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71.
Michael R. Talbot Niels Bo Jensen Tine Lærdal Maria Letizia Filippi 《Journal of Paleolimnology》2006,35(3):467-489
Bulk organic matter (OM) in sediments that accumulated in Lakes Victoria, Albert and Tanganyika during the major transgression
that transformed these basins from a desiccated or lowstand condition at the time of the Last Glacial Maximum to highstand
with overflow in the terminal Pleistocene and early Holocene, show some striking compositional trends. Sediment OM content
(TOC) and Rock-Eval Hydrogen Index (HI) both rise markedly, reflecting increasing primary productivity during the transgression
and enhanced conditions for the preservation of labile OM, In Lake Victoria rising sedimentary sulphur content tracks the
change in HI. Over the same time interval in all three lakes, δ13COM shows a falling trend of 5–8‰ . Nitrogen isotopic and C/N trends are, in contrast, more variable. Relatively high δ15N values at an early stage in the transgression may reflect isotopic effects associated with ammonia loss from an alkaline
waterbody, while high δ15N later in the transgression was probably due to periods when stratification-related denitrification had a particularly strong
impact upon the lakes' N cycle. The observed changes are attributed to the influence of the landscape drowned during the transgression
upon the nutrient budget of the lake, a rise in river-supplied nutrients as the climate became wetter, and an increasing tendency
to stable chemical stratification as the water column deepened. In the case of δ13C, additional contributory factors were changes in the amount and isotopic composition of river-supplied dissolved inorganic
carbon (DIC) as runoff and soil–moisture turnover rates increased, and the replacement of a C4-grassland dominated flora by
mixed woodland-grassland and forest with a significant presence of C3 plants. Distinct geochemical changes are also apparent
in Lakes Victoria and Albert sediments at the end of the transgression which are consistent with a reduction in nutrient loading.
In the case of N this resulted in biological N fixation becoming a major part of the lakes' N cycle, Some of the observed
geochemical variations may be generally applicable as proxy indicators of major transgressions in large lakes.
IDEAL Contribution No. 152. 相似文献
72.
Niels Grobbe Joost van der Neut Evert Slob Kees Wapenaar Carlos Almagro Vidal Guy Drijkoningen 《Geophysical Prospecting》2016,64(2):361-391
Wavefield decomposition forms an important ingredient of various geophysical methods. An example of wavefield decomposition is the decomposition into upgoing and downgoing wavefields and simultaneous decomposition into different wave/field types. The multi‐component field decomposition scheme makes use of the recordings of different field quantities (such as particle velocity and pressure). In practice, different recordings can be obscured by different sensor characteristics, requiring calibration with an unknown calibration factor. Not all field quantities required for multi‐component field decomposition might be available, or they can suffer from different noise levels. The multi‐depth‐level decomposition approach makes use of field quantities recorded at multiple depth levels, e.g., two horizontal boreholes closely separated from each other, a combination of a single receiver array combined with free‐surface boundary conditions, or acquisition geometries with a high‐density of vertical boreholes. We theoretically describe the multi‐depth‐level decomposition approach in a unified form, showing that it can be applied to different kinds of fields in dissipative, inhomogeneous, anisotropic media, e.g., acoustic, electromagnetic, elastodynamic, poroelastic, and seismoelectric fields. We express the one‐way fields at one depth level in terms of the observed fields at multiple depth levels, using extrapolation operators that are dependent on the medium parameters between the two depth levels. Lateral invariance at the depth level of decomposition allows us to carry out the multi‐depth‐level decomposition in the horizontal wavenumber–frequency domain. We illustrate the multi‐depth‐level decomposition scheme using two synthetic elastodynamic examples. The first example uses particle velocity recordings at two depth levels, whereas the second example combines recordings at one depth level with the Dirichlet free‐surface boundary condition of zero traction. Comparison with multi‐component decomposed fields shows a perfect match in both amplitude and phase for both cases. The multi‐depth‐level decomposition scheme is fully customizable to the desired acquisition geometry. The decomposition problem is in principle an inverse problem. Notches may occur at certain frequencies, causing the multi‐depth‐level composition matrix to become uninvertible, requiring additional notch filters. We can add multi‐depth‐level free‐surface boundary conditions as extra equations to the multi‐component composition matrix, thereby overdetermining this inverse problem. The combined multi‐component–multi‐depth‐level decomposition on a land data set clearly shows improvements in the decomposition results, compared with the performance of the multi‐component decomposition scheme. 相似文献
73.
