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.

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.

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.

A multidisciplinary approach in recognizing seep-carbonates:A case study from the Loiano Formation(late Eocene)in the northern Apennines(Italy)

Carbonate deposits related to active seeps are documented along most continental margins and are characterized by peculiar seep-related facies and by chemical(isotope signature),paleontological and mineralogical markers.Their fossil analogues are recognized all over the world by the same features and occurred since the beginning of Phanerozoic.In this paper,we present a new seep outcrop(Castagneto village,Reggio Emilia,northern Italy)belonging to the late Eocene Loiano Formation which,to our knowledge,is in Italy the most ancient seep deposit with not-reworked chemosymbiotic fauna.The outcrop can be roughly divided into two portions,a northern part showing abundant presence of macrofossils in silty carbonate matrix and subhorizontal subdivisions,and a southern part,where macrofossils are almost absent,characterized by sub-vertical internal subdivisions and a clear vertical structure consisting of the rhythmic alternation of light and dark mineralization.Detailed analysis of samples from the southern portion showed the occurrence of authigenic calcite and pyrite,the latter with a peculiar framboidal texture.This feature,together with the occurrence of chemosymbiotic species and~(13)C isotope depletion,suggests possible hydrocarbon-rich fluid-related genesis and provides useful criteria for identifying hydrocarbon-rich fluid-related deposits in geologic units.

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.

Ooidal ironstones in the Meso-Cenozoic sequences in western Siberia: assessment of formation processes and relationship with regional and global earth processes

This study investigates the process of formation of ooidal ironstones in the Upper Cretaceous-Paleogene succession in western Siberia.The formation of such carbonate-based ironstones is a continuing problem in sedimentary geology,and in this study,we use a variety of data and proxies assembled from core samples to develop a model to explain how the ooidal ironstones formed.Research on pyrite framboids and geochemical redox proxies reveals three intervals of oceanic hypoxia during the deposition of marine ooidal ironstones in the Late Cretaceous to the Early Paleogene Bakchar ironstone deposit in western Siberia;the absence of pyrite indicates oxic conditions for the remaining sequence.While goethite formed in oxic depositional condition,chamosite,pyrite and siderite represented hypoxic seawater.Euhedral pyrite crystals form through a series of transition originating from massive aggregate followed by normal and polygonal framboid.Sediments associated with goethite-chamosite ironstones,encompassing hypoxic intervals exhibit positive cerium,negative europium,and negative yttrium anomalies.Mercury anomalies,associated with the initial stages of hypoxia,correlate with global volcanic events.Redox sensitive proxies and ore mineral assemblages of deposits reflect hydrothermal activation.Rifting and global volcanism possibly induced hydrothermal convection in the sedimentary cover of western Siberia,and released iron-rich fluid and methane in coastal and shallow marine environments.This investigation,therefore,reveals a potential geological connection between Large Igneous Provinces(LIPs),marine hypoxia,rifting and the formation of ooidal ironstones in ancient West Siberian Sea.

Pyrite morphology and episodic euxinia of the Ediacaran Doushantuo Formation in South China

The Ediacaran Doushantuo Formation with well-preserved fossil record in South China provides a rare window for our understanding of biological evolution,global carbon cycle,and oceanic redox states.Prominent negativeδ^(13)C anomalies(i.e.,the Shuram excursion)in Ediacaran successions worldwide fundamentally challenge the traditional models of isotopic mass balance.Additionally,conflicting opinions of both oxic and anoxic conditions have been proposed for the deep waters during this period.Here,we present a detailed study of pyrite morphology and carbonate carbon isotope data documented from drill core samples at Songtao County,northeastern Guizhou.Framboid aggregates are the dominant pyrite form in black shale and they can transfer to euhedral crystals through continuous growth of the constituent microcrystals.A positive correlation between microcrystal sizes(d)and framboid diameters(D)is observed,while the different D/d ratios of framboids in argillaceous dolostone and black shale reflect different substrate availability.Electron microprobe analyses reveal no consistent compositional patterns between framboidal and euhedral pyrites.Framboid size distributions of the investigated drill core,in combination with previously published redox data from the intra-shelf Jiulongwan section,shelf margin Zhongling section,and lower slope Wuhe section,suggest that three episodes of marine euxinia have been established throughout the deposition of the Doushantuo Formation.The time lag between the uppermost euxinic interval and the Shuram excursion may arise from the depression of sulfate reduction maintained by other oxidants.

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.