Population growth and its spatial distribution as factors in the deforestation of Nang Rong, Thailand     

Barbara Entwisle  Ronald R. Rindfuss  Philip H. Page 《Geoforum》2008,39(2):879-897

Frontiers constitute a major source of global land cover change hot spots, with forests and grasslands being converted into agricultural uses. As such, frontiers provide an opportunity to see how people manipulate the land and their lives in the context of social, cultural and environmental constraints. This paper examines frontier settlement and land cover change in Nang Rong district, Northeast Thailand for the last half century. It uses a Cellular Automata (CA) model to explore the land cover consequences of alternative patterns of settlement in a setting where people establish dwelling units in nucleated villages and work agricultural plots that surround villages. Forested land around the center of a village is converted into agricultural uses in an inverse relationship to the distance from the village center, but frequently modified by biophysical conditions. Land at the center of the village may be reforested after the village is established as a source of shade as well as fruit and other products. Model variation in land cover change is more sensitive to the spatial reach of village households than their temporal reach, suggesting the important role that technology plays in how villagers travel to their fields (walking versus motorized transit).  相似文献   

The evolution and geography of Jurassic ammonoids     

Kevin N. Page 《Proceedings of the Geologists' Association. Geologists' Association》2008,119(1):35-57

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A carbon and oxygen isotopic analysis of molluscan faunas from the Callovian-Oxfordian boundary at Redcliff Point, Weymouth, Dorset: implications for belemnite behaviour     

Gregory D. Price  Kevin N. Page 《Proceedings of the Geologists' Association. Geologists' Association》2008,119(2):153-160

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Compound paleovalley fills in the Lower Pennsylvanian New River Formation, West Virginia, USA     

Jesse T. Korus  Erik P. Kvale  Kenneth A. Eriksson  R.M. Joeckel   《Sedimentary Geology》2008,208(1-2):15-26

Ancient paleovalley fills are typically interpreted in the rock record using over-generalized models without carefully considering modern analogs, especially in light of recent discoveries. It is now known that many Quaternary paleovalleys are compound in origin, exhibit considerable stratigraphic complexity, contain multiple incisions, and can be orders of magnitude larger than their putative ancient counterparts. Compound paleovalley fills in the Lower Pennsylvanian New River Formation (NRF) are directly comparable to these Quaternary analogs, stimulating a paradigm shift in the interpretation of ancient paleovalleys. In the NRF, multiple laterally- and vertically-juxtaposed fill successions, separated by incision surfaces, record high-frequency fluvial responses to external controls within lower-order sequences. Lowstand incision and sediment bypass, as predicted in sequence stratigraphy, is largely discounted by the available evidence and the definition of regional sequence boundaries is not straightforward. The identification of genetic sequences may be the most effective approach to understanding the NRF and, by inference, many other ancient paleovalleys. Results from this study of the NRF promote a revised model for ancient paleovalleys that incorporates: 1) the pre-eminence of compound architecture, 2) periodic episodes of incision and subaerial exposure occurring in response to high-frequency changes in climate or relative sea level, 3) fluvial downcutting as the primary cause of paleovalley incision, although some sediments are still preserved in a net-erosional regime, and 4) composite, time-transgressive sequence boundaries that may be difficult or impossible to correlate regionally.  相似文献   

Characterization of periodic variations in the GPS satellite clocks   总被引：11，自引：7，他引：4  

