SOIL CATENAS,TROPICAL DEFORESTATION,AND ANCIENT AND CONTEMPORARY SOIL EROSION IN THE PETéN,GUATEMALA     

Timothy Beach 《自然地理学》2013,34(5):378-405

The Petexbatún region of Guatemala's Petén tropical forest lowlands has been largely deforested since the early 1980s as landless peasants have turned even national parks into milpas. The soils of these tropical forest, karst landscapes are known mainly from small-scale maps and archaeological investigations. This paper investigates soil catenas and natural slope processes from the region's last virgin tropical forests and compares these with catenas and processes on slashed and burned slopes. Soils were sampled and analyzed for physical, fertility, and taxonomic measures. This primary forest toposequence forms a typical downward translocation catena, with the thickest soils (>200 cm) in depressions, the thinnest soils on shoulder slopes, and moderately thin soils on the crests and backslopes. Most crest and sloping soils are Lithic Rendolls and contain simple O-A-C horizon sequences, and most soils have little sign of either eluviation or illuviation. A few deeper soils on the slopes are Vertic Rendolls and have moderate slick-ensides, whereas footslopes and seasonally drained depressions are well developed Vertisols. The primary forest soil catena was compared with two slash-and-burn milpas, one with comparable slopes and one with steeper slopes. The comparably sloped milpa had soil thicknesses that were 7.9 to 13.8 cm thinner, truncated horizons, and physical evidence of erosion. The steeper milpa had soil thicknesses that were 11.1 to 18.2 cm thinner and also had truncated horizons and physical evidence of erosion. Where ancient Maya terraces (Late Classic, ca. 550 to 830 A.D.) are present in the slash-and-burn landscape, an average of 25 to 46.5 cm soil remain upslope and 9.3 to 16.1 cm remain downslope. The findings show high soil truncation rates and erosion to bedrock in a decade in one place, though ancient Maya terraces are still holding 2.7 to 3.6 times more soil than the surrounding hillslopes. Despite high modern erosion, lake sediments show surprisingly low soil losses in the Maya Late Classic during the period of the most intensive land use. In contrast, studies at Copán, Honduras and in the northern Petén, where terrace evidence is scant, show high rates of erosion during this period. [Key words: soil catenas, tropical deforestation, milpa, soil erosion, Maya Lowlands.]  相似文献   

METEOROLOGICAL ANALYSIS OF DAILY MAXIMUM GROUND-LEVEL OZONE FOR THE NIAGARA REGION     

Anthony B. Shaw  Timothy J. Griffis 《自然地理学》2013,34(4):371-399

Ground-level ozone has become a problem of major concern in urban airsheds in Canada, owing to its adverse effects on humans and crops. As a secondary pollutant, its formation is dependent on the presence of certain precursor gases in conjunction with appropriate meteorological conditions. Several studies have examined the relationship between maximum concentrations and key meteorological variables at the regional scale during episodic conditions. This study sought to understand this relationship at the local scale using surface and upper-air meteorological data for the Niagara Region. In the methodological approach, factor analysis and linear regression methods were used to determine the best combination of variables that would explain the highest percentage of variance in daily maximum ground-level ozone associated with different event categories. Each event category had a combination of unique meteorological characteristics. Factor analysis yielded seven factors that together constituted 20 of the original 59 variables in the data set. In the application of a series of regression analyses, the thermal factors of the lower atmosphere emerged as the most important variables, followed by variables related to persistence and advection characteristics. The daily maximum temperature was the single most important variable and accounted for the largest percentage of the variance explained, whereas the persistence factor was of secondary importance. The overall results suggest that a small number of surface variables based on the local meteorology of the Niagara Region can be used to estimate daily maximum ground-level ozone.  相似文献   

A disequilibrium compaction model constrained by seismic data and application to overpressure generation in The Eastern Black Sea Basin     

Héctor Marín‐Moreno  Timothy A. Minshull  Rosemary A. Edwards 《Basin Research》2013,25(3):331-347

