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Arnold H. Bouma James M. Coleman Charles E. Stelting Barry Kohl 《Geo-Marine Letters》1989,9(3):161-170
A conceptual sea-level-driven depositional model for individual fanlobes (channel-overbank systems) of the Mississippi Fan
does not permit direct application of the sequence stratigraphic principles of Vail and colleagues. Deep Sea Drilling Project
Leg 96 results suggest that, during initial relative lowering of sea level, the canyon and upper fan channel were formed;
excavated fine-grained slope sediments may have formed a debris flow deposit base for the fanlobe. Continued lowering produced
constructional channel-levee-overbank deposits. Rising relative sea level inhibited input of coarse clastics, and channel
depressions filled with muds. A blanket of (hemi)pelagics represents relative high sea level stand. 相似文献
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A.?BenisekEmail author H.?Kroll L.?Cemi? V.?Kohl U.?Breit B.?Heying 《Contributions to Mineralogy and Petrology》2003,145(1):119-129
A series of high structural state plagioclases (Ab-An) was crystallized from glasses. By exchanging Na for K in KCl melts, metastable K-plagioclases (Or-An) were prepared which possess the same structural state as the starting plagioclases. Both series were investigated at 980 K by lead borate solution calorimetry. Continuing the ideas of Carpenter and McConnell (1984) and Carpenter (1992a), the results can be interpreted as follows. In the high plagioclase series, the enthalpies of solution, jHsol, reflect the schemes of Al,Si ordering: (1) analbite-like (C2/m) ordering in the An-poor region 0hXAnА.2, (2) high albite-like (C1¥) ordering in intermediate plagioclases, and (3) anorthite-like (I1¥) ordering in the An-rich region 0.7AnБ. In regions 1 and 2, jHsol decreases as a function of XAn, but increases in region 3 as a consequence of the C1¥MI1¥ ordering reaction. Therefore, it is not a mixing effect but a compositionally restricted ordering effect which causes the excess enthalpies, jHex, to be positive in the plagioclase binary as a whole. Neglecting the existence of phase transitions at XAn=0.2 and XAn=0.7, jHex can be approximated by a two-parameter Margules model yielding WHAnAb=14Dž kJ/mol and WHAbAn=40Dž kJ/mol. jHsol values of I1¥ plagioclases (XAn>0.7) can be "corrected" for the C1¥MI1¥ ordering effect (Carpenter 1992a). When combining the corrected values with the jHsol data which were actually measured on the C1¥ plagioclases (XAn<0.7), negative excess enthalpies are generated in the plagioclase binary. This may be expected when C1¥ ordering occurs relative to topochemically monoclinic reference states of analbite and hypothetical anorthite devoid of I1¥ order. The solution experiments on the K-plagioclases resulted in similar characteristics as those found for the plagioclases. However, in addition to the ordering effects observed in the plagioclase binary, volume mismatch effects contribute to jHex in the K-plagioclase series. jHex can be represented by a Margules model with WHAnOr=60ᆞ kJ/mol and WHOrAn=91ᆢ kJ/mol when the phase transitions at XAn=0.2 and XAn=0.7 are again neglected. The contribution of the volume mismatch effect to jHex is considerable, as appears from the large difference between the K-plagioclase and the plagioclase Margules parameters. Their difference corresponds to a practically symmetrical dependence of jHexvolmism on composition, with WHvolmism=48ᆡ kJ/mol. 相似文献
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Aqueous oxidation of sulfide minerals to sulfate is an integral part of the global sulfur and oxygen cycles. The current model for pyrite oxidation emphasizes the role of Fe2+-Fe3+ electron shuttling and repeated nucleophilic attack by water molecules on sulfur. Previous δ18O-labeled experiments show that a variable fraction (0-60%) of the oxygen in product sulfate is derived from dissolved O2, the other potential oxidant. This indicates that nucleophilic attack cannot continue all the way to sulfate and that a sulfoxyanion of intermediate oxidation state is released into solution. The observed variability in O2% may be due to the presence of competing oxidation pathways, variable experimental conditions (e.g. abiotic, biotic, or changing pH value), or uncertainties related to the multiple experiments needed to effectively use the δ18O