Pyrite Framboid Size Distributions Research Articles

Large Igneous Province Control on Ocean Anoxia and Eutrophication in the North Sea at the Paleocene–Eocene Thermal Maximum

AbstractThe Paleocene–Eocene Thermal Maximum (PETM) was a global hyperthermal event ∼56 Ma characterized by massive input of carbon into the ocean–atmosphere system and global warming. A leading hypothesis for its trigger is the emplacement of the North Atlantic Igneous Province (NAIP), with extensive extrusion/intrusion of igneous material into nearby sedimentary basins, forcing local uplift and warming‐inducing carbon emissions. It remains unclear if oceanographic changes in the North Sea–Norwegian Sea–Arctic basins, such as anoxia and productivity, were causally linked to local NAIP uplift/activity, and at what time scales these perturbations occurred. To test mechanisms and time scales, we present geochemical proxies (XRF analysis, clay mineralogy, molybdenum isotopes, and pyrite framboid size distribution) in undisrupted marine sediment core E−8X located in the central North Sea. We find evidence for a rapid onset of anoxia/euxinia at the negative carbon isotope excursion from redox proxies, followed by a gradual drawdown of molybdenum/total organic carbon (Mo/TOC) during the PETM main phase indicative of tectonically‐restricted basin likely from NAIP uplift. A short‐lived increase in Mo, pyrite and TOC occurred during a precursor event associated with a sedimentary mercury pulse indicative of volcanic activity. We suggest thermal uplift and flood basalt volcanism tectonically restricted the North Sea and tipped it into an euxinic state via volcanic emission–oceanographic feedbacks inducing eutrophication. This fine temporal separation of tectonic versus climatic geochemical proxies, combined with pulsed NAIP volcanism, demonstrates that Large Igneous Province emplacements can, at least locally, result in ocean biogeochemical feedbacks operating on relatively short timescales.

Pyrite morphological evidence for sedimentary conditions of the Wufeng formation–Longmaxi formation in south-east Chongqing area–insights for high-quality shale gas reservoir formation mechanism

As one of the important minerals in marine shale reservoirs, the development characteristics of pyrite can provide guidance for the exploration of deep shale gas resources and the study of high-quality reservoir development mechanisms. In this study, samples of the Wufeng and Longmaxi Formation in the southeast Chongqing area were selected, and the development characteristics of framboidal pyrite were revealed through a combination of qualitative and quantitative method. High-quality reservoirs of the upper Wufeng Formation and the bottom of the Longmaxi Formation are developed with smaller sizes, weaker variation and a narrower size distribution of pyrite framboids compared with other units of the strata, suggesting relatively stable euxinic conditions during deposition. The development characteristics of framboidal pyrite and some of the key reservoir evaluation parameters such as organic matter content and brittle mineral content, etc., are controlled by similar factors. Therefore, pyrite morphological evidence can be used as a potential indicator of high-quality shale reservoirs.

Palaeoenvironment characteristics of the Ediacaran Doushantuo Shale, Yichang area, South China: Implications for organic matter accumulation mechanism

The organic-rich Ediacaran Doushantuo Shale have been identified as a high-quality source rock and a main shale gas play in the Yichang area, South China, which was characterized by high TOC content, high thermal maturity, enormous cumulative thickness, and relatively high contents of brittle minerals. As the oldest shale gas stratum, the paleo-environmental characteristics and organic matter (OM) accumulation mechanism remain unclear in this special marine setting. In this study, core descriptions, TOC content, X-ray diffraction (XRD) mineralogical analysis, scanning electron microscopy (SEM) observation, and elemental analysis, including the major, trace, and rare earth elements, were combined to identify lithofacies, reconstruct paleo-environmental conditions, and document organic matter (OM) accumulation mechanisms for this potential Ediacaran Doushantuo Shale in the Yichang area, South China. Four major shale lithofacies were identified based on TOC content and mineral composition. The geochemical redox proxies, such as the enrichment factors of Uranium (U-EF) and ratio of organic carbon to total phosphorus (Corg/P), indicate that the Doushantuo Shale was probably deposited in a dysoxic-anoxic conditions, which is consist with the interpretation from size distribution of pyrite framboids. The terrigenous influxes suggest the Doushantuo Shale was deposited in arid climate in absence of chemical weathering. Meanwhile, the productivity proxies (biogenic SiO2, Ba, and P/Al) indicate that the organic-rich Doushantuo Shale was deposited under a relatively low primary productivity. In addition, hydrothermal input has limited influence on organic matter (OM) accumulation of the Doushantuo Shale, which was also reported in the Qinglinkou Section of Yichang area. It can be observed that the organic matter (OM) accumulation mechanism of the Doushantuo Shale, deposited after the Marinoan “Snow Earth” glaciation, in the Yichang area, South China, was mainly controlled by the redox conditions, which is much different from typical marine shales and transitional shales.

