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91.
92.
93.
Molar Tooth Structure: a Contribution from the Mesoproterozoic Gaoyuzhuang Formation, Tianjin City, North China 总被引:2,自引:0,他引:2
Molar-tooth (MT) structure is an enigmatic sedimentary structure consisting of variously-shaped cracks and voids filled with a characteristically uniform, equant calcite microspar. It is globally distributed but temporally restricted to rocks from Neoarchean to Neoproterozoic age. The origin of MT structures has been debated for more than a century and the topic continues to be highly contentious. Some features of MT structure occurring in micritic limestones of the Mesoproterozoic Gaoyuzhuang Formation (ca. 1500 Ma to ca. 1400 Ma), Jixian section, Tianjin City, North China show that: 1) there is a definite interface or lining, rich in organic material and pyrite, between the MT crack-filling calcite microspar and the micritic host rock, which is also rich in organic matter; 2) the micritic host rocks are notable for the absence of stromatolites and microbial laminites; 3) distinctive conglomeratic lag deposits made up of intraclasts of MT microspar result from storm reworking of the MT structures; 4) the MT structure is associated with possible algal megafossils such as Chuaria; 5) the MT microspar is made up of the larger calcite crystal and the MT crack is marked by the diversity of configurations; 6) both the TOC content and the carbon-isotopic value (δ13CPDB) among the host rock, the MT microspar and the possible algae fossil are obviously different. For the forming mechanism of the Gaoyuzhuang MT structure, these features can still indicate that: A) the MT microspar was formed by rapid precipitation and lithification; B) the MT microspar precipitated directly within the cracks; C) the decomposition of organic matter within the host micrite might be the chief mechanism producing gas bubbles; D) microscale gas-sediment interaction led to the generation of the MT cracks and the precipitation of microspar therein; E) the MT cracks might represent the track of migration and expansion of gas bubbles, and that the recrystallization of host micrites cannot be eliminated during forming process of the MT microspar; F) the MT structure is occurred in early diagenetic period; and G) the formation of MT microspars is a complex diagenetic process. Therefore, model of the microbially-induced gas-bubble expansion and migration is the best interpretation for the formation of the MT structure. Effectively, MT structures are a type of sedimentary structure that is formed in the early diagenetic period and is related to microbial activities and organic matter degradation. 相似文献
94.
Mélanie Turpin Laurent Emmanuel John J. G. Reijmer Maurice Renard 《International Journal of Earth Sciences》2011,100(8):1875-1893
Modern aragonite needles are present all along the modern leeward margin of Great Bahama Bank (ODP Leg 166), while Middle
Miocene sediments contain needles only in more distal areas (Sites 1006 and 1007). In contrast to the rimmed, flat-topped
platform topography during the Plio-Pleistocene, the Miocene Great Bahama Bank morphology is a carbonate ramp profile. This
might imply a different location and precipitation type for Miocene aragonite needles. In this study, aragonite needles in
Miocene sediments were isolated using a granulometric separation method. Furthermore, the isolation of the various carbonate
components enables the identification of primary versus diagenetic components. The Miocene aragonite needles are concentrated
in the finest granulometric sediment fractions (<12 μm). The fraction-specific geochemical analyses (δ13C, δ18O and Sr elemental abundance) represent useful tools to assess the possible sources of the aragonite mud. The geochemical
variation of the fractions, rich in pristine aragonite needles, and the characteristics of the needle morphology point to
whiting phenomena as the main sediment source and algal fragmentation as a minor component. Both components indicate shallow-water
environments as the main sediment source area. Ramp-top-related fine-grained particles now present at distal sites were likely
exported as suspended material similar to present-day transport mechanisms. The scarcity of needles at proximal sites is probably
linked to hydrodynamic processes but dissolution and recrystallization processes cannot be excluded. The granulometric separation
approach applied here enables a better characterization of the finest carbonate particles representing an important step towards
the discrimination between primary and diagenetic fine-grained components. 相似文献
95.
