Location Based Social Networks – Definition,Current State of the Art and Research Agenda     

Oliver Roick  Susanne Heuser 《Transactions in GIS》2013,17(5):763-784

The growing penetration of GPS equipped smartphones allowing users to constantly share geographic information on their current whereabouts has led the way to an augmentation of existing Social Network Sites with location‐based features or the creation of new ones exclusively around geographic information. Within these Location Based Social Networks vast amounts of geographic information are allocated, which attracted the attention of researchers with various scientific backgrounds. This article presents a comprehensive definition of this special type of Social Network Site and an overview of research activities, which are currently conducted using the data. Finally, a research agenda for GIScience is drawn from existing research directions.  相似文献   

Effective sea ice area based on a thickness threshold     

Ford  Victoria L.  Frauenfeld  Oliver W.  Nowotarski  Christopher J.  Bombardi  Rodrigo J. 《Climate Dynamics》2021,56(11):3541-3552

As Arctic sea ice declines in response to climate change, a shift from thick multiyear ice to a thinner ice cover is occurring. With this transition, ice thicknesses approach a threshold below which ice no longer insulates the atmosphere from oceanic surface fluxes. While this is well known, there are no estimates of the magnitude of this threshold, nor of the proportion of sea ice area that is below this threshold as ice thins. We determine this threshold by simulating the atmospheric response to varying thicknesses, ranging from 0.0 to 2.0 m and determine that threshold to be 0.40–0.50 m. The resulting “effective” ice area is 4–14% lower than reported total ice area, as 0.39–0.97 × 10⁶ km² of the total ice area falls below the threshold throughout the twentieth century, including during notable ice minima. The atmosphere above large non-insulating ice-covered regions is susceptible to more than 2 °C of warming despite ice presence. Observed mean Arctic Ocean ice thickness is projected to fall below this threshold as early as the mid-2020s. Studies on ocean–atmosphere interactions in relation to sea ice area should focus on this insulating sea ice area, where ice is at least 0.40–0.50 m thick, and treat ice regions below 0.40–0.50 m thickness with caution.

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Aerobic degradation of organic carbon inferred from dinoflagellate cyst decomposition in Southern Ocean sediments     

Monika Kupinska  Oliver Sachs  Eberhard J. Sauter  Karin A.F. Zonneveld 《Quaternary Research》2012,78(1):130-138

Organic carbon (OC) burial is an important process influencing atmospheric CO₂ concentration and global climate change; therefore it is essential to obtain information on the factors determining its preservation. The Southern Ocean (SO) is believed to play an important role in sequestering CO₂ from the atmosphere via burial of OC. Here we investigate the degradation of organic-walled dinoflagellate cysts (dinocysts) in two short cores from the SO to obtain information on the factors influencing OC preservation. On the basis of the calculated degradation index kt, we conclude that both cores are affected by species-selective aerobic degradation of dinocysts. Further, we calculate a degradation constant k using oxygen exposure time derived from the ages of our cores. The constant k displays a strong relationship with pore-water O₂, suggesting that decomposition of OC is dependent on both the bottom- and pore-water O₂ concentrations.  相似文献   

A chironomid-based reconstruction of late glacial summer temperatures in the southern Carpathians (Romania)     

Mónika Tóth  Enikő K. Magyari  Stephen J. Brooks  Mihály Braun  Krisztina Buczkó  Miklós Bálint  Oliver Heiri 《Quaternary Research》2012,77(1):122-131

Late glacial and early Holocene summer temperatures were reconstructed based on fossil chironomid assemblages at Lake Brazi (Retezat Mountains) with a joint Norwegian–Swiss transfer function, providing an important addition to the late glacial quantitative climate reconstructions from Europe. The pattern of the late glacial temperature changes in Lake Brazi show both similarities and some differences from the NGRIP δ¹⁸O record and other European chironomid-based reconstructions. Our reconstruction indicates that at Lake Brazi (1740 m a.s.l.) summer air temperature increased by ~ 2.8°C at the Oldest Dryas/Bølling transition (GS-2/GI-1) and reached 8.1–8.7°C during the late glacial interstade. The onset of the Younger Dryas (GS-1) was characterized by a weak (< 1°C) decrease in chironomid-inferred temperatures. Similarly, at the GS-1/Holocene transition no major changes in summer temperature were recorded. In the early Holocene, summer temperature increased in two steps and reached ~ 12.0–13.3°C during the Preboreal. Two short-term cold events were detected during the early Holocene between 11,480–11,390 and 10,350–10,190 cal yr BP. The first cooling coincides with the Preboreal oscillation and shows a weak (0.7°C) temperature decrease, while the second is characterized by 1°C cooling. Both cold events coincide with cooling events in the Greenland ice core records and other European temperature reconstructions.  相似文献   