The Origin of the Black Shale Series and Bentonites from the Wufeng Formation in the Southwestern Upper Yangtze: Implications for the Convergence of the Yangtze and Cathaysia Blocks in the Late Ordovician

AbstractEarly Paleozoic black organic sediments and bentonites occur widely in the craton basin within the Yangtze block and are generally believed to be genetically related to a specific tectonic setting on the cratonic boundary. However, the intimate relationship between their origins and the dynamic mechanisms are unclear, as exemplified by the genesis of the black shale series and bentonites from the Wufeng Formation during the Ordovician–Silurian transition (OST). In order to reveal the relationship between the Wufeng Formation and the convergence of the Yangtze and Cathaysia blocks (i.e., the intracontinental Kwangsian Orogeny), two stratigraphic sections respectively in Zhaotong area (Northeast Yunnan) and Puyi area (Northwestern Guizhou) that were located in the semi‐restricted inner Yangtze Sea during the OST were systematically studied, on the basis of whole‐rock geochemical composition, pyrite δ34S (δ34Spy), total organic carbon (TOC), stable Sr isotope, pyrite framboid size distribution and zircon U‐Pb age, trace elements. The evidence shows that the paleo‐oceanic environment changed significantly at the turn of the early–late Katian and formed the black shale series in the Wufeng Formation. These acritarch assemblages were formed in the transition process of the Upper Yangtze Basin from passive continental margin basin to foreland basin during this interval. Based on previous research on the genetic relationship between black shale series and plate tectonic movement, a basin‐mountain evolution model suitable for South China in the Late Ordovician is presented. The two bentonites in the Wufeng Formation with U‐Pb ages of 445.5 ± 0.8 Ma and 441.9 ± 2.4 Ma primarily originated from the intermediate–acid volcanic eruption during the collision and convergence between the Yangtze and Cathaysia blocks in the Late Ordovician, the provenance region probably being located in the Jiangnan orogenic belt. Thus, we believe that the appearance of the black shale series and bentonite in the Wufeng Formation at the turn of the early–late Katian may represent the initiation of basin‐mountain transformation and the Kwangsian Orogeny in South China, which provides important evidence for the collision and convergence of the Yangtze and Cathaysia blocks in the Late Ordovician.

Marine redox variation and hydrographic restriction in the early Cambrian Nanhua Basin, South China

The early Cambrian Nanhua Basin is an important archive of information related to contemporaneous marine redox evolution and its relationship to the radiation of early animals. However, the nature of watermass conditions in the Nanhua Basin remains uncertain owing to conflicting redox signals. Here, we generated a redox proxy dataset comprising Fe speciation, redox-sensitive element (RSE), pyrite framboid size distributions, and molybdenum isotopic data (δ98Mo), in combination with proxies for hydrographic restriction (i.e., Co × Mn, Cd/Mo, and Mo/TOC), in order to evaluate environmental conditions in the basinal Yuanjia section. We then compared these data with previous results for outer-shelf (Jinsha, Zhongnancun, and Yangjiaping), slope (Daotuo, Songtao, and Longbizui), and basinal facies (Silikou) to develop an integrated picture of environmental (especially redox) changes in the early Cambrian Nanhua Basin. At Yuanjia, the lower member (LM, 0–28 m) is characterized by weakly euxinic conditions and an intense Fe-Mn shuttle, and the upper member (UM, 28–65 m) by moderately euxinic conditions and a weak Fe-Mn shuttle. Our integrated analysis demonstrates co-existence of oxic surface waters, euxinic mid-depth waters, and ferruginous deep waters in the Nanhua Basin, with euxinic conditions prevailing in shelf-slope facies of the Yangtze Block and basinal facies of the Cathaysia Block but declining in intensity from the LM to the UM. The Nanhua Basin changed from being moderately restricted with strong upwelling during the LM to relatively more restricted with weak upwelling during the UM. We infer that redox fluctuations in the early Cambrian Nanhua Basin were linked to hydrographic restriction, although global factors such as low seawater sulfate concentrations may have played a role also. Our study highlights the need for caution in assuming that redox variation in the early Cambrian Nanhua Basin was representative of contemporaneous global-ocean redox states.

