Geochemical Characteristics of Late Ordovician Shales in the Upper Yangtze Platform, South China: Implications for Redox Environmental Evolution

Changes to the redox environment of seawater in the Late Ordovician affect the process of organic matter enrichment and biological evolution. However, the evolution of redox and its underlying causes remain unclear. This paper analyzed the vertical variability of main, trace elements and δ34Spy from a drill core section (well ZY5) in the Upper Yangtze Platform, and described the redox conditions, paleoproductivity and paleoclimate variability recorded in shale deposits of the P. pacificus zone and M. extraordinarius zone that accumulated during Wufeng Formation. The results showed that shale from well ZY5 in Late Ordovician was deposited under oxidized water environment, and there are more strongly reducing bottom water conditions of the M. extraordinarius zone compared with the P. pacificus zone. Excess silica (SiO2(exc)) and substitution index of paleoproductivity (Y) indicated that the P. pacificus zone had higher paleoproductivity whereas the M. extraordinarius zone was lower. The high productivity level controlled O2 release in the shallow water area as well as the oxidation degree of the P. pacificus zone. The decrease of productivity and the relatively stagnant water mass of the inner Yangtze Sea controlled the formation of relatively reduced water conditions in the M. extraordinarius zone. The chemical index of alteration (CIA) results suggested that palaeoclimatic conditions changed from warm and humid to cold and dry climate from the P. pacificus to the M. extraordinarius zones in the study area. A comparative analysis of the published Fe-S-C data for the Xiushan Datianba section showed that in the P. pacificus zone of the inner Yangtze Sea, warm and humid climate conditions drove high productivity, sulphate flux and low reactive iron flux, which promoted the expansion of oxic ocean-surface waters and mid-depth euxinic waters. In the M. extraordinarius zone, the cold and dry climate with significant uplift of the Xiang’e Submarine High led to the relative sea level decline, resulting in low productivity, sulfate flux and high reactive iron flux, which promoted the expansion of the mid-depth ferruginous waters and the shrinkage of oxic ocean-surface waters. The results offered new insights into the co-evolution of continents and oceans, and explained the role of continental weathering and uplift of the Xiang’e Submarine High in the exchange of sulfate flux and nutrients in the redox environment change of inner Yangtze Sea during the Late Ordovician.

Influences of Progressing Parameters on Flake BaBi4Ti4O15 Powder Synthesized by Molten Salt Synthesis Method

BaBi4Ti4O15 powder was synthesized by molten salt synthesis (MSS) method in NaCl-KCl and Na2SO4-K2SO4 fluxes. The phases of reaction products and the microstructures at different calcined temperatures were detected by X-ray diffraction (XRD) and scanning electron microscope (SEM). As a result, the flaky BaBi4Ti4O15 powder with anisotropy was synthesized by molten salt synthesis method in NaCl-KCl and Na2SO4-K2SO4 fluxes in the range of 850～1050°C. Compared with BaBi4Ti4O15 powder synthesized in the sulfate flux, the anisotropy level of BaBi4Ti4O15 powder which was synthesized in chloride flux increased, the average particle size(APS) of BaBi4Ti4O15 powder synthesized in the chloride flux is slight larger than in the sulfate flux. The thickness of BaBi4Ti4O15 powder was synthesized by molten salt synthesis method in the chloride flux decreased than in the sulfate.

Synthesis of α–Al2O3 platelets using sodium sulfate flux

When powder mixtures of {Al2(SO4)3 + Na2SO4}or {γ–Al2O3 [obtained by heating Al2(SO4)3 at 900 °C for 3 h] + Na2SO4} were heated in an alumina crucible at 1100 °C for 1 h, α–Al2O3 platelets were formed. The powder mixture of {Al2(SO4)3 + 2Na2SO4} yielded aggregations of platelets that were less than 5 μm in size. The size of the aggregations increased in proportion to the amount of Na2SO4, and aggregations of 120 μm were obtained using a mixture of {Al2(SO4)3 + 6Na2SO4}. The powder mixture of {γ–Al2O3 + 2Na2SO4 yielded hexagonal platelets having an average diameter of 3.7 μm and an average thickness of 0.3 μm. In addition to aggregation size, the size of the hexagonal platelets also increased in proportion to the amount of Na2SO4, and platelets having an average diameter of 5 μm were obtained using a mixture of {γ–Al2O3 + 6Na2SO4}.