Lead isotopic compositions of South Sandwich Island volcanic rocks and their bearing on magmagenesis in intra-oceanic island arcs     

Barbara Barreiro 《Geochimica et cosmochimica acta》1983,47(4):817-822

Pb isotope ratios have been measured in 12 volcanic rocks from the South Sandwich Islands. The ranges are ${}^{206}\text{Pb}{}^{204}\text{Pb}\text{= 18.51–18.66;}{}^{207}\text{Pb}{}^{204}\text{Pb}\text{= 15.55–15.64;}{}^{208}\text{Pb}{}^{204}\text{Pb}\text{= 38.42–38.64}$. In ${}^{207}\text{Pb}{}^{204}\text{Pb}\text{-}{}^{206}\text{Pb}{}^{204}\text{Pb}$ and ${}^{208}\text{Pb}{}^{204}\text{Pb}\text{-}{}^{206}\text{Pb}{}^{204}\text{Pb}$ correlation diagrams, the South Sandwich data plot distinctly above the fields for ocean ridge basalts, and yield trends showing apparent mixing with a sedimentary end member similar to South Atlantic pelagic sediments as reported by Chow and Patterson (1962) and this study. Armstrong and Cooper (1971) have likewise shown that volcanics from the Lesser Antilles show mixing trends with North Atlantic sediments in Pb isotope correlation diagrams. The North Atlantic sediments have distinctly higher ${}^{206}\text{Pb}{}^{204}\text{Pb}$ and ${}^{208}\text{Pb}{}^{204}\text{Pb}$ ratios compared to the South Atlantic sediments. The parallel relationships between sediments and volcanic island arc rocks of the North and South Atlantic provide strong evidence for a component of Pb from subducted sediments in the lavas of the west Atlantic basin. In contrast to these data, lavas from the Mariana Arc in the western Pacific show little or no component of Pb from pelagic sediments. The reason for the different behaviors in the two settings is speculative.  相似文献   

Ice core evidence for secular variability and 200-year dipolar oscillations in atmospheric circulation over East Antarctica during the Holocene   总被引：3，自引：0，他引：3  

Barbara Delmonte  Jean Robert Petit  Gerhard Krinner  Valter Maggi  Jean Jouzel  Roberto Udisti 《Climate Dynamics》2005,24(6):641-654

Two Holocene ice core records from East Antarctica (Vostok and EPICA-Dome C) were analysed for dust concentration and size distribution at a temporal resolution of 1 sample per ~50 years. A series of volcanic markers randomly distributed over the common part of the ice cores (from 9.8 to 3.5 kyear BP) ensures accurate relative dating (±33 years). Dust-size records from the two sites display oscillations structured in cycles with sub-millennial and secular scale frequencies that are apparently asynchronous. The power spectra of the composite sum (Σ) of the two dust-size records display spectral energy mostly for 150- to 500-year periodicities. On the other hand, the 200-year band is common to both records and the 200 year components of the two sites are out-of-phase (100-year lead or lag) over ~5.5 kyear, a phenomenon also reflected by a significant (>99% conf. lev.) band in the power spectra of the composite difference (Δ) of the two size records. During long-range transport, mineral dust originating from the Southern Hemisphere continents is graded to a variable extent depending on the altitude and duration of atmospheric transport. Relatively coarse dust is associated with air mass penetration from the middle–lower troposphere and conversely relatively fine dust with upper troposphere air masses or the influence of subsidence over the Antarctic plateau, a hypothesis already proposed for the changes that occurred during the Last Glacial Maximum to Holocene transition (Delmonte et al. 2004b). Moreover, we assume that the overall fluctuation of air mass advection over Antarctica depends on the meridional pressure gradient with respect to low latitudes, i.e. the Antarctic Oscillation (AAO). We therefore suggest a regional variability in atmospheric circulation over East Antarctica. The 150–500 year power spectrum of the composite (Σ) parameter represents the long term variability of the AAO, imprinted by secular internal oscillations probably related to the southern ocean-climatic system. On the other hand, the Δ dust composite parameter suggests a persistent atmospheric dipole over East Antarctica delivering coarser (finer) dust particles alternatively to Vostok and Dome C regions with a bi-centennial periodicity. Indeed, a seesaw phenomenon in dust size distribution was already observed at three East Antarctic sites during the last deglaciation (Delmonte et al. 2004b) and was interpreted as a progressive reduction of the eccentricity of the polar vortex with respect to the geographic south pole. Interestingly, the Δ parameter shows a pronounced 200-year oscillation mode, throwing new light on the unresolved question of a possible relationship between climate and solar activity.  相似文献   

Quantitative extraction and analysis of carriers of magnetization in sediments   总被引：7，自引：0，他引：7  