74.
Niels Lund 《Experimental Astronomy》2005,20(1-3):211-217
With ESA’s INTEGRAL mission gamma-ray astronomy has advanced to the point where major scientific advances must be expected from detailed studies of the many new point sources. The interest in developing focusing telescopes operating in the soft gamma-ray regime up to 1 MeV is therefore mounting rapidly. Telescopes based on Laue diffraction of gamma-rays from crystals appear as one promising route, although the practical difficulties of realizing a large scale Laue lens are certainly not small. In this paper I have attempted to develop an optimized lens design considering the size and mass constraints of a specific medium size launch vehicle. The introduction of the lens mass as a primary design driver has some surprising effects for the choice of material for the crystals and new tradeoff considerations are introduced. 相似文献
75.
Peter John Marcotullio Andrea Sarzynski Jochen Albrecht Niels Schulz Jake Garcia 《Climatic change》2013,121(4):621-634
The purpose of this paper is to describe global urban greenhouse gas emissions by region and sector, examine the distribution of emissions through the urban-to-rural gradient, and identify covariates of emission levels for our baseline year, 2000. We use multiple existing spatial databases to identify urban extent, greenhouse gas emissions (CO2, N2O, CH4 and SF6) and covariates of emissions in a “top-down” analysis. The results indicate that urban activities are significant sources of total greenhouse gas emissions (36.8 and 48.6 % of total). The urban energy sector accounts for between 41.5 and 66.3 % of total energy emissions. Significant differences exist in the urban share of greenhouse gas emissions between developed and developing countries as well as among source sectors for geographic regions. The 50 largest urban emitting areas account for 38.8 % of all urban greenhouse gas emissions. We find that greenhouse gas emissions are significantly associated with population size, density, growth rates, and per capita income. Finally, comparison of our results to “bottom-up” estimates suggest that this research’s data and techniques are best used at the regional and global scales. 相似文献
76.
We present a new, fast and versatile method, the lateral parameter correlation method, of invoking lateral smoothness in model sections of one-dimensional (1D) models. Modern, continuous electrical and electromagnetic methods are capable of recording very large data sets and except for a few cases, standard inversion methodology still relies on 1D models. In environments where the lateral rate of change of resistivity is small, 1D inversion can be justified but model sections of concatenated 1D models do not necessarily display the expected lateral smoothness.
The lateral parameter correlation method has three steps. First, all sounding data are inverted individually. Next, a laterally smooth version of each model parameter, one at a time, is found by solving a simple constrained inversion problem. Identity is postulated between the uncorrelated and correlated parameters and the equations are solved including a model covariance matrix. As a last step, all sounding data are inverted again to produce models that better fit the data, now subject to constraints by including the correlated parameter values as a priori values. Because the method separates the inversion from the correlation it is much faster than methods where the inversion and correlation are solved simultaneously, typically with a factor of 200–500.
