排序方式： 共有35条查询结果，搜索用时 18 毫秒
1.
Brigitta von Rekowski Axel Brandenburg Wolfgang Dobler Anvar Shukurov 《Astrophysics and Space Science》2004,292(1-4):493-500
An axisymmetric model of a cool, dynamo-active accretion disc is applied to protostellar discs. Thermally and magnetically driven outflows develop that are not collimated within 0.1 AU. In the presence of a central magnetic field from the protostar, accretion onto the protostar is highly episodic, which is in agreement with earlier work. 相似文献
2.
Ivanka Stefanova Nadja Ognjanova-Rumenova Wolfgang Hofmann Brigitta Ammann 《Journal of Paleolimnology》2003,30(1):95-111
Diatoms, Cladocera, and chironomids preserved in the sediments of Lake Dalgoto were studied to reconstruct the history of the lake ecosystem in the context of the vegetation history as represented by the pollen stratigraphy. Younger Dryas silty sediments at the base of the core are characterized by low diversity of aquatic organisms. The transition to the Holocene is indicated by a sharp change from silt to clay-gyttja. The migration and expansion of trees at lower elevations between 10200 and 8500 14C-yr BP, along with higher diversities and concentrations of aquatic organisms and the decreased proportion of north-alpine diatoms, point to rapidly rising summer temperatures. After 6500 14C-yr BP the expansion of Pinus mugo in the catchment coincides with signs of natural eutrophication as recorded by an increase of planktonic diatoms. In the late Holocene (4000–0 14C-yr BP) Pinus peuce and Abies are reduced and Picea expands. Cereal grains and disturbance indicators suggest late-Holocene human modification of the vegetation. 相似文献
3.
CHRISTIAN-D. SCH?NWIESE 《Global Change & Human Health》2002,3(1):2-2
Editorial Board
Editors 相似文献4.
In order to evaluate the Holocene palaeoenvironmental evolution of the Ugii Nuur basin, central Mongolia, investigations on chemical and mineralogical properties of lacustrine sediments were carried out on a 630 cm sediment core from lake Ugii Nuur. The interpretation of the record is based on a principal component analysis （PCA） of the elemental composition of the samples. The results show that lacustrine deposition started at 10.6 kyr BP. Low lake level conditions were identified during the Early Holocene （10.6-7.9 kyr BP）. The Mid Holocene （7.9-4.2 kyr BP） was characterized by generally higher lake levels and thus higher moisture supply, but it experienced strong climatic fluctuations. Arid conditions prevailed from 4.2-2.8 kyr BP and were followed by a stable, more humid phase until today. 相似文献
5.
Moritz Nykamp Fabian Becker Ricarda Braun Nadja Pöllath Daniel Knitter Joris Peters Brigitta Schütt 《地球表面变化过程与地形》2021,46(2):430-442
This study presents a meta-analysis of radiocarbon ages for the environs of Göbekli Tepe – one of the oldest monumental structures worldwide – using cumulative probability functions to diachronically assess phases of geomorphodynamic activity as controlled by natural or anthropogenic drivers. We employ sediment cascades as a heuristic framework to study the complex responses of the geomorphological system to various triggers at local to supra-regional scales. Possible triggers include climatic variability as documented by supra-regional hydroclimatic proxy data, regional demographic trends, and local to regional socioeconomic developments such as the emergence of sedentism or the introduction and dispersal of livestock herding. Our results show that phases of intensified geomorphodynamic activity occurred between ca. 7.4–7.0 and 5.8–3.3 ka BP. These phases roughly coincide with phases of population growth in southern Turkey and climatic variations in Turkey and the Levant. The phase between ca. 5.8–3.3 ka BP also corresponds to the time when organized agriculture and the seeder plough were introduced. Also, the identified phases are in agreement with the general trend of varying geomorphodynamic activity in the Eastern Mediterranean as driven by human impact and climatic change. However, neither the Younger Dryas–Holocene transition nor the development of herding during the Pre-Pottery Neolithic left a clear signature. We demonstrate how the different depositional environments in the studied landscape compartments vary with respect to their spatiotemporal coverage and discuss challenges when trying to understand processes that once shaped landscapes of past societies. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd 相似文献
6.
István Major Brigitta Gyökös Marianna Túri István Futó Ágnes Filep András Hoffer Enikő Furu A. J. Timothy Jull Mihály Molnár 《Journal of Atmospheric Chemistry》2018,75(1):85-96
Comprehensive atmospheric studies have demonstrated that carbonaceous particles are one of the main components of atmospheric aerosols over Europe. The aim of our study was to establish an automated elemental analyser interfaced to a stable isotope mass spectrometer (EA-IRMS) method at the Hertelendi Laboratory of Environmental Studies (HEKAL), as a suitable method of quantification of total carbon mass in individual PM2.5 aerosol samples. Total carbon (TC) mass and simultaneous stable isotopic ratios were determined for both test standard and genuine aerosol samples. Finally, the results were compared to the ones obtained independently by an alternative sealed tube combustion method developed previously at HEKAL. The TC recovery tests of standard material prepared by the sealed tube method confirmed at least a carbon recovery yield of 92% for a broad range of carbon mass (100–2000 μg). The stable isotopic results confirmed that sealed tube method is reproducible and suitable to be used as a reference to verify our new EA-IRMS method. The EA-IRMS TC measurements of genuine aerosols gave on average 3% higher carbon recovery yield, relative to the uncorrected results of the sealed tube method. The comparison of the stable isotopic results by the two methods for aerosols also showed minimal differences. Consequently, the possibility of simultaneous TC and stable isotopic analyses makes the EA-IRMS method a very attractive alternative for continuous measurement of aerosols, with an accuracy and reliability similar to other commercial devices. 相似文献
7.
