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121.
Characterizing the dynamics of fluvial sediment sources over space and time is often critical in identifying human impacts on fluvial systems. Upland interfluve and subsoil sources of suspended sediment at Loco Bayou, Texas, were distinguished using 226Ra/232Th, 226Ra/230Th and, 228Ra/232Th. Source contributions were apportioned at three stations during within-bank and flood flows. 137Cs and 210Pbxs (excess 210Pb) were used to determine floodplain sedimentation; suspended sediment 210Pbxs/137Cs data mirrored results of Ra/Th, showing dominance of subsoil sources during within-bank flows, changing to interfluve sources during flood. This trend corresponds spatially to influx of sediment from ephemeral tributaries, reflecting mobilization of stored interfluvial sediments during flood stage. Upper basin sedimentation was similar but markedly less at the lowermost station. These results indicate (1) modified ephemeral tributaries store sediment derived from sheet wash, discharging them during flood, and (2) southernmost Loco Bayou is episodically re-worked, resulting in significantly reduced local rates of sedimentation. 相似文献
122.
Karlyn S. Westover Sherilyn C. Fritz Tatyana A. Blyakharchuk Herbert E. Wright 《Journal of Paleolimnology》2006,35(3):519-541
The sedimentary diatom records of three shallow lakes in the Altai Mountains, southern Siberia, were examined to assess the
nature and timing of Holocene environmental changes. Few paleoenvironmental records, especially reconstructions not based
on pollen, have been reported from this region. The lakes differ in elevation, annual precipitation, and catchment vegetation.
Diatom assemblages in all lakes were dominated for the entire period of record by small benthic species of Pseudostaurosira Williams & Round, Staurosira Ehrenberg, and Staurosirella Williams & Round. Planktonic taxa only occur in very low abundances (<5%). The most diverse diatom flora was found in Dzhangyskol,
which is situated at the lowest elevation within a forested catchment. A lack of detailed information on the ecological preferences
of the dominant taxa and the complexity of environmental drivers make direct interpretation of the diatom record difficult.
However, other proxies suggest that dramatic shifts in dominance between Staurosira elliptica and Staurosirella pinnata in Grusha Ozero reflect millennial-scale variability in climate. Together, chironomids and diatoms provide evidence of a
cooling possibly correlative to the Younger Dryas Stade and subsequent early-Holocene warming consistent with pollen evidence
of afforestation, which also is likely linked to increased humidity. By ~6000 cal year BP, the transition to a cooler, more
continental climate had begun. The diatom record of Akkol shows significantly less variation in diatom community composition,
but biogenic silica accumulation rates, a proxy for diatom productivity, appear to reflect climatic variability driven by
insolation trends over the past 8000 years. Long-term variability in Dzhangyskol is not clearly linked to climate. 相似文献
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Lavas and intrusives from Ayios Mamas, at the western end of the Limassol Forest Complex, south of the Arakapas Fault Belt, Troodos, comprise three groups: (A) a basal sequence of pillow lavas (intruded by dikes and sills) with the phenocryst assemblage PLAG+CPX+MT±OL; (B) a middle sequence of pillows and massive flows [CPX+PLAG±OL]; and (C) an upper sequence mostly of pillows and breccia [OL+OPX+CPX+SP]. The lower group is tholeiitic, and the upper two groups boninitic. Group C whole-rock and glass Mg-numbers exceed 72 and range up to 78, with corresponding olivine compositions of Fo92. They are strongly depleted in high-field strength elements, and on the basis of Cr-Y variation and major element mass balances, are interpreted to reflect partial melting of a basalt-depleted (CPX-) harzburgite source. Group A tholeiites, in contrast, are interpreted to derive from a more fertile lherzolite source. Magmaphile and compatible trace element variation is best modelled after an open magma system, involving periodic replenishment, tapping, and fractionation, although neither tholeiites nor boninites represent steady-state compositions. The boninites belong to a CaO-rich, SiO2-poor variant, resembling those from Guam, but contrasting with those from Bonin and Cape Vogel. MORB-normalized enrichment of low-field strength elements K, Sr, Ba, and Rb in all groups indicates subduction-derived source contamination. It is proposed that the Ayios Mamas section reflects splitting of a volcanic arc, prior to back-arc spreading in an environment analogous to that of the present-day Andaman Sea. 相似文献
128.
H. A. Bridgman R. C. Schnell G. A. Herbert B. A. Bodhaine S. J. Oltmans 《Journal of Atmospheric Chemistry》1989,9(1-3):49-70
In April 1986, a well-instrumented NOAA WP-3D research aircraft conducted three flights in the Canadian Arctic tied to the Canadian Atmospheric Environment Service baseline station in Alert, Northwest Territories. Two of the flights were coordinated with the National Aeronautical Establishment of Canada Twin Otter and the University of Washington C-131 research aircraft. The haze observed in the Canadian Arctic was well-aged and mixed throughout the troposphere in concentrations well below those observed during the previous weeks in the Alaskan Arctic. Over the ice, beneath the surface temperature inversion, ozone was generally depleted to near zero. Over the coast at Alert, there is evidence that topography and downslope winds reduce the strength of the inversion, thus allowing lower tropospheric gases and aerosols to mix down to the surface. At the top of the troposphere, an aerosol-depleted region was observed. In the lower stratosphere, aerosol concentrations were elevated above those observed in the troposphere. 相似文献
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Thermodynamics of sodium feldspar II: Experimental results and numerical calculations 总被引:1,自引:0,他引:1
The free energy, entropy and enthalpy of sodium feldspar in thermal equilibrium and in metastable states are derived from investigations of the heat capacities of albite, analbite, ordered and disordered Or31. The lattice strains of all stable and metastable states are calculated from the two-order parameter theory published in the preceding paper. This approach also allows one to distinguish between the influence of Al, Si order and of the displacive lattice distortion on the thermodynamic behaviour of albite. The thermal cross-over between high albite and low albite is found to be a smooth function of temperature. 相似文献