Mark W. Hounslow  Barbara A. Maher 《Geophysical Journal International》1996,124(1):57-74

  相似文献   

Paleorainfall Reconstructions from Pedogenic Magnetic Susceptibility Variations in the Chinese Loess and Paleosols   总被引：2，自引：0，他引：2  

Barbara A. Maher  Roy Thompson 《Quaternary Research》1995,44(3)

The rock magnetic properties of the Chinese loess and paleosols constitute a unique and sensitive record of East Asian paleoclimate through the Quaternary Period. Systematic variations in the concentration and grain size of the magnetic minerals in these sediments have produced systematic variations in the magnetic susceptibility signal, which can be easily and rapidly measured at many sites across the Loess Plateau. Variations in many other rock magnetic properties can be used to identify the key shifts in ferrimagnetic grain size, but magnetic susceptibility alone is sufficiently sensitive to record stadial and interstadial climate stages, as well as glaciations and interglaciations. Past changes in rainfall and monsoon activity for this region are reconstructed from the susceptibility variations. The susceptibility record is calibrated using the modern relationship between rainfall and pedogenic susceptibility on the Loess Plateau. Our rainfall reconstructions identify enhanced summer monsoonal activity in the Chinese Loess Plateau region in the early Holocene and the last interglaciation. In the presently semiarid western area of the plateau, annual precipitation in interglacial times was up to 80% higher than at present; in the more humid southern and eastern areas, values were up to 20% higher than today's levels. During the last glaciation, precipitation decreased across the entire plateau, typically by 25%. The relationship between pedogenic susceptibility, climate, and weathering age was examined over the Northern Hemisphere temperate zone and the observed positive correlation between rainfall and susceptibility indicates that climate, rather than soil age, is the predominant factor that controls pedogenic susceptibility enhancement in loess soils.  相似文献   

Mackenzie delta: Environmental interactions and implications of development: proceedings of the workshop on the mackenzie delta, 17–18 October 1989, Saskatoon,Saskatchewan edited by P. Marsh and C. S. L. Ommanney,Symposium No. 4, National Hydrology Research Institute,Environment Canada, 1991. No. of pages: xiv + 195. ISBN 0-662-17686-3     

Barbara A. Kennedy 《地球表面变化过程与地形》1993,18(1):84-84

  相似文献   

Seismic structure of the upper mantle beneath the United States by three-dimensional inversion of body wave arrival times     

Barbara A. Romanowicz 《Geophysical Journal International》1979,57(2):479-506

Summary. Teleseismic P and S arrival times to North American stations are obtained from the ISC bulletins for the 10-yr period 1964–73, and relative travel-time delays are calculated with respect to standard tables. Station anomalies as well as variations of the delays with azimuth and epicentral distance from station are analysed, and the location of the velocity anomalies responsible for them is discussed. Inversion of the P delays to infer upper mantle velocity structure down to a depth of 700 km is obtained using three-dimensional blocks, as proposed by Aki, Christofferson & Husebye. Three layers can be resolved in this depth range. It is found that the heterogeneities responsible for the travel-time delays are primarily located in the first 250 km of the upper mantle, and that they correlate with surface features. Significant heterogeneities subsist to depths of at least 700 km and their broad scale pattern also correlates with the surface features: in the third layer (500 to 700 km depth) there is an increase of velocity from the West to the East of the United States, while the second layer (250 to 450 km depth) exhibits a reversed pattern. A tentative interpretation of these deeper anomalies is made, as being due mainly to topography of the major upper mantle discontinuities, near 400 and 650 km depth.  相似文献   

Isotopic and molecular distributions of biochemicals from fresh and buried <Emphasis Type="Italic">Rhizophora mangle</Emphasis> leaves†     

Barbara?J?SmallwoodEmail author  Matthew?J?Wooller  Myrna?E?Jacobson  Marilyn?L?Fogel 《Geochemical transactions》2003,4(1):38

