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21.
Diffusion profiles in minerals are increasingly used to determine the duration of geological events. For this purpose, the
distinction between growth and diffusion zoning is critical; it requires the understanding of complex features associated
with multicomponent diffusion. Seed-overgrowth interdiffusion experiments carried out in the range 1,050–1,250°C at 1.3 GPa
have been designed to quantify and better understand Fe–Mg–Ca interdiffusion in garnet. Some of the diffusion profiles measured
by analytical transmission electron microscope show characteristic features of multicomponent diffusion such as uphill diffusion,
chemical solitary waves, zero-flux planes and complex diffusion paths. We implemented three different methods to calculate
the interdiffusion coefficients of the D matrix from the experimental penetration curves and determined that with Ca as the dependent component, the crossed coefficients
of the D matrix are negative. Experiments and numerical simulations indicate that: (1) uphill diffusion in garnet can be observed
indifferently on the three components Fe, Mg and Ca, (2) it takes the form of complementary depletion/repletion waves and
(3) chemical waves occur preferentially on initially flat concentration profiles. Derived D matrices are used to simulate the fate of chemical waves in time, in finite crystals. These examples show that the flow of
atoms in multicomponent systems is not necessarily unidirectional for all components; it can change both in space along the
diffusion profile and in time. Moving zero-flux planes in finite crystals are transitory features that allow flux reversals
of atoms in the diffusion zone. Interdiffusion coefficients of the D matrices are also analyzed in terms of eigenvalues and eigenvectors. This analysis and the experimental results show that
depending on the composition of the diffusion couple, (1) the shape of chemical waves and diffusion paths changes; (2) the
width of the diffusion zone for each component may or may not be identical; and (3) the width of diffusion calculated at a
given D and duration may greatly vary. D matrices were retrieved from thirteen sets of diffusion profiles. Data were cast in Arrhenius relations. Linear regressions
of the data yield activation energies equal to 368, 148, 394, 152 kJ mol−1 at 1 bar and frequency factors Do equal to 2.37 × 10−6, −4.46 × 10−16, −1.31 × 10−5, 9.85 × 10−15 m2 s−1 for [(D)\tilde]FeFeCa \tilde{D}_{FeFe}^{Ca} , [(D)\tilde]FeMgCa \tilde{D}_{FeMg}^{Ca} , [(D)\tilde]MgFeCa \tilde{D}_{MgFe}^{Ca} , [(D)\tilde]MgMgCa \tilde{D}_{MgMg}^{Ca} , respectively. These values can be used to calculate interdiffusion coefficients in Fe–Mg–Ca garnets and determine the duration
of geological events in high temperature metamorphic or magmatic garnets. 相似文献
22.
Phytoplankton dynamics and cyanobacterial dominance in Murchison Bay of Lake Victoria (Uganda) in relation to environmental conditions 总被引:2,自引:0,他引:2
Sigrid Haande Thomas RohrlackRonald P. Semyalo Pål BrettumBente Edvardsen Anne Lyche-SolheimKai Sørensen Petter Larsson 《Limnologica》2011,41(1):20-29
Murchison Bay is a shallow embayment in the north-western part of Lake Victoria, strongly influenced by urban pollution from the Ugandan capital Kampala. Two stations, representing the semi-enclosed innermost part of the bay and the wider outer part of the bay, were sampled in the period from April 2003 to March 2004, in order to assess the phytoplankton community and the nutrient status in the bay. Murchison Bay was highly eutrophic with average concentrations (n=25) of total phosphorous >90 μg L−1 and total nitrogen >1100 μg L−1 in the inner part of the bay. The phytoplankton community was dominated by a variety of cyanobacterial species and diatoms. Cyanobacteria were dominant in the whole bay, whereas diatoms were more abundant in the outer part of the bay. Moreover, the proportion of N-fixing species like Anabaena sp. was higher in the outer part of the bay, whereas species like Microcystis sp. were more abundant in the inner part of the bay. The phytoplankton community, especially in the outer part of the bay, may be influenced by light limitation. Low NO3-N concentrations in the bay may also indicate a possible N-limitation, thus favouring growth of N-fixing cyanobacteria. The open bay is, however, a complex system, and additional environmental factors and loss processes most likely affect the phytoplankton community. 相似文献
23.
In climatology and hydrology, univariate Extreme Value Theory has become a powerful tool to model the distribution of extreme events. The Generalized Pareto Distribution (GPD) is routinely applied to model excesses in space or time by letting the two GPD parameters depend on appropriate covariates. Two possible pitfalls of this strategy are the modeling and the interpretation of the scale and shape GPD parameters estimates which are often and incorrectly viewed as independent variables. In this note we first recall a statistical technique that makes the GPD estimates less correlated within a Maximum Likelihood (ML) estimation approach. In a second step we propose novel reparametrizations for two method-of-moments particularly popular in hydrology: the Probability Weighted Moment (PWM) method and its generalized version (GPWM). Finally these three inference methods (ML, PWM and GPWM) are compared and discussed with respect to the issue of correlations. 相似文献
24.