Locating and quantifying overpressures are essential to understand basin evolution and hydrocarbon migration in deep basins and thickly sedimented continental margins. Overpressures influence sediment cohesion and hence fault slip in seismically active areas or failure on steep slopes, and may drive catastrophic fluid expulsion. They also represent a significant drilling hazard. Here, we present a method to calculate the pore pressure due to disequilibrium compaction. Our method provides an estimate of the compaction factor, surface porosity and sedimentation rate of each layer in a sediment column using a decompaction model and the constraints imposed by seismic data and geological observations. For a range of surface porosities, an ad hoc iterative equation determines the compaction factor that gives a calculated layer thickness that matches the observed thickness within a tolerance. The surface porosity and compaction factor are then used to obtain a density profile and a corresponding estimate of P‐wave velocity (V_p). The selected parameters are those that give a good match with both the observed and calculated layer thicknesses and V_p profiles. We apply our method to the centre of the Eastern Black Sea Basin (EBSB), where overpressures have been linked to a low‐velocity zone (LVZ) at ca. 5500–8500 m depth. These overpressures were generated by the relatively high sedimentation rate of ca. 0.28 m ka^?1 of the low permeability organic‐rich Maikop formation at 33.9–20.5 Ma and an even higher sedimentation rate of ca. 0.85 m ka^?1 at 13–11 Ma. We estimate a maximum pore pressure of ca. 138 MPa at ca. 8285 m depth, associated with a ratio of overpressure to vertical effective stress in hydrostatic conditions () of ca. 0.7. These values are lower than those presented in a previous study for the same area.  相似文献   

Direct displacement-based seismic design of steel eccentrically braced frame structures     

Timothy John Sullivan 《Bulletin of Earthquake Engineering》2013,11(6):2197-2231

This paper details a direct displacement-based design procedure for steel eccentrically braced frame (EBF) structures and gauges its performance by examining the non-linear dynamic response of a series of case study EBF structures designed using the procedure. Analytical expressions are developed for the storey drift at yield and for the storey drift capacity of EBFs, recognising that in addition to link beam deformations, the brace and column axial deformations can provide important contributions to storey drift components. Case study design results indicate that the ductility capacity of EBF systems will tend to be relatively low, despite the large local ductility capacity offered by well detailed links. In addition, it is found that while the ductility capacity of EBF systems will tend to reduce with height, this is not necessarily negative for seismic performance since the displacement capacity for taller EBF systems will tend to be large. To gauge the performance of the proposed DBD methodology, analytical models of the case study design solutions are subject to non-linear time-history analyses with a set of spectrum-compatible accelerograms. The average displacements and drifts obtained from the NLTH analyses are shown to align well with design values, confirming that the new methodology could provide an effective tool for the seismic design of EBF systems.  相似文献   

Sectional response of T-shaped RC walls   总被引：1，自引：1，他引：0  

Eleni Smyrou  Timothy Sullivan  Nigel Priestley  Michele Calvi 《Bulletin of Earthquake Engineering》2013,11(4):999-1019

Deformation quantities such as strain, curvature and displacement are of paramount importance in seismic design within a performance-based procedure that aims to control the structural response at predefined levels of inelastic action. Given the importance of curvature expressions independent of strength for the design process, and for the particular case of T-shaped walls, the curvature trends at yield, serviceability and ultimate limit state are determined in graphical and analytical form. The comprehensive set of equations proposed in this work are strength independent and allow the reliable computation of limit-state curvatures, essential in a displacement-based design approach, and thus the realistic estimation of appropriate ductility factors in the design of T-shaped walls. Furthermore, results regarding the section properties of T-shaped walls, such as the elastic stiffness and the moment capacity for opposite directions of loading, offer additional information on T-shaped walls.  相似文献   

Inferring network paths from point observations     

Timothy C. Matisziw  Emre Demir 《International journal of geographical information science》2013,27(10):1979-1996

