Results from a Multi-disciplinary Sedimentary Pilot Study of Tectonic Lake Iznik (NW Turkey) – Geochemistry and Paleolimnology of the Recent Past     

Sven O. Franz  Lorenz Schwark  Cathrin Brüchmann  Burkhard Scharf  Ralf Klingel  Jon D. Van Alstine  Namık Çagatay  Umut B. Ülgen 《Journal of Paleolimnology》2006,35(4):715-736

A limnogeological reconnaissance study was carried out on Lake Iznik, located in the southeast of the Marmara region of Turkey, involving a seismic survey and collection of short sediment cores. This lake is located on the middle branch of the North Anatolian Fault Zone (NAFZ), a transform plate boundary between the Eurasian and Anatolian Plates. It is, therefore, tectonically active and offers an opportunity to investigate the interplay of sedimentary and seismo-tectonic processes, as well as climate change and human impact in the region. Short cores of the three sub-basins, maximum length of 35.5 cm, recovered non-laminated, blackish clays and silts with varying amounts of biogenic and minerogenic (allochthonous, autochthonous) material, which documented almost the last 80 years of deposition and environmental history. High sedimentation rates in the deeper core sections are accompanied by changes in land use (conversion of woodland to farmland) in the northern areas of Lake Iznik, which caused the deposition of more weathered material (high K/Na ratios) and higher contents of Mn in the lake. A tendency towards eutrophic conditions within the last 20 years is indicated by high nutrient content (N, TOC, P), decreasing C/N-ratios, and characteristic diatom and cladoceran associations. Also increased pollution is revealed by higher Pb, Cu, and Zn contents and increased supply of human and animal faeces (high coprostanol content) during the last two decades. But simultaneous lower sedimentation rates towards the core tops complicate the reconstruction of recent and past eutrophication and pollution states of Lake Iznik. This requires an extension of the pilot study and deeper sediment cores, to recover non-anthropogenic influenced sediment levels.  相似文献   

Generation of mid-ocean ridge basalts at pressures from 1 to 7 GPa     

Dean C Presnall  Gudmundur H GudfinnssonMichael J Walter 《Geochimica et cosmochimica acta》2002,66(12):2073-2090

We propose a model for the generation of average MORBs based on phase relations in the CaO-MgO-Al₂O₃-SiO₂-CO₂ system at pressures from 3 to 7 GPa and in the CaO-MgO-Al₂O₃-SiO₂-Na₂O-FeO (CMASNF) system at pressures from ∼0.9 to 1.5 GPa. The MELT seismic tomography (Forsyth et al., 2000) across the East Pacific Rise shows the largest amount of melt centered at ∼30-km depth and lesser amounts at greater depths. An average mantle adiabat with a model-system potential temperature (T_p) of 1310°C is used that is consistent with this result. In the mantle, additional minor components would lower solidus temperatures ∼50°C, which would lower T_p of the adiabat for average MORBs to ∼1260°C. The model involves generation of carbonatitic melts and melts that are transitional between carbonatite and kimberlite at very small melt fractions (<0.2%) in the low-velocity zone at pressures of ∼2.6 to 7 GPa in the CMAS-CO₂ system, roughly the pressure range of the PREM low-velocity zone. These small-volume, low-viscosity melts are mixed with much larger volumes of basaltic melt generated at the plagioclase-spinel lherzolite transition in the pressure range of ∼0.9 to 1.5 GPa.In this model, solidus phase relations in the pressure range of the plagioclase-spinel lherzolite transition strongly, but not totally, control the major-element characteristics of MORBs. Although the plagioclase-spinel lherzolite transition suppresses isentropic decompression melting in the CMAS system, this effect does not occur in the topologically different and petrologically more realistic CMASNF system. On the basis of the absence of plagioclase from most abyssal peridotites, which are the presumed residues of MORB generation, we calculate melt productivity during polybaric fractional melting in the plagioclase-spinel lherzolite transition interval at exhaustion of plagioclase in the residue. In the CMASN system, these calculations indicate that the total melt productivity is ∼24%, which is adequate to produce the oceanic crust. The residual mineral proportions from this calculation closely match those of average abyssal peridotites.Melts generated in the plagioclase-spinel lherzolite transition are compositionally distinct from all MORB glasses, but do not have a significant fractional crystallization trend controlled by olivine alone. They reach the composition field of erupted MORBs mainly by crystallization of both plagioclase and olivine, with initial crystallization of either one of these phases rapidly joined by the other. This is consistent with phenocryst assemblages and experimental studies of the most primitive MORBs, which do not show an olivine-controlled fractionation trend. The model is most robust for the eastern Pacific, where an adiabat with a T_p of ∼1260°C is supported by the MELT seismic data and where the global inverse correlation of (FeO)₈ with (Na₂O)₈ is weak. Average MORBs worldwide also are well modeled. A heterogeneous mantle consisting of peridotite of varying degrees of major-element depletion combined with phase-equilibrium controls in the plagioclase-spinel lherzolite transition interval would produce the form of the global correlations at a constant T_p, which suggests a modest range of T_p along ridges. Phase-composition data for the CMASNF system are presently not adequate for quantitative calculation of (FeO)₈-(Na₂O)₈-(CaO/Al₂O₃)₈ systematics in terms of this model. The near absence of basalts in the central portion of the Gakkel Ridge suggests a lower bound for T_p along ridges of ∼1240°C, a potential temperature just low enough to miss the solidus for basalt production at ∼0.9 GPa. An upper bound for T_p is poorly constrained, but the complete absence of picritic glasses in Iceland and the global ridge system suggests an upper bound of ∼1400°C. In contrast to some previous models for MORB generation that emphasize large potential temperature variations in a relatively homogeneous peridotitic mantle, our model emphasizes modest potential temperature variations in a peridotitic mantle that shows varying degrees of heterogeneity. Calculations indicate that melt productivity changes from 0 to 24% for a change in T_p from 1240 to 1260°C, effectively producing a rapid increase to full crustal thickness or decrease to none as ridges appear and disappear.  相似文献   