label to differentiate sulfate-oxygen sources. To examine the role of O2 and Fe3+ in determining the final incorporation of O2 oxygen in sulfate produced during pyrite oxidation, we designed a set of aerated, abiotic, pH-buffered (pH = 2, 7, 9, 10, and 11), and triple-oxygen-isotope labeled solutions with and without Fe3+ addition. While abiotic and pH-buffered conditions help to eliminate variables, triple oxygen isotope labeling and Fe3+ addition help to determine the oxygen sources in sulfate and examine the role of Fe2+-Fe3+ electron shuttling during sulfide oxidation, respectively.Our results show that sulfate concentration increased linearly with time and the maximum concentration was achieved at pH 11. At pH 2, 7, and 9, sulfate production was slow but increased by 4× with the addition of Fe3+. Significant amounts of sulfite and thiosulfate were detected in pH ? 9 reactors, while concentrations were low or undetectable at pH 2 and 7. The triple oxygen isotope data show that at pH ? 9, product sulfate contained 21-24% air O2 signal, similar to pH 2 with Fe3+ addition. Sulfate from the pH 2 reactor without Fe3+ addition and the pH 7 reactors all showed 28-29% O2 signal. While the O2% in final sulfate apparently clusters around 25%, the measurable deviations (>experimental error) from the 25% in many reaction conditions suggest that (1) O2 does get incorporated into intermediate sulfoxyanions (thiosulfate and sulfite) and a fraction survives sulfite-water exchange (e.g. the pH 2 with no Fe3+ addition and both pH 7 reactors); and (2) direct O2 oxidation dominates while Fe3+ shuttling is still competitive in the sulfite-sulfate step (e.g. the pH 9, 10, and 11 and the pH 2 reactor with Fe3+ addition). Overall, the final sulfate-oxygen source ratio is determined by (1) rate competitions between direct O2 incorporation and Fe3+ shuttling during both the formation of sulfite from pyrite and from sulfite to final sulfate, and (2) rate competitions between sulfite and water oxygen exchange and the oxidation of sulfite to sulfate. Our results indicate that thiosulfate or sulfite is the intermediate species released into solution at all investigated pH and point to a set of dynamic and competing fractionation factors and rates, which control the oxygen isotope composition of sulfate derived from pyrite oxidation. 相似文献
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Samples were collected for foraminiferal studies by the Johnson Sea-Link I and II manned submersibles on the Louisiana continental slope. This paper documents that the mud, extruded onto the sea floor from depth by four mud volcanoes, ranges in age from Miocene to Pleistocene based on studies of the planktonic foraminiferal fauna. The vents are in water depths ranging from 300 to 690 m located in Garden Banks Block 382, Green Canyon Blocks 143 and 272, and Mississippi Canyon Block 929. Two mud volcanoes in GB 382 and MC 929 also have rich fossil foraminiferal microfaunas. We suggest that the extrusion of fossil sediments onto the sea floor during the Quaternary is a reasonable explanation for frequent occurrences of displaced fossil microfaunas encountered at depth in wells drilling on the flanks of salt diapirs in the slope environment. Results of this study have important implications for age dating subsurface sediments in bathyal locations. 相似文献
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We present a detailed model of stray-light suppression in the spectrometer channels of the Ultraviolet Coronagraph Spectrometer (UVCS) on the SOHO spacecraft. The control of diffracted and scattered stray light from the bright solar disk is one of the most important tasks of a coronagraph. We compute the fractions of light that diffract past the UVCS external occulter and non-specularly pass into the spectrometer slit. The diffracted component of the stray light depends on the finite aperture of the primary mirror and on its figure. The amount of non-specular scattering depends mainly on the micro-roughness of the mirror. For reasonable choices of these quantities, the modeled stray-light fraction agrees well with measurements of stray light made both in the laboratory and during the UVCS mission. The models were constructed for the bright H i Lyα emission line, but they are applicable to other spectral lines as well. 相似文献
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