We investigated the temporal dynamics of dust entrainment in the Bodélé Depression, Central Sahara, to better understand the intra-annual variability of aerosol emission in the world's largest dust source. The linkages between dust entrainment and large-scale meteorological factors were examined by correlating several meteorological variables in the Mediterranean and Africa north of the equator with the aerosol concentrations in the Bodélé Depression separately for winter and summer. The methodological tools applied are NCEP/NCAR reanalysis data and the aerosol index of the Total Ozone Mapping Spectrometer (TOMS-AI), available for 15 years from 1978 to 1993. We found that dust mobilisation during the Harmattan season is highly dependent on air pressure variability in the Mediterranean area. High pressure to the north of the Bodélé intensifies the NE trade winds, leading to an increased entrainment of dust in the Bodélé Depression. In summer, dust mobilization cannot be explained by the large scale meteorological conditions. This highlights the importance of local to regional wind systems linked to the northernmost position of the intertropical convection zone (ITCZ) during this time. 相似文献
8.
9.
Maurits Lindström Johan Ekvall Stefan E. Hagenfeldt Brigitta Säwe Erik F. F. Sturkell 《International Journal of Earth Sciences》1991,80(1):201-204
The Lockne impact crater south of Östersund formed in the early Middle Cambrian with a diameter of 7 km. It is identified by its rim wall of crushed Precambrian basement granite, by fragments of impact melt, and by grains of shocked quartz. The exceptional preservation, in particular of the rim wall, is due to a complicated geological history, the first stage of which consisted of burial by marine sediments. This stage lasted until the Middle Ordovician, or over 50 million years. An early Caradoc lowering of the sea-level may have induced debris flows that stripped the rim wall of much of its sedimentary cover. Because normal marine sedimentation recommenced soon after this event, the structure was not seriously damaged, as it was buried again. The Caledonian orogeny emplaced an overthrust nappe as ultimate protection, which was removed by a recent erosion episode from all but the center of the structure.Structures formed by the impact of extraterrestrial bodies are very rare throughout much of Europe, because such structures are neither well preserved nor displayed in young mountain belts or sedimentary basins. However, northern Europe has several ascertained structures of this kind (Svensson &Wickman, 1965;Svensson, 1968;Bruun &Dahlman, 1982;Kala et al., 1984;Flodén et al., 1986;Wickman, 1988). Unfortunately, the hitherto known structures are either poorly preserved or hidden by younger deposits. We are reporting the discovery of a well-preserved exposed and accessible impact structure that has escaped the notice of geologists although important features of it have been described and puzzled over by generations of researchers (Wiman, 1900;Hadding, 1927;Thorslund, 1940;Lindström et al., 1983).The structure is located in the Lockne area to the south of Östersund in central Sweden. It has a diameter of 7 km and its center is near Tramsta on the northwest shore of Lake Locknesjön (Fig. 1). Its middle is covered by folded Lower to Middle Ordovician Orthoceratite Limestone with the sheared Lower Ordovician Töyen Shale at its base. This local expanse of deformed rock is the remainder after erosion of an extensive nappe of overthrust rocks emplaced during the Caledonian orogeny. The good preservation of the impact structure is due to the nappe cover, which had to be removed before erosion could attack the underlying structures.The rim of the impact crater is outlined by a wall of strongly shattered fragments of Proterozoic crystalline rocks (Fig. 2), which formed the local bedrock (Strömberg et al., 1984) at the time of impact. The rim wall is best preserved along the western part of the structure. It was referred to as »arkose-like breccia« byThorslund (1940), who interpreted it as the result of continental weathering, but the components rather show evidence of intense crushing than of weathering (Simon, 1987a). The »arkose-like breccia« does not contain components derived from the lower Palaeozoic deposits of the area. 相似文献
10.
Michael H. POELCHAU Thomas KENKMANN Klaus THOMA Tobias HOERTH Anja DUFRESNE Frank SCHÄFER 《Meteoritics & planetary science》2013,48(1):8-22
Abstract– The MEMIN research unit (Multidisciplinary Experimental and Modeling Impact research Network) is focused on analyzing experimental impact craters and experimental cratering processes in geological materials. MEMIN is interested in understanding how porosity and pore space saturation influence the cratering process. Here, we present results of a series of impact experiments into porous wet and dry sandstone targets. Steel, iron meteorite, and aluminum projectiles ranging in size from 2.5 to 12 mm were accelerated to velocities of 2.5–7.8 km s?1, yielding craters with diameters between 3.9 and 40 cm. Results show that the target’s porosity reduces crater volumes and cratering efficiency relative to nonporous rocks. Saturation of pore space with water to 50% and 90% increasingly counteracts the effects of porosity, leading to larger but flatter craters. Spallation becomes more dominant in larger‐scale experiments and leads to an increase in cratering efficiency with increasing projectile size for constant impact velocities. The volume of spalled material is estimated using parabolic fits to the crater morphology, yielding approximations of the transient crater volume. For impacts at the same velocity these transient craters show a constant cratering efficiency that is not affected by projectile size. 相似文献