Abstract The atmospheric model of Danard et al. (1983) is used to investigate the changes in heat, mass and momentum fluxes at the air‐sea interface in Hudson Bay when the seasonal sea surface temperature is varied. Comparisons of model predictions with data from a meteorological buoy located 400 km offshore showed that the model predicted the variations in wind speed and air temperature fairly well but underestimated their magnitudes. In addition it provided offshore heat and mass fluxes for which no direct observations were available. The most important parameter determining air‐sea fluxes is the temperature difference between air and water. This determines the stability and the degree of vertical convection of the air. In the spring the colder water stabilizes the air, which depresses vertical convection. This reduces wind stress and evaporation while increasing the heat flux into the water. During the fall, the opposite occurs. The sea surface temperature is thus buffered against man‐made changes. When the temperature is decreased, for example, as the result of hydroelectric development in surrounding watersheds, the heat flux into the water increases while the wind stress decreases. Both effects increase the sea surface temperature, opposing the initial decrease. A one‐degree depression of sea surface temperature in summer is slowly offset by increased heating and no noticeable change in temperature remains at the end of the fall. 相似文献
96.
Maurice Craig 《Mathematical Geology》1993,25(8):1003-1013
Pointwise division in the Fourier domain can be used to deconvolve the effects of aircraft altitude upon gridded, two-dimensional, radiometric count data. The appropriate divisor function, expressed as a triple integral by Gunn, is developed as a convergent series suitable for its numerical evaluation. For various flying heights, graphs display in cross-section the corresponding, radially symmetric, divisor function. 相似文献
97.
98.
David H. Cohen Maurice A. Leutenegger Kevin T. Grizzard Catherine L. Reed Roban H. Kramer Stanley P. Owocki 《Monthly notices of the Royal Astronomical Society》2006,368(4):1905-1916
X-ray line-profile analysis has proved to be the most direct diagnostic of the kinematics and spatial distribution of the very hot plasma around O stars. The Doppler-broadened line profiles provide information about the velocity distribution of the hot plasma, while the wavelength-dependent attenuation across a line profile provides information about the absorption to the hot plasma, thus providing a strong constraint on its physical location. In this paper, we apply several analysis techniques to the emission lines in the Chandra High Energy Transmission Grating Spectrometer (HETGS) spectrum of the late-O supergiant ζ Ori (O9.7 Ib), including the fitting of a simple line-profile model. We show that there is distinct evidence for blueshifts and profile asymmetry, as well as broadening in the X-ray emission lines of ζ Ori. These are the observational hallmarks of a wind-shock X-ray source, and the results for ζ Ori are very similar to those for the earlier O star, ζ Pup, which we have previously shown to be well fit by the same wind-shock line-profile model. The more subtle effects on the line-profile morphologies in ζ Ori, as compared to ζ Pup, are consistent with the somewhat lower density wind in this later O supergiant. In both stars, the wind optical depths required to explain the mildly asymmetric X-ray line profiles imply reductions in the effective opacity of nearly an order of magnitude, which may be explained by some combination of mass-loss rate reduction and large-scale clumping, with its associated porosity-based effects on radiation transfer. In the context of the recent reanalysis of the helium-like line intensity ratios in both ζ Ori and ζ Pup, and also in light of recent work questioning the published mass-loss rates in OB stars, these new results indicate that the X-ray emission from ζ Ori can be understood within the framework of the standard wind-shock scenario for hot stars. 相似文献
99.
The gravitational-wave spectrum emitted by a non-axisymmetric torus rotating at an angular velocity ΩT, is derived in terms of a structure function representing a combination of sausage-tilt modes in the torus in the limit of an incompressible fluid. The analysis of the gravitational-wave spectrum is then applied to a model proposed recently, in which a highly magnetized torus interacts with a stellar mass, Kerr black hole via poloidal field lines that connect the torus and the horizon. The deformation of the torus results from global magnetic instabilities when the magnetic field strength inside the torus exceeds a few times 1015 Gauss. The dynamics of the system is illustrated using a non-MHD toy model. It is found that, quite generally, most of the gravitational-wave energy emitted is in the frequency range of sensitivity of LIGO and Virgo. 相似文献
100.