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981.
V. I. Slysh 《Astronomy Reports》2008,52(5):343-351
A model for non-uniform source of synchrotron radiation with a power-law radial distribution of the magnetic field and relativistic-electron density along one-or two-sided jets is described. Non-relativistic jets with both constant cross sections (collimated jets) and cross sections that are proportional to distance (conical jets) are considered. Formulas that can be used to determine source parameters from the spectral index, source size, and index of the relativistic-electron energy spectrum based on multi-frequency observations are obtained. In the case of a conical jet, these formulas coincide with the analogous formulas for a spherical source obtained by A.P. Marscher. Relations that can be used to estimate the magnetic-field strength from the brightness temperature in the self-absorbed region are also obtained. As examples, the inhomogeneous-source model is applied to the compact radio sources at the centers of the Milky Way, Sgr A*, and the low-luminosity galactic nuclei M81* and M87*, which are associated with supermassive black holes. The inner radius of the radiation region is determined. For Sgr A*, this distance turns out to be comparable to the gravitational radius, smaller than the radius of the last stable orbit for a non-rotating black hole, and consistent with the radius of the last stable orbit expected for a rotating black hole. The inner radii in M81* and M87* are ~15 R S , an order of magnitude larger than for Sgr A*. Estimates of the magnetic field at the inner radius are 400 G for M81*, 0.65–5.3 kG for Sgr A*, and 20–100 kG for M87*. These magnetic fields and the Blandford-Znajek model for the radiation of a rotating black hole are used to estimate the rotational speed of the black holes, which are in agreement with the characteristic variability time scales for these three objects. However, the accuracy of these estimates is modest, and is limited primarily by the accuracy of interferometric measurements at millimeter wavelengths. 相似文献
982.
We have obtained the stellar velocity dispersion in three mutually perpendicular directions in the halos and cores of clusters as a function of time for several non-stationary open-cluster models. During the dynamical evolution of the open-cluster models, the velocity dispersions undergo oscillations that do not decay during 5–10 violent-relaxation timescales, τ vr . We estimated the time for synchronization of the rotation of the open-cluster models and their motion around the center of the Galaxy, t s , which, depending on the model parameters, is t s ? (5–27)τ vr . Synchronization mechanisms for the models are discussed. The disruption of the systems in the force field of the Galaxy is strongly affected by tidal friction. We have also estimated the time for the formation of a spherical stellar-velocity distribution in the cluster models, t σ ? (6 ? 25)τ vr . The impact of instability in the stellar motions in a cluster on the formation of a spherical velocity distribution in the open-cluster models is discussed. We have noted a tendency for a weakening of the dependence of the coarse phase density of the cluster on small initial perturbations of the stellar phase coordinates in the model cluster cores for times about five times longer than the violent-relaxation time. 相似文献
983.
Many Gulf of Mexico estuaries have low ratios of water volume to bottom surface area, and benthic processes in these systems
likely have a major influence on system structure and function. The purpose of this study was to determine the spatiotemporal
distribution of biomass and community composition of subtidal benthic microalgal (BMA) communities in Galveston Bay, TX, USA,
compare BMA community composition and biomass to phytoplankton in overlying waters, and estimate the potential contribution
of BMA to the trophodynamics in this shallow, turbid, subtropical estuary. The estimates of BMA biomass (mean = 4.21 mg Chl
a m−2) for Galveston Bay were within the range of the reported values for similar Gulf of Mexico estuaries. BMA biomass in the
central part of the bay was essentially homogeneous, whereas biomass at the seaward and upper bay ends of the transect were
significantly lower. Peridinin, fucoxanthin, and alloxanthin were the three carotenoids with the highest concentrations, with
fucoxanthin having the highest mean concentration (1.82 mg m−2). The seaward and landward ends of the transect differed from the central region of the bay with respect to the relative
abundances of chlorophytes, cyanobacteria, and photosynthetic bacteria. Benthic microalgal community composition also showed
a gradual shift over time due to changes in the relative abundances of photosynthetic bacteria, cryptophytes, dinoflagellates,
and cyanobacteria. Major changes in community composition occurred in the spring months (March to April). On an areal basis,
BMA biomass in Galveston Bay occurred at minor concentrations (16.5%) relative to phytoplankton. Furthermore, the concentrations
of carotenoid pigments for phytoplankton and BMA (fucoxanthin, alloxanthin, and zeaxanthin) were correlated (r = 0.48 to 0.61), suggesting a close linkage between microalgae in the water column and sediments. The contribution of BMA
to the primary productivity of the deeper waters (>2 m) of Galveston Bay is probably very small in comparison to shallower
waters along the bay margins. The significant similarities in the community composition of phytoplankton and BMA illustrate
the potential importance of deposition and resuspension processes in this turbid, shallow estuary. 相似文献
984.
