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874.
V.B. Bazarova L.M. Mokhova M.A. Klimin L.A. Orlova K.Yu. Bazarov 《Russian Geology and Geophysics》2008,49(12):978-985
Climatic changes in southeastern Transbaikalia in the Middle and Late Holocene and their influence on alluvial sedimentation environments are reconstructed from the results of study of the Ilya floodplain sediments (Alkhanai National Park). At the beginning of the Subboreal period, the regional climate became more arid, which led to a significant increase in steppe species communities in the landscapes. Intense climate aridization also took place at the beginning and at the end of the Subatlantic period of the Holocene. The alluvial-sedimentation rate increased during the weakening of aridization and decreased during cooling and the intensification of aridization. The obtained regional data are compared with data on the adjacent areas and the global climatic changes in the Northern Hemisphere. 相似文献
875.
We present the results of twenty-year observations of a complete sample of 68 flat-spectrum radio sources with flux densities S 3.9 GHz > 200 mJy carried out at centimeter wavelengths with the RATAN-600 radio telescope. Since 1995, we have observed simultaneously at six frequencies between 0.97 and 21.7 GHz. Of the 56 sources identified with optical objects, 41 are quasars with redshifts between 0.293 and 3.263. Based on our analysis of the spectral shapes, we divide the sources into four classes. Changes of spectral class for individual sources are fairly rare. Based on the light curves and spectra, in most cases, a flare’s evolution is in accordance with a model in which the variations result from the evolution of a shock in the radio jet. The main result of our study is that there is no redshift dependence for the true linear sizes of the radiating regions, the variability indices derived for all 20 years of data or for individual flares, or the peak frequencies of the spectra of the compact radio emission. We suggest that this testifies to an absence of cosmological evolution of the sample quasars, at least to z ≈ 3. 相似文献
876.
V. M. Danilov 《Astronomy Reports》2008,52(11):888-899
The frequencies for homologous and non-homologous density flucutations in unisolated, ellipsoidal models of open clusters are determined. These fluctuations are stable. In the cases considered, the frequencies of non-homologous fluctuations are higher than those for homologous fluctuations of the ellipsoidal models. The instability of the proper fluctuations of the phase density at the center of a homogeneous ellipsoid whose central characteristics correspond to the core parameters of a dynamical, numericalmodel cluster is demonstrated. The development of instability of the phase-density flucutations at the center of such an ellipsoid leads to the domination of fluctuations with the frequency γ 1, corresponding to the growth increment γ 2 for this instability. Estimates of γ 1 and γ 2 agree with estimates obtained for the core of the corresponding dynamical, numerical model cluster. Astrophysical applications of the results are discussed. 相似文献
877.
Mirdita Zone ophiolites and associated sediments in Albania reveal Neotethys Ocean origin 总被引:1,自引:1,他引:0
Hans-Jürgen Gawlick Wolfgang Frisch Lirim Hoxha Paulian Dumitrica Leopold Krystyn Richard Lein Sigrid Missoni Felix Schlagintweit 《International Journal of Earth Sciences》2008,97(4):865-881
The Mirdita Ophiolite Zone in Albania is associated with widespread mélanges containing components of up to nappe-size. We
dated matrix and components of the mélange by radiolarians, conodonts, and other taxa. The components consist of radiolarites,
pelagic limestones and shallow-water limestones, all of Triassic age, as well as ophiolites. Triassic radiolarite as a primary
cover of ophiolite material proves Middle Triassic onset of Mirdita ocean-floor formation. The mélange contains a turbiditic
radiolarite-rich matrix (“radiolaritic flysch”), dated as Late Bajocian to Early Oxfordian. It formed as a synorogenic sediment
during west-directed thrusting of ophiolite and sediment-cover nappes representing ocean floor and underplated fragments of
the western continental margin. The tectonic structures formed during these orogenic events (“Younger Kimmeridian or Eohellenic
Orogeny”) are sealed by Late Jurassic platform carbonates. The geological history conforms with that of the Inner Dinarides
and adjoining areas; we therefore correlate the Mirdita-Pindos Ophiolite Zone with the Vardar Zone and explain its present
position by far-distance west-directed thrusting. 相似文献
878.
Identifying Source Soils in Contemporary Estuarine Sediments: A New Compound-Specific Isotope Method 总被引:2,自引:0,他引:2
M. M. Gibbs 《Estuaries and Coasts》2008,31(2):344-359
A new method is proposed for the identification and apportionment of contemporary source soils contributing to estuarine sediments.
The method uses compound-specific isotopic analysis of naturally occurring biomarkers (fatty acids) derived from plants to
link source soils to land use within a single catchment. For identification and apportionment of source soils in the estuarine
samples, the method uses the isotopic mixing model, IsoSource. The feasible proportions obtained from IsoSource are then scaled
to allow for the percent organic carbon in the source soils. With this approach, the estimation of each source soil contribution
to a location in the estuary is independent of any degradation of the biomarkers through microbial or biogeochemical processes.
Identification relies on the evaluation of the sediment sample relative to a “library” of reference source soils from different
land use within the catchment. Selection of potential sources is geographically constrained by the requirement for a natural
linkage between each source soil and the sediment site sampled. A case study, using this method, mapped the distribution of
three main land use source soils (pasture, native forest, and pine forest) across the river delta in a small estuary fringed
with mangroves. Rather than being uniformly distributed, the results indicated that the source soil contributions varied markedly
across the delta, raising concerns about the validity of taking single cores to characterize the sediments of an estuary.
Coupling the source apportionment results with land use data indicated that the mean percent contribution of pine forest soil
in the river delta sediments was almost three times greater than the percent land use area of pine forest in the catchment.
Furthermore, isotopic signatures indicated that most of the pine forest soil came from the much smaller areas exposed to erosion
by clear cut harvesting and that the soil contribution from recently harvested areas of pine forest could be as much as 20
times greater than that land use area in this catchment. This is the first method that can identify and apportion, by land
use on a catchment scale, the sources of soil contributing to the sediment at a location of an estuary. The results are given
as a “best estimate”, within definable limits, of the proportional contribution of each potential source soil. Information
obtained using this method will allow development of management strategies to alter land use practices to reduce the sediment
load to rivers, and thus, the impact on the aquatic ecosystem downstream in estuaries. 相似文献
879.
We developed light requirements for eelgrass in the Pacific Northwest, USA, to evaluate the effects of short- and long-term
reductions in irradiance reaching eelgrass, especially related to turbidity and overwater structures. Photosynthesis-irradiance
experiments and depth distribution field studies indicated that eelgrass productivity was maximum at a photosynthetic photon
flux density (PPFD) of about 350–550 μmol quanta m−2 s−1. Winter plants had approximately threefold greater net apparent primary productivity rate at the same irradiance as summer
plants. Growth studies using artificial shading as well as field monitoring of light and eelgrass growth indicated that long-term
survival required at least 3 mol quanta m−2 day−1 on average during spring and summer (i.e., May-September), and that growth was saturated above about 7 mol quanta m−2 day−1. We conclude that non-light-limited growth of eelgrass in the Pacific Northwest requires an average of at least 7 mol quanta
m−2 day−1 during spring and summer and that long-term survival requires a minimum average of 3 mol quanta m−2 day−1. 相似文献
880.
Jonas Ardö Meelis Mölder Bashir Awad El-Tahir Hatim Abdalla Mohammed Elkhidir 《Carbon balance and management》2008,3(1):1-18