Macroids from mixed siliciclastic-carbonate high-frequency sequences of the Agrio Formation (Lower Cretaceous), Neuquén Basin: Palaeoecological and palaeoenvironmental constraints

Macroids, cm-sized biogenically coated grains made of associations of metazoans or protozoans, accumulate in some high-frequency sedimentary sequences of the upper Hauterivian to lower Barremian Agua de la Mula Member of the Agrio Formation in the Neuquén Basin (northern Patagonia, Argentina). The macroids studied are sub-spheroidal and sub-discoidal in shape, 2–3 cm in diameter and 1–2 cm in height. Each macroid is characterised by a rough, weakly bored surface, encrusted by oysters, serpulids, and small, dome-shaped bryozoan colonies. The growth arrangement consists of locally concentric, agglutinated encrusting foraminifers growing above and around skeletal nuclei. The accumulations of macroids on top of some starvation hemi-sequences during the latest Hauterivian 3rd-order transgressive systems tract help in understanding the palaeoenvironmental conditions at the onset of high-frequency regressions, characterised by low but persistent sedimentation rates and a generally low-energy environment. Mineralisation of sand-sized glauconite and apatite grains suggests ephemeral periods of reduced sedimentation or non-deposition. Pyrite framboid size-distribution indicates the upper dysoxic zone with oxygen restrictions. The re-establishment of terrigenous sediment input following transgression promoted eutrophication of the water column. Under such palaeoenvironmental conditions, benthic life was represented mainly by species able to thrive with eutrophicated, oxygen-poor waters at the sediment-water interface. This setting excludes activity of borers during the formation of macroids. Macroids result pivotal in understanding palaeoecology of inherent benthic communities and in refining the interpretation of palaeoenvironmental parameters in mixed siliciclastic-carbonate ramp systems.

Preservation of organic carbon in the Cretaceous Hue Shale on the North Slope of Alaska: Insights from pyrite morphology

The Cretaceous Hue Shale is a condensed mudstone section of the Brookian sequence with type II and type III kerogen, and it is one of the major source rocks that fill the world-class conventional oil fields on the North Slope, Alaska. The bottom section of the Hue Shale is referred to as Gamma Ray Zone (GRZ) or Highly Radioactive Zone (HRZ) and believed to be the most organic-rich section. The thermal maturity increases from immature to over oil window towards the southwest. Although, the research on the Hue Shale as an unconventional shale reservoir is still at an early stage, where exploration activities are very limited. The Hue Shale thickens to the east in general. However, regional variations of thickness and more importantly total organic carbon (TOC) have been reported.In this research, a suite of nineteen core samples of the Hue Shale are selected. These cores are collected from six wells covering a wide spectrum of thermal maturity with four wells in the National Petroleum Reserve Alaska and two wells in the state land. A combined geochemical and petrographic methodology is utilized to address the regional redox conditions and the preservation of organic carbon. TOC, Tmax, hydrogen index, and oxygen index are acquired through pyrolysis to characterize the abundance, thermal maturity, the origin, and the evolution of organic matter. Scanning electron microscope (SEM) coupled with energy dispersive X-ray spectroscopy (EDS) is used to investigate the mineralogy and diagenesis of the Hue Shale. Pyrite crystal forms, and pyrite framboid size distributions are used as an indicator of the redox conditions during deposition. The results show substantial variance in oxygen levels in the water column. High abundance and small sizes of pyrite framboids, which indicates a depleted oxygen environment, show better preservation potential of organic matter.

Geochemical characteristics of organic carbon and pyrite sulfur in Ordovician-Silurian transition shales in the Yangtze Platform, South China: Implications for the depositional environment

