conditions on the out‐of‐plane response of brick masonry walls in buildings with RC slabs          下载免费PDF全文

Marco Tondelli  Katrin Beyer  Matthew DeJong 《地震工程与结构动力学》2016,45(8):1337-1356

In modern unreinforced masonry buildings with stiff RC slabs, walls of the top floor are most susceptible to out‐of‐plane failure. The out‐of‐plane response depends not only on the acceleration demand and wall geometry but also on the static and kinematic boundary conditions of the walls. This paper discusses the influence of these boundary conditions on the out‐of‐plane response through evaluation of shake table test results and numerical modelling. As a novum, it shows that the in‐plane response of flanking elements, which are orthogonal to the wall whose out‐of‐plane response is studied, has a significant influence on the vertical restraint at the top of the walls. The most critical configuration exists if the flanking elements are unreinforced masonry walls that rock. In this case, the floor slabs can uplift, and the out‐of‐plane load‐bearing walls loose the vertical restraint at the top. Numerical modelling confirms this experimentally observed behaviour and shows that slab uplift and the difference in base and top excitation have a strong influence on the out‐of‐plane response of the walls analysed. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

Trans‐dimensional Bayesian inversion of controlled‐source electromagnetic data in the German North Sea          下载免费PDF全文

Romina A. S. Gehrmann  Jan Dettmer  Katrin Schwalenberg  Martin Engels  Stan E. Dosso  Asli Özmaral 《Geophysical Prospecting》2015,63(6):1314-1333

This paper presents the first controlled‐source electromagnetic survey carried out in the German North Sea with a recently developed seafloor‐towed electrical dipole–dipole system, i.e., HYDRA II. Controlled‐source electromagnetic data are measured, processed, and inverted in the time domain to estimate an electrical resistivity model of the sub‐seafloor. The controlled‐source electromagnetic survey targeted a shallow, phase‐reversed, seismic reflector, which potentially indicates free gas. To compare the resistivity model to reflection seismic data and draw a combined interpretation, we apply a trans‐dimensional Bayesian inversion that estimates model parameters and uncertainties, and samples probabilistically over the number of layers of the resistivity model. The controlled‐source electromagnetic data errors show time‐varying correlations, and we therefore apply a non‐Toeplitz data covariance matrix in the inversion that is estimated from residual analysis. The geological interpretation drawn from controlled‐source electromagnetic inversion results and borehole and reflection seismic data yield resistivities of ～1 Ωm at the seafloor, which are typical for fine‐grained marine deposits, whereas resistivities below ～20 mbsf increase to 2–4 Ωm and can be related to a transition from fine‐grained (Holocene age) to unsorted, coarse‐grained, and compacted glacial sediments (Pleistocene age). Interface depths from controlled‐source electromagnetic inversion generally match the seismic reflector related to the contrast between the different depositional environments. Resistivities decrease again at greater depths to ～1 Ωm with a minimum resistivity at ～300 mbsf where a seismic reflector (that marks a major flooding surface of late Miocene age) correlates with an increased gamma‐ray count, indicating an increased amount of fine‐grained sediments. We suggest that the grain size may have a major impact on the electrical resistivity of the sediment with lower resistivities for fine‐grained sediments. Concerning the phase‐reversed seismic reflector that was targeted by the survey, controlled‐source electromagnetic inversion results yield no indication for free gas below it as resistivities are generally elevated above the reflector. We suggest that the elevated resistivities are caused by an overall decrease in porosity in the glacial sediments and that the seismic reflector could be caused by an impedance contrast at a thin low‐velocity layer. Controlled‐source electromagnetic interface depths near the reflector are quite uncertain and variable. We conclude that the seismic interface cannot be resolved with the controlled‐source electromagnetic data, but the thickness of the corresponding resistive layer follows the trend of the reflector that is inclined towards the west.  相似文献   

Cyclic tensile-compressive tests on thin concrete boundary elements with a single layer of reinforcement prone to out-of-plane instability     

Angelica Rosso  Lisandro A. Jiménez-Roa  João Pacheco de Almeida  Aydee Patricia Guerrero Zuniga  Carlos A. Blandón  Ricardo L. Bonett  Katrin Beyer 《Bulletin of Earthquake Engineering》2018,16(2):859-887

