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841.
842.
Xinan Mu Steve Kershaw Yue Li Li Guo Yuping Qi Alan Reynolds 《Journal of Asian Earth Sciences》2009,36(6):434
High-resolution δ13CCARB analysis of the Permian–Triassic boundary (PTB) interval at the Laolongdong section, Beibei, near the city of Chongqing, south China, encompasses the latest Permian and earliest Triassic major facies changes in the South China Block (SCB). Microbialites form a distinctive unit in the lowermost 190 cm above the top of the Changhsing Formation (latest Permian) at Laolongdong, comparable to a range of earliest Triassic sites in low latitudes in the Tethyan area. The data show that declining values of δ13CCARB, well-known globally, began at the base of the microbialite. High positive values (+3 to 4 ppt) of δ13CCARB in the Late Permian are interpreted to indicate storage of 12C in the deep waters of a stratified ocean, that was released during ocean overturn in the earliest Triassic, contributing to the distinctive fall in isotope values; this interpretation has been stated by other authors and is followed here. The δ13CCARB curve shows fluctuations within the microbialite unit, which are not reflected in the microbialite structure. Comparisons between microbialite branches and adjacent micritic sediment show little difference in δ13CCARB, demonstrating that the microbialite grew in equilibrium with surrounding seawater. The Early Triassic microbialites are interpreted to be a response to upwelling of bicarbonate-rich poorly oxygenated water in low latitudes of Tethys Ocean, consistent with current ocean models for the PTB interval. However, the decline of δ13CCARB may be due to a combination of processes, including productivity collapse resulting from mass extinction, return of deep water to ocean surface, oxidation of methane released from methane hydrate destabilisation, and atmospheric deterioration. Nevertheless, build-up of bicarbonate-rich anoxic deep waters may be expected as a result of the partial isolation of Tethys, due to continental geography; release of bicarbonate-rich deep water, by ocean upwelling, in the earliest Triassic may have been an inevitable consequence of this combination of circumstances. 相似文献
843.
The Wilcox aquifer is a major groundwater resource in the northern Gulf Coastal Plain (lower Mississippi Valley) of the USA, yet the processes controlling water chemistry in this clastic aquifer have received relatively little attention. The current study combines analyses of solutes and stable isotopes in groundwater, petrography of core samples, and geochemical modeling to identify plausible reactions along a regional flow path ~300 km long. The hydrochemical facies evolves from Ca-HCO3 upgradient to Na-HCO3 downgradient, with a sequential zonation of terminal electron-accepting processes from Fe(III) reduction through SO4 2? reduction to methanogenesis. In particular, decreasing SO4 2? and increasing δ34S of SO4 2? along the flow path, as well as observations of authigenic pyrite in core samples, provide evidence of SO4 2? reduction. Values of δ13C in groundwater suggest that dissolved inorganic carbon is contributed both by oxidation of sedimentary organic matter and calcite dissolution. Inverse modeling identified multiple plausible sets of reactions between sampled wells, which typically involved cation exchange, pyrite precipitation, CH2O oxidation, and dissolution of amorphous Fe(OH)3, calcite, or siderite. These reactions are consistent with processes identified in previous studies of Atlantic Coastal Plain aquifers. Contrasts in groundwater chemistry between the Wilcox and the underlying McNairy and overlying Claiborne aquifers indicate that confining units are relatively effective in limiting cross-formational flow, but localized cross-formational mixing could occur via fault zones. Consequently, increased pumping in the vicinity of fault zones could facilitate upward movement of saline water into the Wilcox. 相似文献
844.
845.
Alan E. Rubin 《Meteoritics & planetary science》1988,23(4):333-337
Abstract— Ureilites are modeled as impact-melt products of CV-chondrite-like material. This model is consistent with the brecciated nature and cumulate textures of ureilites, O-isotopic constraints (which indicate ureilite derivation from an isotopically heterogeneous body like the CV-chondrite parent), the high abundance of planetary-type noble gases, and the relatively high concentrations of siderophile and chalcophile elements (indicating incomplete separation of metal-sulfide from silicate). Each ureilite may have been derived from a separate cratering event. 相似文献
846.
Nine twentieth-century ordinary chondrite falls from China are described and classified. They include: Nantong (H6), Zaoyang (H5), Zhaodong (L4), Qidong (L-LL5), Raoyang (L6), Sheyang (L6), Guangnan (L6), Suizhou (L6) and Nan Yang Pao (L6). Kamacite in Qidong is rare and contains much more Co (15 mg/g) than is characteristic of L-group chondrites; Qidong may be a member of a chondrite group intermediate in its properties between L and LL. Zhaodong, Qidong, Raoyang, Sheyang and Suizhou have several olivine and/or low-Ca pyroxene grains with aberrant Fel(Fe + Mg) ratios; it is probable that these five chondrites are fragmental breccias. The lack of correlation between shock facies and occurrence of aberrant silicate grains suggests that breccia lithification caused only minimal shock effects in many meteorites. Alternatively, postshock annealing may have resulted in the recrystallization of shock-indicating phases, leading to assignment of a shock facies that is lower than that present immediately after the shock event. 相似文献
847.
Nightly means for extinction were determined from the recorded real-time extinction values of observations made on the 7-inch Transit Circle Telescope at Black Birch Astrometric Observatory (BBAO). The residual effects of the 15 June 1991 Mount Pinatubo eruption in the Philippines increased the extinction by about 0.25 magnitudes per airmass. 相似文献
848.
849.
850.