The use of geographically referenced point data, such as that obtained from global positioning systems (GPS), is rapidly increasing. However, due to error and uncertainty inherent in most geographic datasets, the ability to accurately associate these point locations with other layers of geographic data is still a challenge. One difficulty in particular is how to associate spatially and temporally referenced point-based observations of a network activity with a network topology such that a continuous network path can be best inferred. In this article, an optimization method for inferring a network path from a temporal sequence of point observations of location is presented. An application to GPS data is provided to highlight various characteristics of the proposed modeling approach relative to several other available techniques.  相似文献   

Emission-line diagnostics of low-metallicity active galactic nuclei     

Brent A. Groves  Timothy M. Heckman  Guinevere Kauffmann 《Monthly notices of the Royal Astronomical Society》2006,371(4):1559-1569

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First Astrobiology Science Conference NASA Ames Research Center Moffett Field,California 2000 April 3–5     

Timothy A. Kral 《Meteoritics & planetary science》2000,35(3):444-444

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Ozone abundance on Mars from infrared heterodyne spectra: I. Acquisition, retrieval, and anticorrelation with water vapor     

Kelly Fast  Theodor Kostiuk  Fred Espenak  John Annen  David Buhl  Michael F. A'Hearn  Timothy A. Livengood  Guido Sonnabend  Frank Schmülling 《Icarus》2006,181(2):419-431

Observations of ozone on Mars were made using the Goddard Space Flight Center's Infrared Heterodyne Spectrometer and Heterodyne Instrument for Planetary Wind and Composition at the NASA Infrared Telescope Facility. Ozone is an important observable tracer of martian photochemistry. Infrared heterodyne spectroscopy with spectral resolution ?¹⁰⁶ is the only technique that directly measures ozone in the martian atmosphere from the surface of the Earth. Ozone column abundances down to the martian surface were acquired in seven data sets taken between 1988 and 2003 at various orbital positions (LS=40°, 74°, 102°, 115°, 202°, 208°, 291°). Ozone abundances are compared with those retrieved using ultraviolet techniques, showing good agreement. Odd hydrogen (HO_X) chemistry predicts anticorrelation of ozone and water vapor abundances. Retrieved ozone abundances consistently show anticorrelation with corresponding water vapor abundances, providing strong confirmation of odd hydrogen activity. Deviation from strict anticorrelation between the observed total column densities of ozone and water vapor suggests that constituent vertical distribution is an additional, significant factor.  相似文献   

Ozone abundance on Mars from infrared heterodyne spectra: II. Validating photochemical models     

Kelly Fast  Theodor Kostiuk  Tilak Hewagama  Timothy A. Livengood  Sebastien Lebonnois 《Icarus》2006,183(2):396-402

Ozone is an important observable tracer of martian photochemistry, including odd hydrogen (HO_x) species important to the chemistry and stability of the martian atmosphere. Infrared heterodyne spectroscopy with spectral resolution ?¹⁰⁶ provides the only ground-based direct access to ozone absorption features in the martian atmosphere. Ozone abundances were measured with the Goddard Infrared Heterodyne Spectrometer and the Heterodyne Instrument for Planetary Wind and Composition at the NASA Infrared Telescope Facility on Mauna Kea, Hawai'i. Retrieved total ozone column abundances from various latitudes and orbital positions (LS=40°, 74°, 102°, 115°, 202°, 208°, 291°) are compared to those predicted by the first three-dimensional gas phase photochemical model of the martian atmosphere [Lefèvre, F., Lebonnois, S., Montmessin, F., Forget, F., 2004. J. Geophys. Res. 109, doi:10.1029/2004JE002268. E07004]. Observed and modeled ozone abundances show good agreement at all latitudes at perihelion orbital positions (LS=202°, 208°, 291°). Observed low-latitude ozone abundances are significantly higher than those predicted by the model at aphelion orbital positions (LS=40°, 74°, 115°). Heterogeneous loss of odd hydrogen onto water ice cloud particles would explain the discrepancy, as clouds are observed at low latitudes around aphelion on Mars.  相似文献