Béatrice Allard Michaël Danger Loïc Ten-Hage Gérard Lacroix 《Aquatic Sciences - Research Across Boundaries》2011,73(1):113-126
The effects of food web structure on the quantity and biochemical composition of seston, zooplankton and recently deposited
sediment in experimental freshwater mesocosms were examined. Food web structure was manipulated by addition of zooplanktivorous
fish. Biochemical characterisations were carried out using lipid biomarkers (sterols, fatty acids, chlorophyll-derived compounds
and long-chain alkanediols). Fish addition decreased zooplankton biomass and increased seston biomass and deposited sediment
through a trophic cascade. Fish presence strongly influenced the biochemical characteristics of seston and sediment. In contrast,
food web structure had a minor impact on the lipid biomarker composition of zooplankton. Although the relative abundance of
sterols in the different compartments did not differ strongly between treatments, sterol profiles in seston and sediment depended
on food web structure. The predominance of Δ7-sterols in seston and sediment in the fish treatment indicated a major contribution of Chlorophyceae. In contrast, the distribution
of sterols in seston and sediment in the fishless treatment, dominated by cholesterol, indicated a major zooplanktonic input.
The distribution of fatty acids and the relative abundance of chlorophyll-derived compounds and long-chain alkanediols agreed
with the predominant contribution of phytoplankton or zooplankton to seston and sediment in the two treatments. The relative
abundance of bacterial biomarkers suggested that the contribution of bacteria was rather low. The high relative abundance
of polyunsaturated fatty acids (PUFAs) and the absence of stanols in sediments suggested low microbial reworking of organic
matter in the recently accumulated sediments. The trophic cascade, generated by the addition of fish, increased the relative
abundance of PUFAs in deposited organic matter, thus enhancing sediment quality and potential degradability. 相似文献
25.
Serkan Üner Gülçin Özürlan Ağaçgözgü Doğa Düşünür Doğan 《Geophysical Prospecting》2019,67(8):2176-2195
Western Anatolia hosts many low-to-moderate and high-temperature geothermal sources in which active faults play a dominant role to control the recharge and the discharge of geothermal fluid. In this study, we used the two-dimensional geoelectric structure of Kütahya Hisarcık geothermal field, and created a conceptual hydrogeophysical model that includes faults, real topographical variations and geological units. The temperature distribution and fluid flow pattern are also investigated. The depth extension of Hisarcık Fault, electrical basement and low resistivity anomalies related to the presence of geothermal fluid are determined by using resistivity studies in the area. Numerical simulations suggest that Hisarcık fault functioning as a fluid conduit primarily enables hot fluid to be transported from depth to the surface. It is shown that the locations of predicted outflow vents coincide with those of hot springs in the area. 相似文献
26.
In the Makran subduction zone, earthquake focal mechanisms and geodetic data indicate that the deforming prism currently experiences N–S compression. However, palaeostress inversions performed on normal faults observed along the coast reveal local stress components consistent with N‐S extension. Previously proposed mechanisms such as gravitational collapse are not favoured by N–S compression and surface uplift. We propose that the observed kinematics result from transient stress reversals following large earthquakes. During the interseismic period (now), the region experiences N–S compression. However, following a large reverse rupture on the subduction interface, stresses in the inner wedge relax, enabling a brief period of extensional faulting before a compressive stress state is re‐established. This mechanism, also observed in other subduction zones, requires low overall stresses in the upper plate and that the margin ruptures in large megathrust earthquakes that result in nearly complete stress drops. 相似文献
27.
F. Tuba Kadirioğlu Recai F. Kartal Tuğbay Kılıç Doğan Kalafat Tamer Y. Duman Tuba Eroğlu Azak Selim Özalp Ömer Emre 《Bulletin of Earthquake Engineering》2018,16(8):3317-3338
Many catalogues, agency reports and research articles have been published on seismicity of Turkey and its surrounding since 1950s. Given existing magnitude heterogeneity, erroneous information on epicentral location, event date and time, this past published data however is far from fulfilling the required standards. Paucity of a standardized format in the available catalogues have reinforced the need for a refined and updated catalogue for earthquake related hazard and risk studies. During this study, ~37,000 earthquakes and related parametric data were evaluated by utilizing more than 41 published studies and, an integrated database was prepared in order to analyse all parameters acquired from the catalogues and references for each event. Within the scope of this study, the epicentral locations of M ≥ 5.0 events were firstly reappraised based on the updated Active Fault Map of Turkey. An improved catalogue of 12.674 events for the period 1900–2012 was as a result recompiled for the region between 32–45N° and 23–48E° by analyzing in detail accuracy of all seismological parameters available for each event. The events consist of M ≥ 4.0 are reported in several magnitude scales (e.g. moment magnitude, Mw; surface wave magnitude, MS; body-wave magnitude mb; local magnitude ML and duration magnitude Md) whereas the maximum focal depth reaches up to 225-km. In order to provide homogenous data, the improved catalogue is unified in terms of Mw. Fore-and aftershocks were also removed from the catalogue and completeness analyses were performed both separately for various tectonic sources and as a whole for the study region of interest. Thus, the prepared homogenous and declustered catalogue consisting of 6573 events provides the basis for a reliable input to the seismic hazard assessment studies for Turkey and its surrounding areas. 相似文献
28.
S. Akkar T. Azak T. Çan U. Çeken M. B. Demircioğlu Tümsa T. Y. Duman M. Erdik S. Ergintav F. T. Kadirioğlu D. Kalafat Ö. Kale R. F. Kartal K. Kekovalı T. Kılıç S. Özalp S. Altuncu Poyraz K. Şeşetyan S. Tekin A. Yakut M. T. Yılmaz M. S. Yücemen Ö. Zülfikar 《Bulletin of Earthquake Engineering》2018,16(8):3197-3228
A review on the historical evolution of seismic hazard maps in Turkey is followed by summarizing the important aspects of the updated national probabilistic seismic hazard maps. Comparisons with the predecessor probabilistic seismic hazard maps as well as the implications on the national design codes conclude the paper. 相似文献
29.