Fe and O isotope composition of meteorite fusion crusts: Possible natural analogues to chondrule formation?          下载免费PDF全文

Dominik C. Hezel  Graeme M. Poole  Jack Hoyes  Barry J. Coles  Catherine Unsworth  Nina Albrecht  Caroline Smith  Mark Rehkämper  Andreas Pack  Matthew Genge  Sara S. Russell 《Meteoritics & planetary science》2015,50(2):229-242

Meteorite fusion crust formation is a brief event in a high‐temperature (2000–12,000 K) and high‐pressure (2–5 MPa) regime. We studied fusion crusts and bulk samples of 10 ordinary chondrite falls and 10 ordinary chondrite finds. The fusion crusts show a typical layering and most contain vesicles. All fusion crusts are enriched in heavy Fe isotopes, with δ⁵⁶Fe values up to +0.35‰ relative to the solar system mean. On average, the δ⁵⁶Fe of fusion crusts from finds is +0.23‰, which is 0.08‰ higher than the average from falls (+0.15‰). Higher δ⁵⁶Fe in fusion crusts of finds correlate with bulk chondrite enrichments in mobile elements such as Ba and Sr. The δ⁵⁶Fe signature of meteorite fusion crusts was produced by two processes (1) evaporation during atmospheric entry and (2) terrestrial weathering. Fusion crusts have either the same or higher δ¹⁸O (0.9–1.5‰) than their host chondrites, and the same is true for Δ¹⁷O. The differences in bulk chondrite and fusion crust oxygen isotope composition are explained by exchange of oxygen between the molten surface of the meteorites with the atmosphere and weathering. Meteorite fusion crust formation is qualitatively similar to conditions of chondrule formation. Therefore, fusion crusts may, at least to some extent, serve as a natural analogue to chondrule formation processes. Meteorite fusion crust and chondrules exhibit a similar extent of Fe isotope fractionation, supporting the idea that the Fe isotope signature of chondrules was established in a high‐pressure environment that prevented large isotope fractionations. The exchange of O between a chondrule melt and an ¹⁶O‐poor nebula as the cause for the observed nonmass dependent O isotope compositions in chondrules is supported by the same process, although to a much lower extent, in meteorite fusion crusts.  相似文献   

Formation of intracratonic basins by lithospheric shortening and phase changes: a case study from the ultra‐deep East Barents Sea basin     

Sébastien Gac  Ritske S. Huismans  Nina S. C. Simon  Yuri Y. Podladchikov  Jan Inge Faleide 《地学学报》2013,25(6):459-464

The origin of large subsidence in intracratonic basins is still under debate. We propose a new and self‐consistent model for the formation of those basins, where lithospheric shortening/buckling triggers metamorphism and densification of crustal mafic heterogeneities. We use a forward thermo‐mechanical finite element technique to evaluate this mechanism for the typical example of the East Barents Sea basin (EBB) where a very large and compensated subsidence, accommodating an up to 20‐km‐thick sediment succession, is observed. The lower crust in the dynamic model is modelled with petrologic‐consistent densities for a wet mafic gabbroic composition that depend on pressure and temperature taking into account dehydration at high PT conditions. The model successfully explains the main characteristics of the EBB, notably the large anomalous and fast subsidence during the Late Permian–Early Triassic, its present‐day geometry and the absence of a significant gravity anomaly.  相似文献   

Upper-mantle velocity structure beneath the Siberian platform   总被引：1，自引：0，他引：1  

Keith Priestley  John Cipar  Anatoli Egorkin  Nina Pavlenkova 《Geophysical Journal International》1994,118(2):369-378

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Holocene Paleoenvironmental Change in the Kenyan Central Rift as Indicated by Micromammals from Enkapune Ya Muto Rockshelter     

Curtis W. Marean  Nina Mudida  Kaye E. Reed 《Quaternary Research》1994,41(3)

An assemblage of micromammals, recovered from the Holocene levels of a rockshelter at 2400 m in the montane forest of the Mau Escarpment, were examined with the goal of testing and contributing to prior reconstructions of paleoenvironments in the Central Rift Valley of Kenya. Species representation in the assemblage is consistent with a drying of the Rift Valley lakes in the middle Holocene and suggests a decrease in forest accompanied by expanding grasslands near the site. Changes in the abundance of grassland species suggests an increase in the frequency of fires, probably the result of pastoral burning. The body size of the root rat (Tachyoryctes splendens) decreases from the early Holocene to the middle Holocene, and this may indicate increasing aridity or increasing temperature. We compare measures of species diversity (number of taxa, species richness, and the Shannon diversity index) for both micromammals and macromammals since species diversity may change with paleoenvironmental change. The macromammals show no changes in species diversity that are assignable to paleoenvironmental change, while the micromammals show a trend toward decreasing diversity from the early to middle Holocene, and then show an increase in diversity during the peak of the middle Holocene dry phase, though sample size effects may be confounding the patterning.  相似文献   

Water chemistry and sedimentological observations in littlefield lake,michigan: Implications for lacustrine marl deposition     

Nina M. Duston  Robert M. Owen  Bruce H. Wilkinson 《Environmental Geology》1986,8(4):229-236