A macroscopic damage‐plastic constitutive law for modeling quasi‐brittle fracture and ductile behavior of concrete     

P. J. Sánchez  A. E. Huespe  J. Oliver  G. Diaz  V. E. Sonzogni 《国际地质力学数值与分析法杂志》2012,36(5):546-573

A new phenomenological macroscopic constitutive model for the numerical simulation of quasi‐brittle fracture and ductile concrete behavior, under general triaxial stress conditions, is presented. The model is particularly addressed to simulate a wide range of confinement stress states, as also, to capture the strong influence of the mean stress value in the concrete failure mechanisms. The model is based on a two‐surface damage‐plastic formulation. The mechanical behavior in different domains of the stress space is separately described by means of a quasi‐brittle or ductile material response:

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World Stress Map Database as a Resource for Rock Mechanics and Rock Engineering     

Arno Zang  Ove Stephansson  Oliver Heidbach  Silke Janouschkowetz 《Geotechnical and Geological Engineering》2012,30(3):625-646

Knowledge of the in situ stress state is of key importance for rock engineering. We inform the reader about the World Stress Map (WSM) database and its application to rock mechanics and rock engineering purpose, and in particular the orientation of maximum horizontal stress. We discuss the WSM and the quality ranking system of stress orientation data. We show one example of discrete-measured and computed-smoothed stress orientations from central and northern Europe with respect to relative plate velocity trajectories. We give first insights into ongoing development of a second, more Quantitative World Stress Map database which compiles globally rock-type specific stress magnitudes versus depth. We discuss the vertical stress component, and the lateral stress coefficient versus depth for different rock types. We display stress magnitudes in 2D and 3D stress space, and investigate stress ratios in relation to depth, lithology and tectonic faulting regime.  相似文献   

The legacy of crystal-plastic deformation in olivine: high-diffusivity pathways during serpentinization   总被引：1，自引：1，他引：0  

Oliver Plümper  Helen E. King  Christian Vollmer  Quentin Ramasse  Haemyeong Jung  Håkon Austrheim 《Contributions to Mineralogy and Petrology》2012,163(4):701-724

Crystal-plastic olivine deformation to produce subgrain boundaries composed of edge dislocations is an inevitable consequence of asthenospheric mantle flow. Although crystal-plastic deformation and serpentinization are spatio-temporally decoupled, we identified compositional readjustments expressed on the micrometric level as a striped Fe-enriched ( [`(X)]<sub>\textFe</sub> \bar{X}_{\text{Fe}}  = 0.24 ± 0.02 (zones); 0.12 ± 0.02 (bulk)) or Fe-depleted ( [`(X)]_\textFe \bar{X}_{\text{Fe}}  = 0.10 ± 0.01 (zones); 0.13 ± 0.01 (bulk)) zoning in partly serpentinized olivine grains from two upper mantle sections in Norway. Focused ion beam sample preparation combined with transmission electron microscopy (TEM) and aberration-corrected scanning TEM, enabling atomic-level resolved electron energy-loss spectroscopic line profiling, reveals that every zone is immediately associated with a subgrain boundary. We infer that the zonings are a result of the environmental Fe²⁺Mg₋₁ exchange potential during antigorite serpentinization of olivine and the drive toward element exchange equilibrium. This is facilitated by enhanced solid-state diffusion along subgrain boundaries in a system, which otherwise re-equilibrates via dissolution-reprecipitation. Fe enrichment or depletion is controlled by the silica activity imposed on the system by the local olivine/orthopyroxene mass ratio, temperature and the effect of magnetite stability. The Fe-Mg exchange coefficients K_\textD^{\textAtg/\textOl} K_{\text{D}}^{{{\text{Atg}}/{\text{Ol}}}} between both types of zoning and antigorite display coalescence toward exchange equilibrium. With both types of zoning, Mn is enriched and Ni depleted compared with the unaffected bulk composition. Nanometer-sized, heterogeneously distributed antigorite precipitates along olivine subgrain boundaries suggest that water was able to ingress along them. Crystallographic orientation relationships gained via electron backscatter diffraction between olivine grain domains and different serpentine vein generations support the hypothesis that serpentinization was initiated along olivine subgrain boundaries.  相似文献   