Global climate change and tectonism are principal causes of spatial and temporal differences in Ordovician-Silurian (OS) transition sedimentary environments of China's Upper Yangtze Platform. However, the relative significance of each of these drivers is not always obvious. This paper describes results of a geochemical study of a drill core section (Well X3) recovered from Xiang'e Submarine High in the central part of the Upper Yangtze Platform. This work focused on shale deposits of the Wufeng Formation and Longmaxi Formation that accumulated during the OS transition. Geochemical characteristics, including total organic carbon (TOC), pyrite S (Spy), stable carbon and sulfur isotopes, and Mo and U enrichment values, were combined with pyrite framboid size distribution data to reconstruct the OS paleoenvironmental evolution of this region of the Upper Yangtze Platform. The studied succession appears to record the transition from lower Katian-Hirnantian shale deposited under sulfate-limited conditions to Rhuddanian organic-rich shale and finally to organic-depleted Aeronian sediment. The depositional history of these deposits was controlled by changes in water depth and watermass chemistry as a result of changing sea level. The sea-level history of the studied succession was largely controlled by global climate change related to Hirnantian glaciation and Rhuddanian warming and tectonism associated with the Kwangsian Orogeny that resulted in uplift of the Xiang'e Submarine High. Similar geochemical investigation of the Pengshui and Shuanghe sections, also in the Upper Yangtze Platform, demonstrate the dominant role of global climate change and tectonism in controlling organic carbon content and thickness of potential shale gas targets. Specifically, the low concentrations of organic carbon in thin shale deposits proximal to uplifted regions of the basin pose a major risk to shale gas exploration.

Oceanic water redox conditions of the region between Tethys and Panthalassa during the late early Triassic

Fossil evidence of a complex food-chain in the marine ecosystem after the end-Permian mass extinction (EPME) event has been reported from the Osawa Formation, which consists of upper Olenekian (Spathian) shallow marine, mudstone-dominant strata in the South Kitakami Belt in Northeast Japan. This sedimentary sequence offers an important material that records the paleoenvironment of the oceanic region between the eastern Tethys and Panthalassa. This paper reports detailed records of the oceanic environment from two sections of the Osawa Formation focusing on the redox condition. Parallel, undisturbed laminae suggesting limited benthic activity are prominent in the middle to upper parts of the Osawa Formation, while disturbance of laminae by burrows occurs in the lower and uppermost parts, suggesting active benthic activity in an oxygen-rich depositional environment. The amount of pyrites (euhedral pyrite, pyrite framboids, and pyrite polyframboids) increases in the well-laminated mudstones. A relatively broad size distribution of pyrite framboids indicates that these mudstones were deposited under dysoxic conditions and suggest that the water column was not sulfidic. Redox-sensitive elements such as Mn, V, and U show values close to the average values of the upper continental crust throughout the Osawa Formation. Mo is moderately enriched in the well-laminated and pyrite-abundant mudstone horizons although the degree of enrichment is lower than known examples of modern sulfidic basins. These data suggest that the condition of the bottom water in this oceanic region during the Spathian varied between oxic and dysoxic (weakly oxygenated) and was never sulfidic. Such an oceanic environment characterized by mildly reducing bottom water and oxic water column appears to have been the settings for the recovery and establishment of the Early Triassic marine ecosystem characterized by diverse nektonic motile animals and limited diversity of benthic, in-motile animals.

The diagenetic origin and depositional history of the Cherry Valley Member, Middle Devonian Marcellus Formation

We report δ13C, δ18O, δ34S records coupled with elemental, pyrite morphology, and fluid inclusion data spanning the Cherry Valley Member in the Middle Devonian Marcellus shale from two cores located across a depth gradient in the Appalachian basin. The goal of the study was to evaluate the diagenetic and redox history of this carbonate-interbedded layer within a prominent shale gas system. Three lithological units were identified in the Cherry Valley Member: a nodular clay-rich marlstone lower unit, a fossiliferous limestone middle unit, and a calcareous mudstone upper unit. Low δ13C and δ18O values of the carbonate matrix display a mixing trend among marine carbonate, authigenic carbonate, and late stage diagenetic cementation. The petrology of barite nodules and associated δ34Sbarite values suggest that a shoaling sulfate-methane transition zone created by methane advection contributed to the nodule growth and pyritization. Along with other early and late diagenetic processes, this mechanism provided the main carbonate source for the lower unit due to elevated rates of aerobic methane oxidation. The δ34Spyrite values, U and Mo contents, and pyrite framboid size distributions together constrain the redox history of the Cherry Valley Member. An anoxic-euxinic marine environment rather than a fully oxygenated environment continued through the lower unit in areas with deeper paleo-bathymetry until it was interrupted by an episode of dysoxia during the deposition of the middle unit. Basin-scale anoxia was then renewed during a subsequent 3rd order sea level rise in the upper unit. The entire Cherry Valley Member was ultimately altered by warm burial fluid interaction through vertically distributed veins. The combined body of evidence indicates that the duration and spatial distribution of oxygenated marine conditions during the deposition of Cherry Valley Member within the Marcellus Formation may have been overestimated.

