Sulfur Isotopes of Framboidal Pyrite in the Permian-Triassic Boundary Clay at Meishan Section

Pyrite framboids were found in the Permian-Triassic boundary at Meishan Section,while their sulfur isotopes were determined.The majority of framboids is less than 5μm in diameter,with some large-sized framboids.Also,euhedral gypsums were observed in the boundary clay.The authors suppose that most of the pyrite framboids formed just below the redox boundary and stopped growing after entering the lower water column.The result indicates that it was probably lower dysoxia condition in the temporal ocean.Moreover,the authors also presume that some pyrite was oxidated to sulfates accompanying the fluctuation of redox condition,which would probably be the origin of the negative sulfur isotopes of gypsum and CAS reported before.In addition,sulfur isotope of framboidal pyrite suggests that sulfur is originated from bacterial sulfate reduction in anoxic condition.Therefore, this study confirms that the ocean was widely anoxic during the Permian-Triassic transitional period. However,the redox condition in temporal ocean was probably not stable,with short-term fluctuations.

The petrological characteristics and signif icance of organic-rich shale in the Chang 7 member of the Yanchang Formation,south margin of the Ordos basin,central China

The organic-rich shale of the Chang 7 member is the most important source rock in the Ordos basin.The sedimentary environment and the controlling factors of organic matter enrichment,however,are still in contention.In this investigation,the Yishicun outcrop,located on the south margin of the Ordos basin,has been considered for the study.X-ray diffraction,polarizing microscopy,field emission scanning electron microscopy and cathodoluminescence(CL)were used to investigate the petrological features of the organic-rich shale.The content of volcanic ash and the diameter of pyrite framboid pseudocrystals were measured to illustrate the relationship between oxygen level,ash content and the enrichment of organic matter.It has been found that the diameter of pyrite framboid pseudocrystals has a strong correlation with the total organic carbon,demonstrating that the redox status degree of the water column has a positive impact on the enrichment of organic matter.Additionally,with an increase in the ash content,the content of organic matter increased at first and then decreased,and reached a maximum when the ash content was about 6%,illustrating that the ash input has a double effect on the enrichment of organic matter.

Evolution and diagenetic implications of framboids in the methane-related carbonates of the northern Okinawa Trough

Authigenic carbonate samples were collected from the northern Okinawa Trough.Based on their carbon and oxygen isotopes,these samples were found to be methane-related carbonates precipitated by the anaerobic oxidation of methane(AOM).Petrological analysis revealed numerous framboidal pyrites that had been partly oxidized.In order to trace the variation and diagenetic information of these framboidal pyrites,their diameters and geochemical components were studied using an electron probe.The results showed that their diameters varied from 4μm to 17μm(n=60;geometric mean of 9.9μm)and were of a normal distribution.The diameters of single pyrite that formed the framboidal pyrites varied from 1μm to 2μm.The framboidal pyrites with diameters of 6–14μm accounted for~80%of the total.The geometric mean of 9.9μm indicates that they are probably diagenetic pyrites that were precipitated in a lower dysoxic environment(weakly oxygenated bottom waters).The S/Fe ratio of the framboidal minerals ranged from 0 to 1.67,and the pyrite content of single framboid varied between 0%and 86.4%.Therefore,numerous pyrites were oxygenated to iron oxides or oxyhydroxides,and were retained as pseudomorphism pyrites.The size of framboidal pyrites precipitated in cold seeps can be used to trace the redox environment;however,acquisition of additional data via investigation of different cold seeps is necessary to obtain more persuasive results.

Euxinia as a dominant process during OAEla(Early Aptian)on the Eastern Russian Platform and during OAElb(Early Albian)in the Middle Caspian

A detailed study of the mineral composition and microstructure of the black shales associated with OAEla(Eastern Russian Platform),OAElb(Middle Caspian),and the host rocks has been carried out using X-ray diffraction,scanning electron microscopic,and microprobe analyses.The results provide important constraints for depositional environments in the sedimentary basins.Black shales with pyrite framboids imply euxinic(sulfidic)conditions with increased organic matter preservation.Disintegrating framboids suggest partial or complete dissolution of the organic matter inside the framboids due to increasing water oxygenation.OAEla on the Eastern Russian Platform is heterogeneous as it includes thin interbeds of concretionary coccolith limestones within the interval of bituminous shales,and correlates with the Lower Aptian Rhagodiscus angustus nannofossil zone.The coccolith limestones indicate short intermittent episodes of interrupted stagnation,rapid oxygenation,and restoration of normal marine conditions.The presence of montmorillonite,albite,microcline,and diopside in the bituminous siltstones and in the host siltstones of OAEla on the Eastern Russian Platform,as well as a high content of titanium in the black siltstones correlated with OAElb in the Middle Caspian allow proposing significant input of pyroclastic material into the extant sea.

Deciphering the upper ordovician Wufeng siliceous shale depositional environments(Wuxi,NE Chongqing)based on multi-proxy record

Based on stratigraphic variations of petrology,geochemistry(major,trace elements),and pyrite framboids,we study the sedimentary environment of siliceous rocks of the Wufeng Formation in Wuxi,Northeastern Chongqing,China.Coupled Al2O3/(Al2O3þFe2O3),SiO2/Al2O3,with AleFeeMn values indicate that Wufeng Formation are deposited in a continental margin and influenced by detrital input.UEF-MoEF and V/CreU/Th cross-plots suggest that the siliceous shale was formed in dysoxic to anoxic conditions.Redox proxies imply that the lower,the middle to upper,and the top part of the Wufeng sediments were deposited in an anoxic,dysoxic and oxic environment,respectively.Accordingly,the average size of pyrite framboids are gradually increased from bottom to top,showing that increased oxidisability.A complete vibratory third-order cycle of sea level fluctuations during the Wufeng deposition can be identified.During Wufeng to Longmaxi transition,the sedimentary environment exhibits a short-scale oscillatory pattern and was probably transformed from an outer shelf to inner shelf.Coupled multi-proxies are considered more reliable proxies for deciphering redox conditions in fine-grained sediment.