The growing need for residential housing in Latin American countries has led to the construction of reinforced concrete buildings with wall thicknesses as low as 8–10 cm. Such walls have typically only a single layer of vertical rebars and are therefore particularly susceptible to out-of-plane failure. In order to investigate the response of the corresponding wall boundary elements, twelve reinforced concrete members with a single layer of vertical rebars were tested under tension–compression cycles. The objective was to gain insight into the parameters governing wall instability and out-of-plane failure, namely the thickness, reinforcement ratio, and eccentricity of the longitudinal rebars with respect to the member axis. This paper summarises the results of the test program, where the specimens' response is analysed also at the global and local levels. The results show that the crack pattern has an important influence on the out-of-plane behaviour and the conditions leading to out-of-plane failure are described. Furthermore, the differences between members with a single layer of vertical rebars and members with two layers are discussed. The influence of the parameters considered in the experimental program is addressed, showing that sections with small thickness and large reinforcement content are more prone to out-of-plane failures. Finally, predictions given by existing models are compared to the new experimental data. The entire data set is publicly available.  相似文献   

Axially equilibrated displacement‐based beam element for simulating the cyclic inelastic behaviour of RC members          下载免费PDF全文

Danilo Tarquini  João P. Almeida  Katrin Beyer 《地震工程与结构动力学》2017,46(9):1471-1492

Distributed plasticity beam elements are commonly used to evaluate limit state demands for performance‐based analysis of reinforced concrete (RC) structures. Strain limits are often preferred to drift limits because they directly relate to damage and are therefore less dependent on member geometry and boundary conditions. However, predicting accurately strain demands still represents a major simulation challenge. Tension shift effects, which induce a linear curvature profile in the plastic hinge region of RC columns and walls, are one of the main causes for the mismatch between experimental and numerical estimates of local level quantities obtained through force‐based formulations. Classical displacement‐based approaches are instead suitable to simulate such linear curvature profile. Unfortunately, they verify equilibrium only on an average sense due to the wrong assumption on the axial displacement field, leading to poor deformation and force predictions. This paper presents a displacement‐based element in which axial equilibrium is strictly verified along the element length. The assumed transversal displacement field ensures a linear curvature profile, connecting accurately global displacement and local strain demands. The proposed finite element is validated against two sets of quasi‐static cyclic tests on RC bridge piers and walls. The results show that curvature and strain profiles for increasing ductility demands are significantly improved when axially equilibrated rather than classical displacement‐based or force‐based elements are used to model the structural members. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

Relevance of Intra‐ and Extracellular Cyanotoxins for Drinking Water Treatment     

Jrg Pietsch  Katrin Bornmann  Wido Schmidt 《洁净——土壤、空气、水》2002,30(1):7-15

Bloom‐forming cyanobacteria have been observed in eutrophic waterbodies including drinking water reservoirs all over the world. In this connection investigations about the relevance of intra‐ and extracellular cyanotoxins for drinking water treatment were carried out in laboratory‐ and pilot‐scale experiments. An algae growth phase depended toxin release from cyanobacteria was obtained naturally caused from cultured cyanobacteria (Microcystis aeruginosa) and in a eutrophic reservoir containing Planktothrix rubescens. Results from laboratory‐scale tests using cultivated cyanobacteria and pilot‐scale experiments at a eutrophic reservoir underline the induced toxin release during conventional water treatment. Additional to the known toxin release using pre‐oxidation, it was obtained the first time that the application of flocculation/filtration also effects in toxin release under the conditions investigated, possibly caused by turbulences in pipes and pressure gradients in filters.  相似文献   

Paleorivers and geoarchaeology in the southern Egyptian Sahara     

William P. McHugh  John F. McCauley  C. Vance Haynes  Carol S. Breed  Gerald G. Schaber 《Geoarchaeology》1988,3(1):1-40