Anomalous positive pyrite sulfur isotope in lacustrine black shale of the Yanchang Formation, Ordos Basin: Triggered by paleoredox chemistry changes

The pyrite sulfur isotope in the black shale of the Chang 7 unit are anomalous positive compared with those of the current paradigm, in which the value of pyrite sulfur isotopes should be negative due to sulfur isotopic fractionation through bacterial sulfate reduction (BSR). A series of geological and geochemical analysis (FeT/Al and δ34Spy, average of 0.51 and 6.5‰, respectively) are carried out to confirm that sulfur speciation is derived from sulfate in watermass rather than hydrothermal fluids, and the δ34Spy values are effective records of the original sedimentary environment. Integrated geochemical data including redox sensitive elements (RSE) ratios, ratio of organic carbon to phosphorus (Corg:P, molar, average of 85.4), organic sulfur to carbon (organic S:C, average of 10.47%) and pyrite framboid size distribution (range in 9.7–16.6 μm), are represented to reconstruct the paleoredox chemical conditions during the deposition of the Chang 7 unit black shale. The results suggest that the paleoredox chemical conditions of the middle and lower subunits (the Ch 72 and Ch 73 subunits) were dominated by suboxic environments with intermittent anoxic environments and that changed to oxic with intermittent suboxic environments in the upper subunit (the Ch 71 subunit). Based on the analysis of sulfur isotope fractionation during the sulfur cycle, the paleoredox chemical conditions are confirmed to be the trigger for the anomalous sulfur isotopic compositions. The limited sulfate in watermass and overlying fine-grained sediments resulted in a relatively closed system for BSR, where limited sulfur isotope fractionation lead to the formation of heavy pyrite. Subsequently, due to the dissolved oxygen penetrated into the sediment, the reactivation of anaerobic oxidants resulted in reoxidation of hydrogen sulfide derived from BSR, then to “heavier” pyrite via Rayleigh-type distillation in the Ch 71 subunit.

Depositional environment and geomechanical properties of Cambay Shale: potential reservoir for shale oil and gas in India

This study aims to investigate mineralogy, depositional environment and rock mechanical properties of Cambay Shale Formation in Jambusar-Broach block of South Cambay Basin, India, by an integrated approach of combining wireline logs and laboratory studies. The study involves the analysis of spectral gamma ray (SGR) logs and laboratory studies of core samples using X-ray diffraction (XRD), thin-section analysis and scanning electron microscope-energy-dispersive X-ray spectroscopy (SEM-EDS). The results show that in the Cambay Shale, mineral components are dominated by clay minerals, quartz and siderite with minor amounts of feldspar, mica and pyrite. Both thorium (Th)–potassium (K) cross-plots derived from SGR logs and XRD analysis have revealed that kaolinite and chlorite are the dominant clay minerals. The dominant clay mineralogy and the analysis of thorium to uranium ratio plots indicate the deposition of the shale in marginal marine to marine environment. Pyrite framboid size distribution, used as a redox proxy, indicates deposition under anoxic conditions. The geomechanical property of Cambay Shale has been analysed based on the overall mineral composition of the core samples. Mineralogy-based brittleness index (BI) has been evaluated and found to vary between 0.21 and 0.66, suggesting low to moderately high brittleness. Kaolinite and chlorite, the dominant clay minerals, are the less swelling clays and exhibit a lower plasticity. It may be concluded that these factors might facilitate the intrinsic fracking properties of Cambay Shale.

Millennial-scale ocean redox and δ13C changes across the Permian–Triassic transition at Meishan and implications for the biocrisis

The Permian–Triassic extinction was the largest Phanerozoic mass extinction event, yet its ultimate causes remain unresolved. Ocean redox conditions, global warming, and carbon cycle perturbations owing to massive volcanism and their inter-relationships are keys to resolve the origins of this event. We here assess such relationships using a method to obtain an instantaneous stratigraphic response from high-resolution (millennial-scale) records of carbonate-δ13C and pyrite framboid size distributions across the Permian–Triassic boundary (PTB) at Meishan, China. We report two gradual long-term negative δ13C shifts before and after the PTB. Late Permian ocean redox conditions varied through three successive oxic–dysoxic, dysoxic–euxinic, and high-frequency euxinic stages. Oxic and high-frequency euxinic events occurred in the early Griesbachian and middle Griesbachian, respectively. The short-term euxinia events were not associated with the negative δ13C excursions. Siberian volcanism could have produced large light-carbon inputs, global warming, and low-oxygen conditions at intermediate ocean depths, but it may not directly drive high-frequency euxinia across the PTB. Long-term dysoxia and occasional euxinia well before the PTB stressed mobile macroorganisms and caused gradual extinction before the major extinction event. High-frequency latest Permian euxinia reduced overall biotic adaptability, and caused the initial rapid mass extinction. Increasing temperatures and other deleterious environment factors then reinforced the extinction process. Oxic conditions contributed to biotic recovery immediately after the mass extinction in the early Griesbachian. Further millennial-scale dysoxic/euxinic alternations in the middle Griesbachian would have prolonged Early Triassic biotic recovery.