Development of site-specific design ground motions in Western and Eastern North America     

Walter Silva  Carl Costantino   《Soil Dynamics and Earthquake Engineering》2002,22(9-12):755-764

This paper presents results recently obtained for generating site-specific ground motions needed for design of critical facilities. The general approach followed in developing these ground motions using either deterministic or probabilistic criteria is specification of motions for rock outcrop or very firm soil conditions followed by adjustments for site-specific conditions. Central issues in this process include development of appropriate attenuation relations and their uncertainties, differences in expected motions between Western and Eastern North America, and incorporation of site-specific adjustments that maintain the same hazard level as the control motions, while incorporating uncertainties in local dynamic material properties. For tectonically active regions, such as the Western United States (WUS), sufficient strong motion data exist to constrain empirical attenuation relations for M up to about 7 and for distances greater than about 10–15 km. Motions for larger magnitudes and closer distances are largely driven by extrapolations of empirical relations and uncertainties need to be substantially increased for these cases.

For the Eastern United States (CEUS), due to the paucity of strong motion data for cratonic regions worldwide, estimation of strong ground motions for engineering design is based entirely on calibrated models. The models are usually calibrated and validated in the WUS where sufficient strong motion data are available and then recalibrated for applications to the CEUS. Recalibration generally entails revising parameters based on available CEUS ground motion data as well as indirect inferences through intensity observations. Known differences in model parameters such as crustal structure between WUS and CEUS are generally accommodated as well. These procedures are examined and discussed.  相似文献   

A 1500-year record of climatic and environmental change in ElkLake, Minnesota I: Varve thickness and gray-scale density     

Walter Dean  Roger Anderson  J. Platt Bradbury  David Anderson 《Journal of Paleolimnology》2002,27(3):287-299

The deepest part (29.5 m) of Elk Lake, ClearwaterCounty, northwestern Minnesota, contains a complete Holocene section that iscontinuously varved. The varve components are predominantly autochthonous(CaCO₃, organic matter, biogenic silica, and several iron andmanganese minerals), but the varves do contain a minor detrital-clastic(aluminosilicate) component that is predominantly wind-borne (eolian) andprovides an important record of atmospheric conditions. Singular spectrumanalysis (SSA) and wavelet analysis of varve thickness recognized significantperiodicities in the multicentennial and multidecadal bands that varied inpower (i.e., variable significance) and position (i.e., variable period) withinthe periodic bands. Persistent periodicities of about 10, 22, 40, and 90 years,and, in particular, multicentennial periodicities in varve thickness and otherproxy variables are similar to those in spectra of radiocarbon production, aproxy for past solar activity. This suggests that there may be a solar control,perhaps through geomagnetic effects on atmospheric circulation. Multicentennialand multidecadal periodicities also occur in wavelet spectra of relativegray-scale density. However, gray-scale density does not appear to correlatewith any of the measured proxy variables, and at this point we do not know whatcontrolled gray scale.  相似文献   

