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81.
The saltmarsh sharp-tailed sparrow (Ammodramus caudacutus) and the morphologically similar seaside sparrow (Ammodramus maritimus) occur together in Atlantic coast salt marshes. Over 2 yr we examined food exploitation patterns to gain an understanding
of how these species coexist and to characterize the trophic conditions that allow the maintenance of different mating systems
within the same habitat. In the promiscuous saltmarsh sharp-tailed sparrow, only females feed the young, while the young of
the socially monogramous seaside sparrow are fed by both parents. To determine dietary patterns, we used a nonlethal method
(throat constriction) to obtain food samples from nestlings. Resource states were determined by sweep-netting the vegetation
and by taking cores from the marsh surface. Based on volume, the same 4 prey groups (flies, amphipods, grasshoppers, moths)
composed > 80% of each species diet. The main food of both sparrows was adult and juvenile (larval and pupal) soldier flies
(Stratiomyidae:Odontomyia microstomata). Measures of dietary overlap, based on resource states, indicated that nestling diets did not differ between species on
a seasonal basis or over 2-wk intervals. As the season progressed, each species tracked in parallel changes in resource abundance.
Diets did not diverge from what was expected if food was exploited randomly; diets frequently overlapped significantly more
than expected. Interspecific resource-use patterns were congruent over 2 yr. Based on the results of this study, and on an
evaluation of hypotheses based on life history information from previously published work, we conclude that food usually is
not limiting for these sparrows and that resource abundance has fostered behavioral and dietary opportunism. 相似文献
82.
A new approach is proposed for incorporating solid solution reactions into mass conservation equations describing reaction paths in both closed and open systems. The method is applicable to problems involving advective, dispersive, and diffusive transport in a porous medium. By representing the continuously variable solid solution composition with a discrete set of stoichiometric solids that span composition space, combined with a kinetic formulation of their rates of reaction, a self-determining spatial and temporal evolution of the solid solution concentration and composition is obtained. It is demonstrated that equilibrium of an aqueous solution with a stoichiometric solid derived from a solid solution corresponds to equilibrium of the solid solution itself if and only if equilibrium of the stoichiometric solid is stable. One advantage of this approach is that it is unnecessary to introduce any additional compositional variables to represent the solid solution. Discretization may be over the entire range of composition space, or over some subset depending on the system. A major consequence of the kinetic discrete-composition solid solution representation is that modeling solid solutions is similar to modeling pure mineral phases with the exception of a weighting factor applied to reaction rates of stoichiometric solids corresponding to a common solid solution. With this approach, precipitation leads to a discrete zonation of the solid solution that approximates the continuous variation in composition expected for the actual solid solution. The approach is demonstrated for a hypothetical ideal and non-ideal binary solid solution AxB1−xC for a reaction path formulation and reactive transport involving advection and diffusion. 相似文献
83.
84.
Sadrack Flix Toteu Joseph Penaye Etienne Deloule William Randall Van Schmus Rigobert Tchameni 《Journal of African Earth Sciences》2006,44(4-5):428
Ion microprobe U–Pb dating of zircons from Neoproterozoic volcano-sedimentary sequences in Cameroon north of the Congo craton is presented. For the Poli basin, the depositional age is constrained between 700–665 Ma; detrital sources comprise ca. 920, 830, 780 and 736 Ma magmatic zircons. In the Lom basin, the depositional age is constrained between 613 and 600 Ma, and detrital sources include Archaean to Palaeoproterozoic, late Mesoproterozoic to early Neoproterozoic (1100–950 Ma), and Neoproterozoic (735, 644 and 613 Ma) zircons. The Yaoundé Group is probably younger than 625 Ma, and detrital sources include Palaeoproterozoic and Neoproterozoic zircons. The depositional age of the Mahan metavolcano-sedimentary sequence is post-820 Ma, and detrital sources include late Mesoproterozoic (1070 Ma) and early Neoproterozoic volcanic rocks (824 Ma). The following conclusions can be made from these data. (1) The three basins evolved during the Pan-African event but are significantly different in age and tectonic setting; the Poli is a pre- to syn-collisional basin developed upon, or in the vicinity of young magmatic arcs; the Lom basin is post-collisional and intracontinental and developed on old crust; the tectono-metamorphic evolution of the Yaoundé Group resulted from rapid tectonic burial and subsequent collision between the Congo craton and the Adamawa–Yade block. (2) Late Mesoproterozoic to early Neoproterozoic inheritance reflects the presence of magmatic event(s) of this age in west–central Africa. 相似文献
85.
Pollen data from two sections from a coastal cliff on the western Yamal Peninsula (69°43.27′N, 66°48.80′E) document the environmental history during the Karginsky (Middle Weichselian) interstadial. Low pollen concentrations, high amounts of redeposited pollen, and relatively high presence of Artemisia pollen characterize sediments deposited at about 33,000 14C yr B.P. Grass-sedge plant associations with few other herbs occupied the area during the late Karginsky interstadial. Artemisia pollen may indicate rather xerophytic vegetation and disturbed soils in the area. The dominance of redeposited pollen reflects scarce (disturbed) vegetation cover and low pollen productivity. The climate was relatively cold and dry. Sediments dated to 32,400 14C yr B.P. contain fewer redeposited pollen and concentration of non-redeposited pollen is significantly higher. Pollen contents indicate the dominance of tundra-like grass-sedge vegetation and more humid conditions. Pollen records dated between 30,100 and 25,100 14C yr B.P. also reflect scarce tundra-like vegetation during this interval. The presence of Betula nana and Salix pollen may reflect limited presence of shrub communities. This suggests that the climate was somewhat warmer during the latter part of the interstadial. However, generally the pollen records show that harsh environmental conditions prevailed on the Yamal Peninsula during the Karginsky interstadial. 相似文献
86.