The Ab-Bagh Late Jurassic-Early Cretaceous sediment-hosted Zn-Pb deposit, Sanandaj-Sirjan zone of Iran: Ore geology, fluid inclusions and (S–Sr) isotopes

The Ab-Bagh clastic- and carbonate-hosted stratabound and stratiform Zn-Pb deposit in the Sanandaj-Sirjan zone, in the southeastern corner of the Malayer-Esfahan metallogenic belt of Iran, is typical of several sedimentary–exhalative (SEDEX) deposits in this metallogenic belt. The deposit is hosted in a late Jurassic-early Cretaceous sedimentary sequence. Based on position, there are two Zn-Pb-bearing stratigraphic ore horizons. Ore horizon 1 is hosted by late Jurassic-early Cretaceous black shale and siltstone. The wedge-shaped ore body is located close to a synsedimentary normal fault. Petrographic studies indicates that mineralization comprises three sulfide ore facies: stockwork, bedded and massive ore facies. Ore horizon 2 occurs in early Cretaceous carbonates and comprises a massive-replacement ore facies that is concordant with host rock layering; it is also underlain by a stockwork facies. Textures include framboidal, laminated, breccia, replacement, massive and vein-veinlet. Hypogene sulfide minerals are predominantly sphalerite, galena, pyrite and chalcopyrite. Calcite and quartz are major gangue minerals. Silicification and carbonatization are the main wallrock alteration styles; alteration intensity increases towards the feeder zone. Based on relationships between ore minerals and rock-forming minerals, sulfide mineralization in the Ab-Bagh deposit formed during two main stages: fine-grained sulfide bands (stage I) are intricately interlayered with organic matter-rich beds of black shale and siltstone. Sulfide bands exhibit classic sedimentary textures, such as laminations and bedding, indicative of a synsedimentary origin. Coarser-grained stage II base metal sulfides show breccia and vein-veinlet textures, and are considered to have formed by replacement during sub-seafloor fluid flow. Fluid inclusion microthermometry was carried out on calcite associated with sulfides of the stockwork ore facies. Homogenization temperatures are in the range of 180–301 °C, and the average salinity is 17.93 wt% NaCl eq. The size distribution of pyrite framboids of the bedded ore, geochemistry of host rock and presence of organic matter, collectively suggest anoxic to locally suboxic ambient conditions for the host basin. The δ34S composition of pyrite, sphalerite, and galena ranges from −5.08 to +4.51‰. The highest δ34S values are from massive and stockwork ore facies (+2.66 to + 4.51‰) and the lowest from bedded ore (−5.08 to −4.72‰). In the bedded ore facies, the main sulfur source was bacterially reduced seawater sulfate. Whereas in massive and stockwork ore facies, thermochemically reduced seawater sulfate was the main source of sulfur. The 87Sr/86Sr ratios of calcite indicate that origin of ore forming fluids is Paleozoic radiogenic basement. Collectively, the isotopic, sedimentological, textural, mineralogical, fluid inclusion, and geochemical data for the Ab-Bagh deposit support a vent-proximal sub-seafloor replacement SEDEX ore deposit.

Guadalupian (Middle Permian) ocean redox evolution in South China and its implications for mass extinction

The Guadalupian mass extinction was a long-term biocrisis that started in the early Guadalupian Epoch (Permian Period). Many studies of the causes of the extinction have focused on the late Guadalupian, although the entire Guadalupian should be studied to address its origins. The selective nature of the species removed during the Guadalupian extinction also remains a puzzle. Here, we present pyrite framboid size distributions, pyrite-sulfur isotopic compositions, and C-N-S profiles for the entire Guadalupian from South China to develop a record of ocean redox evolution to help understand the selective extinction. Our results indicate that euxinia was mostly persistent in the deep-water platform basin throughout the Guadalupian, which triggered long-term Early and Middle Guadalupian shallow-water dysoxia and frequent episodic Late Guadalupian euxinia events that resulted in a gradual long-term Early and Middle Guadalupian diversity decrease and selective Late Guadalupian extinction. The combined effect of euxinia and global regression led to selective extinction of taxa that had weakly buffered respiratory physiology and high growth efficiency, such as brachiopods, large fusulinids, and giant bivalves.