Although the supply and fate of suspended sediment is of fundamental importance to the functioning and morphological evolution
of muddy estuaries, reliable sediment budgets have been established in only a few cases. Especially for smaller estuaries,
inadequate bathymetric surveys and a lack of intertidal sedimentation data often preclude estimation of the sediment budget
from morphological change, while instrument-derived residual fluxes typically lie well within the errors associated with measurement
of much larger gross tidal transports. Given suitably long-term records of continuously monitored suspended sediment concentration
(SSC), however, analysis of the major scales of variation in sediment transport and their relation to hydrodynamic and meteorological
forcing permits qualitative testing of hypotheses suggested by directly measured residual fluxes. This paper analyzes data
from a 1-year acoustic Doppler profiler deployment in the Blyth estuary, a muddy mesotidal barrier-enclosed system on the
UK east coast. Flux calculations indicate a small sediment import equivalent to just 1.5% of the gross flood tide transport.
Little confidence can be assigned to either the magnitude or direction of such a small residual when considered in isolation.
However, the inference that the sediment regime is finely balanced is qualitatively supported by the close similarity between
flood-tide and ebb-tide SSC values. Singular spectrum analysis of the SSC time series shows the expectedly large contributions
to the variance in SSC at intratidal and subtidal (semimonthly and monthly) scales but also picks out intermittent variability
that is initially attributed to a combination of non-tidal surge and wind stress forcing. Closer examination of the data through
cross-correlograms and event-scale analysis indicates that local meteorological forcing is the major factor. Acting through
the resuspension of intertidal mudflat sediments at times of strong westerlies, meteorological forcing is directly implicated
in episodic sediment export from the estuary. Thresholding of tide-averaged fluxes using a range of critical wind stress values
further indicates that ‘tide-dominated’ (i.e., low wind stress) and ‘wave-dominated’ (high wind stress) conditions are associated
with sediment import and export. Sediment balance is potentially sensitive to the frequency of high wind stress events, since
the associated sediment exports are several times larger than the average import under calm conditions. Intermittent meteorological
forcing may thus exert an important control on the sedimentary balance of otherwise tidally dominated muddy estuarine systems,
and the role of wind climate should not be overlooked in studies of estuary response to environmental change. 相似文献
985.
986.
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
987.
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
988.
989.
990.
The Tandilia Belt in northeast Argentina includes a Neoproterozoic sequence of sediments (Sierras Bayas Group), in which the Cerro Largo Formation, ca. 750 Ma in age, forms a siliciclastic, shallowing upward succession of subtidal nearshore to tidal flat deposits. Trace fossils Palaeophycus isp. and Didymaulichnus isp. have been described from the upper part of this succession. Specific sedimentary structures consisting of round-crested bulges, arranged in a reticulate pattern, and networks of curved cracks are associated with the trace fossils. These structures are considered to be related to epibenthic microbial mats that once colonized the sediment surface. They reflect stages of mat growth and mat destruction, if compared to analogous structures in modern cyanobacterial mats of peritidal, siliciclastic depositional systems. Also the trace fossils are interpreted as mat-related structures, partly forming components of networks of shrinkage cracks, partly representing the upturned and involute margins of shrinkage cracks or circular openings in desiccating and shrinking, thin microbial mats.
The definition of Didymaulichnus miettensis Young as a Terminal Proterozoic trace fossil is questioned, and it may be considered to interpret the ‘bilobate’ structure as the upturned, opposite margins of microbial shrinkage cracks which have been brought back into contact by compaction after burial. 相似文献