Glacial advances constrained by Be exposure dating of bedrock landslides, Kyrgyz Tien Shan     

Katia Sanhueza-Pino  Oliver Korup  Ralf Hetzel  Henry Munack  Johannes T. Weidinger  Stuart Dunning  Cholponbek Ormukov  Peter W. Kubik 《Quaternary Research》2011,76(3):295-304

Numerous large landslide deposits occur in the Tien Shan, a tectonically active intraplate orogen in Central Asia. Yet their significance in Quaternary landscape evolution and natural hazard assessment remains unresolved due to the lack of "absolute" age constraints. Here we present the first ¹⁰Be exposure ages for three prominent (> 10⁷ m³) bedrock landslides that blocked major rivers and formed lakes, two of which subsequently breached, in the northern Kyrgyz Tien Shan. Three ¹⁰Be ages reveal that one landslide in the Alamyedin River occurred at 11–15 ka, which is consistent with two ¹⁴C ages of gastropod shells from reworked loess capping the landslide. One large landslide in Aksu River is among the oldest documented in semi-arid continental interiors, with a ¹⁰Be age of 63–67 ka. The Ukok River landslide deposit(s) yielded variable ¹⁰Be ages, which may result from multiple landslides, and inheritance of ¹⁰Be. Two ¹⁰Be ages of 8.2 and 5.9 ka suggest that one major landslide occurred in the early to mid-Holocene, followed by at least one other event between 1.5 and 0.4 ka. Judging from the regional glacial chronology, all three landslides have occurred between major regional glacial advances. Whereas Alamyedin and Ukok can be considered as postglacial in this context, Aksu is of interglacial age. None of the landslide deposits show traces of glacial erosion, hence their locations and ¹⁰Be ages mark maximum extents and minimum ages of glacial advances, respectively. Using toe-to-headwall altitude ratios of 0.4–0.5, we reconstruct minimum equilibrium-line altitudes that exceed previous estimates by as much as 400 m along the moister northern fringe of the Tien Shan. Our data show that deposits from large landslides can provide valuable spatio-temporal constraints for glacial advances in landscapes where moraines and glacial deposits have low preservation potential.  相似文献   

Global mass wasting during the Middle Ordovician: Meteoritic trigger or plate-tectonic environment?     

Guido Meinhold  Arzu Arslan  Oliver Lehnert  Gérard M. Stampfli 《Gondwana Research》2011,19(2):535-541

Mass wasting at continental margins on a global scale during the Middle Ordovician has recently been related to high meteorite influx. Although a high meteorite influx during the Ordovician should not be neglected, we challenge the idea that mass wasting was mainly produced by meteorite impacts over a period of almost 10 Ma. Having strong arguments against the impact-related hypothesis, we propose an alternative explanation, which is based on a re-evaluation of the mass wasting sites, considering their plate-tectonic distribution and the global sea level curve. A striking and important feature is the distribution of most of the mass wasting sites along continental margins characterised by periods of magmatism, terrane accretion and continental or back-arc rifting, respectively, related to subduction of oceanic lithosphere. Such processes are commonly connected with seismic activity causing earthquakes, which can cause downslope movement of sediment and rock. Considering all that, it seems more likely that most of this mass wasting was triggered by earthquakes related to plate-tectonic processes, which caused destabilisation of continental margins resulting in megabreccias and debris flows. Moreover, the period of mass wasting coincides with sea level drops during global sea level lowstand. In some cases, sea level drops can release pore-water overpressure reducing sediment strength and hence promoting instability of sediment at continental margins. Reduced pore-water overpressure can also destabilise gas hydrate-bearing sediment, causing slope failure, and thus resulting in submarine mass wasting. Overall, the global mass wasting during the Middle Ordovician does not need meteoritic trigger.  相似文献   

Updating Markov chain models using the ensemble Kalman filter   总被引：1，自引：0，他引：1  

Dean S. Oliver  Yan Chen  Geir Nævdal 《Computational Geosciences》2011,15(2):325-344

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