Frequent euxinia in southern Neo-Tethys Ocean prior to the end-Permian biocrisis: Evidence from the Spiti region, India

In this study, we reconstruct water-column oxygen availability during the deposition of Late Permian shales (Wuchiapingian to approximately early Changhsingian) using the geochemistry and morphology of sedimentary pyrite (FeS2). Deposition of the shales occurred prior to the end-Permian mass extinction within the southern Neo-Tethys Ocean (Spiti region, Himachal Pradesh, India). We found that the variability of pyritic sulfur to organic carbon (Spyr/Corg) and the pyritic sulfur isotopic composition (δ34Spyr) did not record changes in oxygen availability at Spiti sections. This was due to the removal of original pyritic sulfur by modern oxidative weathering and diagenetic pyrite formation which overwhelmed the original δ34Spyr values. However, pyrite framboid size distributions was a reliable recorder of paleoredox conditions. The degree of oxygen limitation varied with multiple transient euxinic events interrupting a predominantly oxic-dysoxic trend near the top of the Gungri Formation upper member (approximately late Wuchiapingian to early Changhsingian). These transient euxinic intervals provide important insight and regional evidence of environmental stress and instability in the southern Neo-Tethys well prior to the end-Permian mass extinction.

Organic matter enrichment of the Late Triassic Yanchang Formation (Ordos Basin, China) under dysoxic to oxic conditions: Insights from pyrite framboid size distributions

Pyrite framboids are spherical or ellipsoidal compact aggregates of submicron pyrite microcrystals. The various lithologies in the Chang 7 Member of the Upper Triassic Yanchang Formation contain abundant pyrite framboids, which provide an opportunity to understand the fluctuation in paleoredox conditions and relative sedimentation rates during deposition. The measured diameters of the pyrite framboids in the Chang 7 Member are characterized by large mean diameters (9.7–16.6 μm) with a wide distribution range that plots in the area indicative of dysoxic to oxic paleoredox conditions during deposition of the Chang 7 sediments. Based on correlation analysis, the strong correlation between paleoredox-sensitive trace elements ratios (V/Cr, U/Th, and Cu/Zn) and the mean size of framboids indicate that the latter can serve as a reliable indicator of paleoredox conditions. The results of the framboid size distribution can also be interpreted through Crystal Size Distribution Theory (CSDT). According to CSDT, the relative sedimentation rate of the Chang 7 Member fluctuated during deposition in parallel with fluctuation in lacustrine levels. Furthermore, the strongly positive correlation between total organic carbon (TOC) and paleoredox conditions indicates that although relatively high sedimentation rate may have contributed to the dysoxic-oxic conditions that were harmful to the preservation of organic matter, but it may also have brought nutrients to increase productivity. Similarly, the correlation between TOC and relative sedimentation rates suggests that the relative sedimentation rates can affect organic matter enrichment in several ways. A high sedimentation rate is favourable for the preservation of organic matter, which can be prevented from oxidation, but large amounts of sediments can dilute the organic matter, which reduces the organic matter abundance in a potential source rock.

Two-stage marine anoxia and biotic response during the Permian–Triassic transition in Kashmir, northern India: pyrite framboid evidence