Theoretical examples show that the method produces laterally smooth model sections where the main influence comes from the well-determined parameters in such a way that problems with equivalence and poor resolution are alleviated. A field example is presented, demonstrating the improved resolution obtained with the lateral parameter correlation method. The method is very flexible and is capable of coupling models from inversion of different data types and information from boreholes. 相似文献
The lateral parameter correlation method has three steps. First, all sounding data are inverted individually. Next, a laterally smooth version of each model parameter, one at a time, is found by solving a simple constrained inversion problem. Identity is postulated between the uncorrelated and correlated parameters and the equations are solved including a model covariance matrix. As a last step, all sounding data are inverted again to produce models that better fit the data, now subject to constraints by including the correlated parameter values as a priori values. Because the method separates the inversion from the correlation it is much faster than methods where the inversion and correlation are solved simultaneously, typically with a factor of 200–500.
Theoretical examples show that the method produces laterally smooth model sections where the main influence comes from the well-determined parameters in such a way that problems with equivalence and poor resolution are alleviated. A field example is presented, demonstrating the improved resolution obtained with the lateral parameter correlation method. The method is very flexible and is capable of coupling models from inversion of different data types and information from boreholes. 相似文献
77.
Niels Fold 《Geoforum》1998,29(4):393-412
Some observers of agro-industrial development in the Third World recognize the nation state as an important locus for mediation of conflicts between different social forces but fail to analyze the regulatory mechanisms and their effect on the dynamics of particular agro-industries. This paper proposes an analytical model to study agro-industrial regulation in a national framework. The purpose is to investigate the dynamics between changing forms of state regulation and particular industry governance system in the vegetable oil industry in Malaysia and Zimbabwe. State regulation and industry governance systems differ in a number of ways between the two countries. However, in each country there is a correspondence between the dominant form of state regulation and the industrial governance system. The study confronts some of the basic assumptions within the food regime approach and stress the need to incorporate the properties of the crops in the analysis of (agro-)industry governance systems. 相似文献
78.
Finn Danielsen Christopher E. Filardi Knud A. Jønsson Victor Kohaia Niels Krabbe Jan B. Kristensen Robert G. Moyle Patrick Pikacha Michael K. Poulsen Mikael K. Sørensen Charles Tatahu Joseph Waihuru Jon Fjeldså 《Singapore journal of tropical geography》2010,31(1):100-114
Anthropogenic habitat changes and the introduction of pigs, dogs, cats and rats have caused a catastrophic decline in the terrestrial biodiversity of Pacific archipelagos. At present, economic globalization and an increased demand for timber are promoting industrial logging and plantation expansion. Commercial logging can be sustainable but in practice it more often leads to land degradation, especially on small flat islands. On large and mountainous islands, however, more modest impacts can be expected as the narrowly endemic species tend to inhabit montane forests where logging is difficult. In this study we use ornithological data collected at different elevations to assess the extent to which the avifauna of Makira, a large mountainous island in Melanesia, will be affected by deforestation of the lowlands, most of which are under timber concessions. Our data suggest that a majority of the endemic bird species use lowland forest to some extent and that this may even apply to species hitherto associated with montane forest. If current commercial forestry programmes are continued, the forest habitat may be disturbed or lost over large parts of Makira, potentially undermining the natural resource base for the local subsistence economy, exacerbating climate change and threatening the integrity of one of the most important areas for biodiversity conservation on earth. Our study highlights the importance of understanding the habitat requirements of endemic species and the urgency of establishing and effectively managing community-based protected areas in suitable lowland forests of the Pacific. 相似文献
79.
80.
Niels Otto Jensen 《Boundary-Layer Meteorology》1978,15(1):95-108
Turbulence and mean flow variables under unstable conditions are examined with special emphasis on the consequences of roughness and surface elevation change. An interpolation formula for
w
2, between neutral and free convection, is shown to bring order to the data. The spectral distribution of vertical wind variance is found to be in good agreement with results over horizontal homogeneous terrain, both with respect to form and position. In particular, the length scale m corresponding to the maximum of nS
w(n) is unchanged. Another turbulent length scale, (k/)z, is found to be substantially reduced in the disturbed zone of the internal boundary layer. To a first approximation, the flow-acceleration effect on the non-dimensional wind shear can be separated from the diabatic effect. 相似文献