Interpretation of Shuttle Imaging Radar (SIR) images acquired during the November 1981 flight of Columbia led to the idea that a paleodrainage system of regional and perhaps transcontinental proportions crossed southern Egypt and northern Sudan prior to the onset of Quaternary aridity. Three seasons of field investigations in two locales (Wadi Arid and Wadi Safsaf, some 70-80 km apart) near the Sudan border of southern Egypt confirmed initial interpretations of the radar images and produced geologic evidence of fluvial deposition in broad “radar river” valleys (designated RR-1) whose presence is now obscured by eolian sand sheets. Development of an integrated, regional river system probably occurred in the late Paleogene or early Neogene. the river courses eventually were disrupted by tectonism, volcanism, and stream piracy. Their broad valleys were almost fully aggraded long before the middle Pleistocene appearance of man in the area. Some of the large paleovalleys on the radar images contain narrow (0.05-2.0 km wide), braided inset channels (designated RR-2). the RR-2 channels, we believe, represent the last episodes of running water in the valleys during the Quaternary pluvials. In Wadi Arid (the type area for the RR-1 valleys) 36 backhoe trenches and manual excavations yielded Acheulian handaxes, flakes, and cores from nearshore alluvial sediments and surface and shallow subsurface locales. These assemblages are typologically Middle to Late Acheulian and date from ca. 0.15th 0.5 million years ago. the unabraded, unrolled, buried artifacts and their geological contexts indicate that human groups were widely present in a subhumid riparian environment along the edges of the ancient valleys, while these localities were undergoing episodic, local aggradation. Scarce Middle Paleolithic and abundant Neolithic assemblages are widespread along the edges of Wadi Arid and bordering interfluves. In Wadi Safsaf, another broad RR-1 valley, geologic evidence from 20 backhoe excavations demonstrates that an RR-2 channel complex is inset into the alluvial fill in the central part of the valley. No artifacts were found in the alluvium excavated there, but Late Acheulian, Middle Paleolithic, and Neolithic surface assemblages are present nearby. Stratigraphic relations indicate the RR-2 channels are late Pleistocene in age; they were already fully aggraded and their surfaces deflated before an eolian sand sheet of Holocene age was deposited on them. Bir Safsaf, on the northern edge of Wadi Safsaf, was a magnet for Late Acheulian populations, and the paleovalley just south of the bir is bordered with extensive alignments of artifacts, mainly handaxes. Some of these handaxes were found in situ, embedded in alluvial sediments, while others appear to have been exposed on the surface by deflation of the valley fill. The accumulated data support a provisional archaeo-geochronological framework for the last half-million years in the Wadi Arid-Wadi Safsaf area. This framework provides a new geomorphic explanation for the widespread distribution of buried and exposed Middle Acheulian and Late Acheulian assemblages. the interpretation emphasizes the importance of exploring the non-oasis zones for their archaeological and geochronological potential. Although our research has literally only scratched the surface, it demonstrates the potential of radar imaging for defining ancient drainage patterns in arid regions and their associated habitats for human occupation.  相似文献   

Ocean iron fertilization: Why further research is needed     

Kerstin Güssow  Alexander Proelss  Andreas Oschlies  Katrin Rehdanz  Wilfried Rickels 《Marine Policy》2010

Despite large uncertainties in the fertilization efficiency, natural iron fertilization studies and some of the purposeful iron enrichment studies have demonstrated that Southern Ocean iron fertilization can lead to a significant export of carbon from the sea surface to the ocean interior. From an economic perspective the potential of ocean iron fertilization (OIF) is far from negligible in relation to other abatement options. Comparing the range of cost estimates to the range of estimates for forestation projects they are in the same order of magnitude, but OIF could provide more carbon credits even if high discount rates are used to account for potential leakage and non-permanence. However, the uncertainty about undesired adverse effects of purposeful iron fertilization on marine ecosystems and biogeochemistry has led to attempts to ban commercial and, to some extent, scientific experiments aimed at a better understanding of the processes involved, effectively precluding further consideration of this mitigation option. As regards the perspective of public international law, the pertinent agreements dealing with the protection of the marine environment indicate that OIF is to be considered as lawful if and to the extent to which it represents legitimate scientific research. In this respect, the precautionary principle can be used to balance the risks arising out of scientific OIF activities for the marine environment with the potential advantages relevant to the objectives of the climate change regime. As scientific OIF experiments involve only comparatively small negative impacts within a limited marine area, further scientific research must be permitted to explore the carbon sequestration potential of OIF in order to either reject this concept or integrate it into the flexible mechanisms contained in the Kyoto Protocol.  相似文献