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  • Copernicus  (2)
  • Geological Society of London  (2)
  • 2015-2019  (4)
  • 1
    Publication Date: 2017-02-23
    Description: Establishing the source(s) of sedimentary material is critical to many geological applications, but is complicated by the ability of some minerals to be recycled. To test the relative utility of current proxies for determining a unique provenance, new samples have been collected from the Namurian Millstone Grit Group of Yorkshire, England. Two K-feldspar 206 Pb/ 204 Pb isotope populations between 12.5 and 15.5 and c. 18.4 are consistent with Archaean–Proterozoic basement and Caledonian granites, respectively. Zircon U–Pb age populations at c. 2700, 2000 – 1000 and 430 Ma reflect a mixture of Archaean basement, overlying Proterozoic sediments and intrusive Caledonian granites, and Hf values in zircons of all ages indicate crystallization from reworked crust. Garnet major element compositions are relatively rich in Fe and low in Ca, indicative of derivation from a granulitic or charnockitic source. Rutile Cr/Nb ratios indicate that source rocks were dominantly metapelitic, and Zr-in-rutile thermometry records two populations representing lower ( c. 650°C) and higher ( c. 800°C) metamorphic grade material. Combining these results with published monazite and muscovite data suggests overall derivation from the Greenland Caledonides, with additional contributions from NE Scotland and western Norway, highlighting the power of multi-proxy provenance work, especially in tectonically and geologically complicated regions. Supplementary material : Sample details, full analytical methods, data tables and references for compilation figures in the text are available at https://doi.org/10.6084/m9.figshare.c.3515457 .
    Print ISSN: 0016-7649
    Topics: Geosciences
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  • 2
    Publication Date: 2017-01-07
    Description: Establishing the source(s) of sedimentary material is critical to many geological applications, but is complicated by the ability of some minerals to be recycled. To test the relative utility of current proxies for determining a unique provenance, new samples have been collected from the Namurian Millstone Grit Group of Yorkshire, England. Two K-feldspar 206 Pb/ 204 Pb isotope populations between 12.5 and 15.5 and c. 18.4 are consistent with Archaean–Proterozoic basement and Caledonian granites, respectively. Zircon U–Pb age populations at c. 2700, 2000 – 1000 and 430 Ma reflect a mixture of Archaean basement, overlying Proterozoic sediments and intrusive Caledonian granites, and Hf values in zircons of all ages indicate crystallization from reworked crust. Garnet major element compositions are relatively rich in Fe and low in Ca, indicative of derivation from a granulitic or charnockitic source. Rutile Cr/Nb ratios indicate that source rocks were dominantly metapelitic, and Zr-in-rutile thermometry records two populations representing lower ( c. 650°C) and higher ( c. 800°C) metamorphic grade material. Combining these results with published monazite and muscovite data suggests overall derivation from the Greenland Caledonides, with additional contributions from NE Scotland and western Norway, highlighting the power of multi-proxy provenance work, especially in tectonically and geologically complicated regions. Supplementary material : Sample details, full analytical methods, data tables and references for compilation figures in the text are available at https://doi.org/10.6084/m9.figshare.c.3515457 .
    Print ISSN: 0016-7649
    Topics: Geosciences
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  • 3
    Publication Date: 2015-12-03
    Description: The complex three-dimensional (3-D) structure of tropical forests generates a diversity of light environments for canopy and understory trees. Understanding diurnal and seasonal changes in light availability is critical for interpreting measurements of net ecosystem exchange and improving ecosystem models. Here, we used the Discrete Anisotropic Radiative Transfer (DART) model to simulate leaf absorption of photosynthetically active radiation (lAPAR) for an Amazon forest. The 3-D model scene was developed from airborne lidar data, and local measurements of leaf reflectance, aerosols, and PAR were used to model lAPAR under direct and diffuse illumination conditions. Simulated lAPAR under clear sky and cloudy conditions was corrected for light saturation effects to estimate light utilization, the fraction of lAPAR available for photosynthesis. Although the fraction of incoming PAR absorbed by leaves was consistent throughout the year (0.80–0.82), light utilization varied seasonally (0.67–0.74), with minimum values during the Amazon dry season. Shadowing and light saturation effects moderated potential gains in forest productivity from increasing PAR during dry season months when the diffuse fraction from clouds and aerosols was low. Comparisons between DART and other models highlighted the role of 3-D forest structure to account for seasonal changes in light utilization. Our findings highlight how directional illumination and forest 3-D structure combine to influence diurnal and seasonal variability in light utilization, independent of further changes in leaf area, leaf age, or environmental controls on canopy photosynthesis. Changing illumination geometry constitutes an alternative biophysical explanation for observed seasonality in Amazon forest productivity without changes in canopy phenology.
    Print ISSN: 1810-6277
    Electronic ISSN: 1810-6285
    Topics: Biology , Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 4
    Publication Date: 2015-02-13
    Description: Vegetation fires are a major driver of ecosystem dynamics and greenhouse gas emissions. Anticipating potential changes in fire activity and their impacts relies first on a realistic model of fire activity (e.g., fire incidence and interannual variability) and second on a model accounting for fire impacts (e.g., mortality and emissions). In this paper, we focus on our understanding of fire activity and describe a new fire model, HESFIRE (Human–Earth System FIRE), which integrates the influence of weather, vegetation characteristics, and human activities on fires in a stand-alone framework. It was developed with a particular emphasis on allowing fires to spread over consecutive days given their major contribution to burned areas in many ecosystems. A subset of the model parameters was calibrated through an optimization procedure using observation data to enhance our knowledge of regional drivers of fire activity and improve the performance of the model on a global scale. Modeled fire activity showed reasonable agreement with observations of burned area, fire seasonality, and interannual variability in many regions, including for spatial and temporal domains not included in the optimization procedure. Significant discrepancies are investigated, most notably regarding fires in boreal regions and in xeric ecosystems and also fire size distribution. The sensitivity of fire activity to model parameters is analyzed to explore the dominance of specific drivers across regions and ecosystems. The characteristics of HESFIRE and the outcome of its evaluation provide insights into the influence of anthropogenic activities and weather, and their interactions, on fire activity.
    Print ISSN: 1726-4170
    Electronic ISSN: 1726-4189
    Topics: Biology , Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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