The BEAM-project: prediction and assessment of mixture toxicities in the aquatic environment     

Thomas Backhaus  Rolf Altenburger  sa Arrhenius  Hans Blanck  Michael Faust  Antonio Finizio  Paola Gramatica  Matthias Grote  Marion Junghans  Wiebke Meyer  Manuela Pavan  Tobias Porsbring  Martin Scholze  Roberto Todeschini  Marco Vighi  Helge Walter  L. Horst Grimme 《Continental Shelf Research》2003,23(17-19):1757

Freshwater and marine ecosystems are exposed to various multi-component mixtures of pollutants. Nevertheless, most ecotoxicological research and chemicals regulation focus on hazard and exposure assessment of individual substances only, the problem of chemical mixtures in the environment is ignored to a large extent. In contrast, the assessment of combination effects has a long tradition in pharmacology, where mixtures of chemicals are specifically designed to develop new products, e.g. human and veterinary drugs or agricultural and non-agricultural pesticides. In this area, two concepts are frequently used and are thought to describe fundamental relationships between single substance and mixture effects: Independent Action (Response Addition) and Concentration Addition. The question, to what extent these concepts may also be applied in an ecotoxicological and regulatory context may be considered a research topic of major importance, as the concepts would allow to make use of already existing single substance toxicity data for the predictive assessment of mixture toxicities. Two critical knowledge gaps are identified: (a) There is a lack of environmental realism, as a huge part of our current knowledge about the applicability of the concepts is restricted to artificial situations with respect to mixture composition or biological effect assessment. (b) The knowledge on what exactly is needed for using the concepts as tools for the predictive mixture toxicity assessment is insufficient. Both gaps seriously hamper the necessary, scientifically sound consideration of mixture toxicities in a regulatory context.In this paper, the two concepts will be briefly introduced, the necessity of considering the toxicities of chemical mixtures in the environment will be demonstrated and the applicability of Independent Action and Concentration Addition as tools for the prediction and assessment of mixture toxicities will be discussed. An overview of the specific aims and approaches of the BEAM project to fill in the identified knowledge gaps is given and first results are outlined.  相似文献   

Measurement of Carbon Dioxide Emissions Plumes from Prudhoe Bay, Alaska Oil Fields     

Steven B. Brooks  Timothy L. Crawford  Walter C. Oechel 《Journal of Atmospheric Chemistry》1997,27(2):197-207

Large carbon dioxide plumes with concentrations up to 45 ppm aboveambient levels were measured about 15 km downwind of the Prudhoe Bay, Alaskamajor oil production facilities, located at 70° N Lat. above the ArcticCircle. The measured emissions were 1.3 × 10³ metrictons (C) hour^-1 (11.4× 10⁶ metric tons(C) year^-1), six times greater than the combustion emissionsassumed by Jaffe and coworkers in J. Atmos. Chem. 20 (1995), 213–227,based on 1989 reported Prudhoe Bay oil facility fuel consumption data, andfour times greater than the total C emissions reported by the oil facilitiesfor the same months as the measurement time periods. Variations in theemissions were estimated by extrapolating the observed emissions at a singlealtitude for all tundra research transect flights conducted downwind of theoil fields. These 30 flights yielded an average emission rate of1.02 × 10³ metric tons (C) hour^-1 with astandard deviation of 0.33 × 10³. These quantity ofemissions are roughly equivalent to the carbon dioxide emissions of7–10 million hectares of arctic tussock tundra (Oechel and Vourlitis,Trends in Ecol. Evolution 9 (1994), 324–329).  相似文献   

The l:200,000-scale map of the near-surface mineral resources of the Federal Republic of Germany     

Walter Lorenz 《Natural Resources Research》1997,6(1):5-10

For those who deal with aspects of regional planning that are affected by the extraction of near-surface mineral resources, a simple map that shows the distribution of these resources accompanied by explanatory notes is essential; a preliminary 1:1 million-scale map was published in 1982. The Geological Surveys of the Federal States of Germany, in conjunction with the Federal Institute for Geosciences and Natural Resources, are compiling a series of maps that will cover the country at a scale of 1:200,000. When completed by the end of the next decade, this set of maps will consist of 57 sheets, each of which will be accompanied by explanatory notes. By the end of 1995, 17 sheets had been published BGR reports on the status of a Federal mineral-resource mapping program.  相似文献   