Kenneth L. Senior  Jim R. Ray  Ronald L. Beard 《GPS Solutions》2008,12(3):211-225

The clock products of the International Global Navigation Satellite Systems (GNSS) Service (IGS) are used to characterize the timing performance of the GPS satellites. Using 5-min and 30-s observational samples and focusing only on the sub-daily regime, approximate power-law stochastic processes are found. The Block IIA Rb and Cs clocks obey predominantly random walk phase (or white frequency) noise processes. The Rb clocks are up to nearly an order of magnitude more stable and show a flicker phase noise component over intervals shorter than about 100 s. Due to the onboard Time Keeping System in the newer Block IIR and IIR-M satellites, their Rb clocks behave in a more complex way: as an apparent random walk phase process up to about 100 s and then changing to flicker phase up to a few thousand seconds. Superposed on this random background, periodic signals have been detected in all clock types at four harmonic frequencies, n × (2.0029 ± 0.0005) cycles per day (24 h coordinated universal time or UTC), for n = 1, 2, 3, and 4. The equivalent fundamental period is 11.9826 ± 0.0030 h, which surprisingly differs from the reported mean GPS orbital period of 11.9659 ± 0.0007 h by 60 ± 11 s. We cannot account for this apparent discrepancy but note that a clear relationship between the periodic signals and the orbital dynamics is evidenced for some satellites by modulations of the spectral amplitudes with eclipse season. All four harmonics are much smaller for the IIR and IIR-M satellites than for the older blocks. Awareness of the periodic variations can be used to improve the clock modeling, including for interpolation of tabulated IGS products for higher-rate GPS positioning and for predictions in real-time applications. This is especially true for high-accuracy uses, but could also benefit the standard GPS operational products. The observed stochastic properties of each satellite clock type are used to estimate the growth of interpolation and prediction errors with time interval.  相似文献   

Oyster Predation by Black Drum Varies Spatially and Seasonally     

Kenneth M. Brown  Gerald J. George  Gary W. Peterson  Bruce A. Thompson  James H. CowanJr. 《Estuaries and Coasts》2008,31(3):597-604

Oyster reefs (Crassostrea virginica) supply important ecosystem services to estuarine habitats in the northern Gulf of Mexico, but little is known of the role of fish predators in controlling their structure or areal cover on soft sediments. At two sites and during fall and spring, we employed gill nets and trot lines to remove black drum (Pogonias cromis) from experimental reefs, and assessed oyster survival in comparison to control reefs. Numbers and biomass of black drum removed from reefs varied seasonally, among sites, and among removal methods. In the fall, black drum were rare at one site and abundant at the other, but did not significantly lower oyster survival on control reefs at either site. In the spring, black drum were common at both sites, and significantly lowered oyster survival on control reefs. Oysters and epizoic hooked mussels comprised roughly a third of the fishes’ diet, and oyster mortality was closely related to the percentage of drum feeding on oysters. There was little evidence of mortality from other predators of seed oysters like stone crabs or Southern oyster drills, and a repeated measures analysis of variance indicated black drum biomass was significantly depressed on experimental reefs during the experiments. Black drum thus appear to be potentially important predators on oyster reefs, but more work needs to be done on what factors explain the temporal and spatial variation in their abundance and oyster consumption.  相似文献   

Quantitative Analysis of H₂OComa Images Using a Multiscale MHD Model with Detailed Ion Chemistry     

Roman M. Häberli  Michael R. Combi  Tamas I. Gombosi  Darren L. De Zeeuw  Kenneth G. Powell 《Icarus》1997,130(2):373-386

The observation of ions created by ionization of cometary gas, either by ground-based observations or byin situmeasurements can give us useful information about the gas production and composition of comets. However, due to the interaction of ions with the magnetized solar wind and their high chemical reactivity, it is not possible to relate measured ion densities (or column densities) directly to the parent gas densities. In order to quantitatively analyze measured ion abundances in cometary comae it is necessary to understand their dynamics and chemistry. We have developed a detailed ion–chemical network of cometary atmospheres. We include production of ions by photo- and electron impact-ionization of a background neutral atmosphere, charge exchange of solar wind ions with cometary atoms/molecules, reactions between ions and molecules, and dissociative recombination of molecular ions with thermal electrons. By combining the ion–chemical network with the three-dimensional plasma flow as computed by a new fully three-dimensional MHD model of cometary plasma environments (Gombosiet al.1996) we are able to compute the density of the major cometary ions everywhere in the coma. The input parameters for our model are the solar wind conditions (density, speed, temperature, magnetic field) and the composition and production rate of the gas. We applied our model to Comet P/Halley in early March 1986, for which the input parameters are reasonably well known. We compare the resulting column density of H₂O⁺with ground-based observations of H₂O⁺from DiSantiet al.(1990). The results of our model are in good agreement with both the spatial distribution and the absolute abundance of H₂O⁺and with their variations with the changing overall water production rate between two days. The results are encouraging that it will be possible to obtain production rates of neutral cometary constituents from observations of their ion products.  相似文献   