A combination of both water chemistry and sedimentological information was used to investigate the carbonate-producing mechanism in Littlefield Lake, a small lake located in Isabella County, central Michigan. Data on temperature, dissolved oxygen, pH, calcium carbonate (CaCO₃) saturation, alkalinity, calcium, and magnesium were obtained on a monthly basis over a 13-month period, with each parameter determined at 1m intervals over a depth range of 20m. The loss of dissolved carbon dioxide (CO₂) from warm surface waters during direct degassing, and to a lesser extent during photosynthetic uptake by lacustrine macrophytes and phytoplankton during the summer, results in massive precipitation of the low-magnesium calcite which predominates in all Littlefield Lake sedimentary facies However, despite the fact that carbonate precipitation in this rather typical temperate-region marl lake is directly related to, and may be driven by, seasonal variation in these physiochemical parameters, most calcite forms as encrustations around cyanophytic and chlorophytic macrophytes. Such relationships demonstrate that carbonate precipitation in marl lakes may result from complex interactions between both biochemical and physiochemical processes. As such, marl formation in this, and probably many other calcareous lake systems, can not be simply ascribed to one or the other of these two general mechanisms.  相似文献   

Tracing of water movement through the unsaturated zone of a coarse gravel aquifer by means of dye and deuterated water     

Nina Mali  Janko Urbanc  Albrecht Leis 《Environmental Geology》2007,51(8):1401-1412

To better understand the movement and transport of water and pollution through the coarse gravel unsaturated zone, the presented research was conducted to estimate water flow and transport processes with a tracing experiment in a lysimeter in the Selniška Dobrava. A combined tracing experiment was performed with deuterated water and the fluorescent dye—uranine. The fastest and dominant flow velocities were calculated based on injection time, the first tracer appearance time and the time of highest concentration. Mean flow velocity and vertical dispersion were estimated by an analytical best-fit method using one-dimensional convection–dispersion model. Deuterium was confirmed as an ideal conservative tracer and a more suitable tracer than dye (uranine) for the study of water flow in the unsaturated zone of a coarse gravel aquifer. The retardation factor of the dye as compared with deuterium was 1.13–1.75, which is in agreement with previously published results. Artificial tracers, especially deuterated water, were also identified as a very useful tool to assess other properties and differences in water flow in the unsaturated zone of a coarse gravel aquifer such as velocity and dispersion.  相似文献   

环境与健康的评估问题和空间分析   总被引：1，自引：0，他引：1  

林绍颜 《地理学报》2001,56(2):239-247

评估环境污染对人民健康影响的研究需要结合空间，时间和统计分析，并吸引不同学科背景的研究人员合作研究，可是，在这个研究领域内，最重要的问题是研究结论的不可靠性程度很高，不同的研究方法会导致不同的结论，一个有系统的研究架构拥有不同的空间分析方法将会减少结论的可疑程度，有效的环保政策是需要可靠的研究成果，文中首先介绍在环境与健康的评估研究领域的四大重要问题，指出这项研究需要这项研究需要空间分析和地理信息系统方法，从而提出一个空间分析的研究框架和内裹的方法，通过路易斯安纳州的一个污染地点和肿瘤发病率关系研究实例，指出这项研究领域的各种问题和空间分析架构的重要性。  相似文献   

Weather categorization in climate change research: Preliminary results     

Ivana Nemešová  Nina Klimperová  Radan Huth  Jaroslava Kalvová 《Studia Geophysica et Geodaetica》1992,36(4):370-375

Summary The results of an objective weather categorization are presented. The 9 meteorological variables recorded daily during winter seasons of 1961–66 at Prague-Clementinum represent the input dataset. The principal component analysis and a few clustering procedures have been evaluated. 5 component solution and the average linkage clustering method were found optimal. The winter days have been grouped, according to their meteorological character, into 14 clusters. The warm categories represent 20% of the time and the cold categories less than 15% of the days. The mean maps of 1000 hPa and 500 hPa are shown for a few selected categories. Clustering techniques applied to long-time instrumental series can provide a better basis for attempting to detect temperature changes which have taken place over a long time span.  相似文献   

旱作地膜小麦玉米带田一膜两用热效应研究     

仇化民  王宁珍  马妮娜 《干旱气象》1995,(4)

旱作地膜小麦—玉米带田一膜两用技术热效应显著，可提高地温２—６℃，保证了小麦安全越冬，促进了作物早发。热资源利用率高达０．９９７，比单作提高２５％—４０％，变作物一年一熟为一年两熟。  相似文献   

Use of remotely sensed land use classification for a better evaluation of micrometeorological flux measurement sites     

L. M. Reithmaier  M. Göckede  T. Markkanen  A. Knohl  G. Churkina  C. Rebmann  N. Buchmann  T. Foken 《Theoretical and Applied Climatology》2006,84(4):219-233

Summary Long-term flux measurement sites are often characterized by a heterogeneous terrain, which disagrees with the fundamental theoretical assumptions for eddy-covariance measurements. An evaluation procedure to assess the influence of terrain heterogeneity on the data quality has been developed by G?ckede et al. (2004), which combines existing quality assessment tools for flux measurements with analytic footprint modeling. In addition to micrometeorological input data, this approach requires information defining the land use structure and the roughness of the surrounding terrain. The aim of this study was to improve the footprint based site evaluation approach by using high-resolution land use maps derived by Landsat ETM+ and ASTER satellite data. The influence of the grid resolution of the maps on the results was examined, and four different roughness length classification schemes were tested. Due to numerical instabilities of the analytic footprint routine, as an additional footprint model a Lagrangian stochastic footprint routine (Rannik et al., 2003) was employed. Application of the approach on two German FLUXNET sites revealed only weak influence of the characteristics of the land use data when the land use structure was homogeneous. For a more heterogeneous site, use of the more detailed land use maps derived by remote sensing methods resulted in distinct differences indicating the potential of remote sensing for improving the flux measurement site evaluation.  相似文献