Genesis and distribution of mineral waters as a consequence of recent lithospheric dynamics: the Rhenish Massif,Central Europe     

Franz May  Stephan Hoernes  Horst J. Neugebauer 《International Journal of Earth Sciences》1996,85(4):782-799

Three major, interdependent processes control the genesis and distribution of mineral and thermal waters in the Rhenish Massif, Central Europe: (a) Magmatic processes in the upper mantle provide most of the CO₂ to produce bicarbonate waters in shallow aquifers. (b) Extension of the brittle upper crust enables the ascent of sodium chloride waters. (c) Uplift and erosion shape the massif's relief, which determines the extent of flow systems and the distribution of thermal springs. The chemistry of mineral waters further depends on the aquifers' mineral composition. A comprehensive set of hydrological, chemical, tectonic and geophysical data on the Rhenish Massif has been compiled. It was used to classify the mineral waters and to map the spatial distribution of water properties. The composition of cuttings from several representative wells producing different water types shows that the hydrothermal alteration of the aquifer rocks consists mainly of kaolinization of chlorite and dissolution of feldspar. Numerical transport simulations favour two modes of groundwater flow: topography-driven flow and the pressure-driven ascent of basement brines along active faults. Thermal convection is less important.  相似文献   

The color of pumice: case study on a trachytic fall deposit, Meidob volcanic field, Sudan   总被引：1，自引：0，他引：1  

H. Paulick  G. Franz 《Bulletin of Volcanology》1997,59(3):171-185

 This paper examines the cause of color variations of trachytic pumices which are essentially uniform in chemical composition and proposes a geological model for their formation. A pyroclastic sequence of distinct subunits with brown, buff, and black pumices was deposited during the 5000-B.P. eruption of a tuff ring in the central Meidob volcanic field (Sudan). Subunits of buff pumices locally contain minor amounts of streaky pumice with pale-gray and dark-gray domains. The combined evidence of petrographic studies, chemical analyses of whole pumices and groundmass separates, electron microprobe analyses, optical spectroscopy, transmission electron microscopy, and magnetic susceptibility measurements show that color variations of the pumice clasts are related to the size and distribution of Fe³⁺-rich oxide microcrysts. Buff pumice and light-gray domains of streaky pumice have a colorless, transparent groundmass with very few microcrysts. Dark-gray domains of streaky pumice contain abundant hematite and/or magnetite microcrysts visible in thin section within a transparent, colorless glass groundmass. The groundmass of the black pumice clasts is brown in thin section which is most likely caused by submicroscopic magnetite microcrysts. Brown pumice clasts have a mixed groundmass consisting of brown domains and domains with opaque microcrysts in transparent glass. Variations in the eruption dynamics have been inferred from lithological observations. Subunits of black pumices are related to eruption pulses with low magma discharge and high water/magma mass ratio, whereas subunits of buff pumice were deposited during eruption pulses with high magma discharge and low water/magma mass ratio. Brown pumices represent the top part of the magma body, and the initial stage of the eruption probably had a low magma discharge. Streaky pumices are interpreted as the product of syn-eruptive mixing of Fe³⁺-rich oxide microcryst-bearing magma and microcryst-free magma. Received: 3 February 1997 / Accepted: 28 July 1997  相似文献   

Observations of borehole-source amplitude reduction due to casing     

Richard L. Gibson  Walter R. Turpening  rea Born  & Roger M. Turpening 《Geophysical Prospecting》1997,45(1):1-20

Theoretical results show that the amplitude of a borehole source is reduced when the well in which it is operated is cased and cemented. This reduction is a strong function of the formation velocity and is more weakly dependent on the direction of propagation of the wave travelling from source to receiver and on the diameter of the borehole itself. We have tested these predictions with data gathered in a cross-hole seismic experiment conducted in two stages in 1990 and 1991. The source and receivers were located in the MIT/Stech 1-21A and MIT/Burch 1-20B wells at the Earth Resources Laboratory (ERL) test site in Michigan (USA). Though the source well (MIT/Stech 1-21A) was uncased in 1990, a steel casing and cement were added prior to the collection of data in 1991. Several receiver positions were reoccupied to compare data collected with the source in open and cased holes. Using a velocity model for this area and borehole diameter measurements from a calliper log, a compensation factor can be computed that will adjust the data collected in 1991 to have amplitudes comparable to that collected in the first stage of the experiment. The accuracy of the results demonstrates the validity of the theory, which can be very useful in better understanding seismic waveforms recorded in cross-hole experiments.  相似文献