Compact,variable sources on the Sun at 2 centimeter wavelength     

Robert F. Willson  Kenneth R. Lang 《Solar physics》1988,114(1):93-104

Very Large Array (VLA) observations of compact transient sources on the Sun at 2 cm wavelength are presented. These sources have angular sizes of 5–25, brightness temperatures of T _B 1–3 × 10⁵ K, and lifetimes ranging between a few minutes to several hours. The emission originates in regions of diffuse plage and quiet Sun, where the photospheric magnetic fields are relatively weak (H 100 G). In some cases the 2 cm radiation may be explained as the thermal bremsstrahlung of a dense (N _e 10¹⁰ cm^-3) plasma in the transition region. For other sources, the relatively high circular polarization ( _c 40–50 %) suggests a nonthermal emission mechanism, such as the gyrosynchrotron radiation of mildly relativistic electron with a power-law spectrum.  相似文献   

Denudation rates of a subequatorial orogenic belt based on estimates of sediment yields: evidence from the Paleozoic Appalachian Basin,USA          下载免费PDF全文

Kenneth A. Eriksson  Brian W. Romans 《Basin Research》2017,29(1):2-15

The Upper Mississippian (ca. 325 Ma) Pride Shale and Glady Fork Member in the Central Appalachian Basin comprise an upward‐coarsening, ca. 60‐m‐thick succession of prodeltaic‐delta front, interlaminated fine‐grained sandstones and mudstones gradational upwards into mouth‐bar and distributary‐channel sandstones. Analysis of laminae bundling in the Pride Shale reveals a hierarchy of tidal cycles (semi‐diurnal, fortnightly neap‐spring) and a distinct annual cyclicity resulting from seasonal fluvial discharge. These tidal rhythmites thus represent high‐resolution chronometers that can be used in basin analysis. Annual cycles average 10 cm in thickness, thus the bulk of the Pride Shale‐Glady Fork Member in any one vertical section is estimated to have accumulated in ca. 600 years. Progradational clinoforms are assumed to have had dips of 0.3–3° with a median dip of 1.7°; the latter infilled a NE‐SW oriented foreland trough up to 300 km long by 50 km wide in the relatively short time period of 90 kyr. The total volume of sediment in the Pride basin is ca. 900 km³ which, for an average sediment density of 2700 kg m^?3, equates to a total mass of ca. 2.4 × 10⁶ Mt. Thus, mass sediment load can be estimated as 27 Mt yr^?1. For a drainage basin area of 89 000 km², based on the scale of architectural channel elements and cross‐set thicknesses in the incised‐valley‐fill deposits of the underlying Princeton Formation, suspended sediment yields are estimated at ca. 310 t km^?2 yr^?1 equating to a mechanical denudation rate of ca. 0.116 mm yr^?1. Calculated sediment yields and inferred denudation rates are comparable to modern rivers such as the Po and Fly and are compatible with a provenance of significant relief and a climate characterized by seasonal, monsoonal discharge. Inferred denudation rates also are consistent with average denudation rates for the Inner Piedmont Terrane of the Appalachians based on flexural modelling. The integration of stratigraphic architectural analysis with a novel chronometric application highlights the utility of sedimentary archives as a record of Earth surface dynamics.  相似文献   

Nitrogen Retention in Salt Marsh Systems Across Nutrient-Enrichment,Elevation, and Precipitation Regimes: a Multiple-Stressor Experiment     

Autumn Oczkowski  Cathleen Wigand  Alana Hanson  Erin Markham  Kenneth M. Miller  Roxanne Johnson 《Estuaries and Coasts》2016,39(1):68-81

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