Rhizophora mangle L. (red mangrove) is the dominant species of mangrove in the Americas. At Twin Cays, Belize (BZ) red mangroves are present in a variety of stand structures (tall >5 m in height, transition ~2–4 m and dwarf ~1–1.5 m). These height differences are coupled with very different stable carbon and nitrogen isotopic values[1] (mean tall δ ¹³C = -28.3‰, δ ¹⁵N = 0‰; mean tall δ ¹³C = -25.3‰, δ ¹⁵N = -10‰). To determine the utility of using these distinct isotopic compositions as 'biomarkers' for paleoenvironmental reconstruction of mangrove ecosystems and nutrient availability, we investigated the distribution and isotopic (δ ¹³C and δ ¹⁵N) composition of different biochemical fractions (water soluble compounds, free lipids, acid hydrolysable compounds, individual amino acids, and the residual un-extractable compounds) in fresh and preserved red mangrove leaves from dwarf and tall trees. The distribution of biochemicals are similar in dwarf and tall red mangrove leaves, suggesting that, regardless of stand structure, red mangroves use nutrients for biosynthesis and metabolism in a similar manner. However, the δ ¹³C and δ ¹⁵N of the bulk leaf, the biochemical fractions, and seven amino acids can be used to distinguish dwarf and tall trees at Twin Cays, BZ. The data support the theory that the fractionation of carbon and nitrogen occurs prior to or during uptake in dwarf and tall red mangrove trees. Stable carbon and nitrogen isotopes could, therefore, be powerful tools for predicting levels of nutrient limitation at Twin Cays. The δ ¹³C and δ ¹⁵N of biochemical fractions within preserved leaves, reflect sedimentary cycling and nitrogen immobilization. The δ ¹⁵N of the immobilized fraction reveals the overlying stand structure at the time of leaf deposition. The isotopic composition of preserved mangrove leaves could yield significant information about changes in ecosystem dynamics, nutrient limitation and past stand structure in mangrove paleoecosystems.  相似文献   

Towards 3D map generation from digital aerial images     

Lukas Zebedin  Andreas Klaus  Barbara Gruber-Geymayer  Konrad Karner 《ISPRS Journal of Photogrammetry and Remote Sensing》2006,60(6):413

This paper describes the fusion of information extracted from multispectral digital aerial images for highly automatic 3D map generation. The proposed approach integrates spectral classification and 3D reconstruction techniques. The multispectral digital aerial images consist of a high resolution panchromatic channel as well as lower resolution RGB and near infrared (NIR) channels and form the basis for information extraction.Our land use classification is a 2-step approach that uses RGB and NIR images for an initial classification and the panchromatic images as well as a digital surface model (DSM) for a refined classification. The DSM is generated from the high resolution panchromatic images of a specific photo mission. Based on the aerial triangulation using area and feature-based points of interest the algorithms are able to generate a dense DSM by a dense image matching procedure. Afterwards a true ortho image for classification, panchromatic or color input images can be computed.In a last step specific layers for buildings and vegetation are generated and the classification is updated.  相似文献   

Turbulent fluxes from Helipod flights above quasi-homogeneous patches within the LITFASS area     

Jens Bange  Thomas Spieß  Marcus Herold  Frank Beyrich  Barbara Hennemuth 《Boundary-Layer Meteorology》2006,121(1):127-151

Turbulent fluxes of sensible and latent heat were measured with the helicopter-borne turbulence probe Helipod over a heterogeneous landscape around the Meteorological Observatory Lindenberg during the STINHO-2 and LITFASS-2003 field experiments. Besides the determination of area-averaged heat fluxes, the analysis focused on different aspects of the response of the turbulent structure of the convective boundary layer (CBL) on the surface heterogeneity. A special flight pattern was designed to study flux profiles both over quasi-homogeneous sub-areas of the study region (representing the major land use types—forest, farmland, water) and over a typical mixture of the different surfaces. Significant differences were found between the heat fluxes over the individual surfaces along flight legs at about 80 m above ground level, in agreement with large-aperture scintillometer measurements. This flux separation was still present during some flights at levels near the middle of the CBL. Different scales for the blending height and horizontal heterogeneity were calculated, but none of them could be identified as a reliable indicator of the mixing state of the lower CBL. With the exception of the flights over water, the latent heat flux measurements generally showed a larger statistical error when compared with the sensible heat flux. Correlation coefficients a nd integral length scales were used to characterise the interplay between the vertical transport of sensible and latent heat, which was found to vary between ‘fairly correlated’ and ‘decoupled’, also depending on the soil moisture conditions.  相似文献   

Towards urbanisation of the non-hydrostatic numerical weather prediction model Lokalmodell (LM)     

Lina Neunhäuserer  Barbara Fay  Matthias Raschendorfer 《Boundary-Layer Meteorology》2007,124(1):81-97

Numerical weather prediction models are increasingly employed for providing meteorological data for urban air quality applications. Model resolution, physiographic parameters and surface-layer parameterisations need to be adapted to the requirements of the urban boundary layer. The Lokalmodell of the German Weather Service was triple-nested down to a horizontal grid resolution of 1.1 km, urbanised physiographic parameters were implemented, and an additional anthropogenic heat source was introduced. Results of a sensitivity study for a spring dust episode in Helsinki show a clear urbanisation effect of these measures on temperature, humidity and the partitioning of surface fluxes, leading to an increased Bowen ratio and heat storage and an urban heat island effect.  相似文献