W. Roland Gehrels Katie Szkornik Jason R. Kirby William A. Marshall Jørn B.T. Pedersen 《Quaternary Research》2006,66(2):288-302
Cores and exposed cliff sections in salt marshes around Ho Bugt, a tidal embayment in the northernmost part of the Danish Wadden Sea, were subjected to 14C dating and litho- and biostratigraphical analyses to reconstruct paleoenvironmental changes and to establish a late Holocene relative sea-level history. Four stages in the late Holocene development of Ho Bugt can be identified: (1) groundwater-table rise and growth of basal peat (from at least 2300 BC to AD 0); (2) salt-marsh formation (0 to AD 250); (3) a freshening phase (AD 250 to AD 1600?), culminating in the drying out of the marshes and producing a distinct black horizon followed by an aeolian phase with sand deposition; and (4) renewed salt-marsh deposition (AD 1600? to present). From 16 calibrated AMS radiocarbon ages on fossil plant fragments and 4 calibrated conventional radiocarbon ages on peat, we reconstructed a local relative sea-level history that shows a steady sea-level rise of 4 m since 4000 cal yr BP. Contrary to suggestions made in the literature, the relative sea-level record of Ho Bugt does not contain a late Holocene highstand. Relative sea-level changes at Ho Bugt are controlled by glacio-isostatic subsidence and can be duplicated by a glacial isostatic adjustment model in which no water is added to the world's oceans after ca. 5000 cal yr BP. 相似文献
87.
The páramo is a neotropical alpine ecosystem that covers more than 75,000 km2 of the northern Andes of Colombia, Ecuador, Venezuela, and Peru. It provides important environmental services: more than 10 million people in the Andean highlands benefit from the water supply and regulation function, which is attributed to the volcanic soils that underlie the ecosystem. The soils are also major carbon sinks of global significance. Severe land use changes and soil degradation threaten both the hydrology and carbon sink function. Nevertheless, soil genesis and properties in the páramo is rather poorly understood, nor are their ecological functions well documented. The impact of the geomorphology of the páramo on soil genesis was studied in the rio Paute basin, south Ecuador. Two toposequences were described and analysed. In each toposequence, four pedons were selected representing summit, backslope, undrained plain situation, and valley bottom positions in the landscape. The soils are classified as Hydric Andosols in the World Reference Base for Soil Resources and Epiaquands or Hydrudands in Soil Taxonomy. They are very acidic and have a high organic matter content, high P deficiency, and Al toxicity. Their water content ranges from 2.64 g g− 1 at saturation, down to 1.24 g g− 1 at wilting point, resulting in a large water storage capacity. Two major soil forming processes are identified: (1) volcanic ash deposition and (2) accumulation of organic carbon. Volcanic ash deposits may vary in depth as a result of regional geomorphological factors such as parent material, orientation, slope, and altitude. Organic carbon accumulation is an interaction of both waterlogging, which depends on the position in the landscape, and the formation of organometallic complexes with Al and Fe released during volcanic ash breakdown. Despite the high variability in parent material and topography, the soil is characterised by a notable homogeneity in physico-chemical properties. Statistical analysis reveals that only topographic location has a slight but significant influence on soil pH as well as the organic matter content, saturated conductivity and water retention at high pressure. Finally, the exceptional properties of these soils provide useful insights to improve classification of the Andosols reference group of the FAO World reference Base for Soil Resources. 相似文献
88.
89.
Seismic Hazards Assessment for Radioactive Waste Disposal Sites in Regions of Low Seismic Activity 总被引:4,自引:0,他引:4
Clark H. Fenton John Adams Stephen Halchuk 《Geotechnical and Geological Engineering》2006,24(3):579-592
A comparative tectonic quiescence and lack of earthquakes make the stable centres of continents attractive for siting long-term
radioactive waste storage facilities. The low rates of deformation in such regions, however, make it difficult to characterize
their long-term seismotectonic behaviour, leading to uncertain estimates for the very low probability hazard estimates required
by society. In an attempt to overcome the deficiency of both contemporary seismicity and paleoseismic data in central Canada,
we have used earthquake histories from regions with similar seismotectonic characteristics from around the world. Substituting
space for time, we estimate a long-term rate per 106 km2 of 0.004 magnitude ≥6 earthquakes per annum, of which 33–100% might rupture to the surface. 相似文献
90.
Daniel G. Brown Rick Riolo Derek T. Robinson Michael North William Rand 《Journal of Geographical Systems》2005,7(1):25-47
The use of object-orientation for both spatial data and spatial process models facilitates their integration, which can allow exploration and explanation of spatial-temporal phenomena. In order to better understand how tight coupling might proceed and to evaluate the possible functional and efficiency gains from such a tight coupling, we identify four key relationships affecting how geographic data (fields and objects) and agent-based process models can interact: identity, causal, temporal and topological. We discuss approaches to implementing tight integration, focusing on a middleware approach that links existing GIS and ABM development platforms, and illustrate the need and approaches with example agent-based models. 相似文献