Abstract Although expanded ocean anoxia has long been believed to be a direct killing mechanism causing mortality of organisms during the Permian-Triassic mass extinction, little has been published on the extent and timing of this anoxia in Gondwana. The Guryul Ravine section in Kashmir, northern India, is a classic Permian–Triassic boundary (PTB) section containing high-quality marine sedimentary and fossil records, and thus provides a unique opportunity to study the redox conditions associated with the biotic crisis in the Gondwana region. Here, high-resolution biotic and redox data were generated from Kashmir to achieve an improved understanding of the nature of environmental stresses associated with the Earth's largest biocatastrophe. Our study, which evaluates pyrite framboid size and morphology, reveals two pronounced stages of oceanic oxygen deficiency, in the assigned latest Permian Hindeodus praeparvus–Clarkina meishanensis Zone and the earliest Triassic Isarcicella staeschei Zone. Updated marine invertebrate fossil records show three sharp species richness declines at Guryul Ravine. The first decline occurred within uppermost Permian storm beds and is interpreted to represent a facies control, in which a storm-agitated environment was inhospitable for benthos. The latter two biotic declines coincided with two marine anoxic events, as documented by pyrite framboid size distributions. The same two anoxic events are also recognized from PTB beds in the adjacent, relatively shallower Barus Spur section in Kashmir, in which newly obtained faunal data help to constrain placement of the PTB. The present study represents a new report of the two-stage pattern of oceanic anoxia during the Permian–Triassic transition. We propose that the two anoxic events at Guryul Ravine correlate precisely with anoxic events in the Meishan GSSP and some sections in South China, suggesting that this event sequence might have been characteristic of the Permian–Triassic transition in some specific geological settings. The close relationship between oxygen depletion and species richness decline suggests that the former were an important contributor to the latter. In addition, we find that many framboids exhibit surface oxidation, reducing their overall size. However, our statistical analysis suggests that the mean oxidation-related reduction in size is

Rapid biotic rebound during the late Griesbachian indicates heterogeneous recovery patterns after the Permian-Triassic mass extinction

New fossil data from two Early Triassic (Griesbachian to Dienerian) sections from South China show unusually high levels of both benthic and nektonic taxonomic richness occurring in the late Griesbachian. In total, 68 species (including 26 newly originated species) representing mollusks, brachiopods, foraminifers, conodonts, ostracods, and echinoderms occur in the late Griesbachian, indicating well-established and relatively complex marine communities. Furthermore, the nekton shows higher origination rates than the benthos. Analyses of the sedimentary facies and size distribution of pyrite framboids show that this high-diversity interval is associated with well-oxygenated environments. In contrast to the previously suggested scenario, which inferred that persistently harsh environmental conditions impeded the biotic recovery during the Early Triassic, our new findings, combined with recent work, indicate a fitful regional recovery pattern after the Permian-Triassic crisis, resulting in three main diversity highs: late Griesbachian–early Dienerian, early–middle Smithian, and Spathian. The transient rebound episodes were therefore influenced by both extrinsic local (e.g., redox condition, temperature) and intrinsic (e.g., biological tolerances, origination rates) parameters.

Redox conditions and authigenic mineralization related to cold seeps in central Guaymas Basin, Gulf of California

Highlights • In cold seeps of Guaymas Basin, aragonite, barite and pyrite precipitated from modified seawater. • Aragonite is highly depleted in 13C suggesting formation via anaerobic oxidation of methane. • Barite formed through mixing of reducing, Ba-rich seep fluids with a 34S-rich sulfate pool. • Pyrite framboids formed under anoxic-sulfidic water via microbial sulfate reduction. Abstract Authigenic carbonate crusts, surface muds and bivalve shell fragments have been recovered from inactive and active recently discovered cold seep sites in central Guaymas Basin. In this study, for first time, redox conditions and fluid sources involved in mineral precipitation were investigated by analyzing the mineralogy and textures of surface samples, along with skeletal contents, and C, O and S isotopes variations. The δ13C values of aragonitic bivalve shells and non-skeletal carbonate from some surface muds (1‰ to −3.7‰ V-PDB) suggest that carbonate precipitated from ambient dissolved inorganic carbon, whereas fibrous aragonite cement and non-skeletal carbonate from other sites are highly depleted in 13C (down to −47.6‰ V-PDB), suggesting formation via anaerobic oxidation of methane, characteristic of methane seepage environments. δ18O in most of the carbonates varies from +1.4‰ to +3.2‰ V-PDB, indicating that they formed from slightly modified seawater. Some non-skeletal carbonate grains from surface muds have lower δ18O values (−12.5‰ to −8.2‰ V-PDB) reflecting the influence of 18O-depleted pore water. Size distribution of pyrite framboids (mean value: 3.1 μm) scattered within diatomaceous sinter suggests formation from anoxic-sulfidic bottom waters. δ34S in pyrite is of −0.3‰ V-CDT compared to +46.6‰ V-CDT in barite, thus implying a fluid sulfate−sulfide fractionation of 21.3‰ that argues in favor of microbial sulfate reduction as the processes that mediated pyrite framboid formation, in a semi-closed system. Barite formation occurred through the mixing of reducing and Ba-rich seep fluids with a 34S-enriched sulfate pool that resulted from microbial sulfate reduction in a semi-closed system. The chemical composition of aragonite cement, barite and pyrite suggest mineral precipitation from modified seawater. Taken together, our data suggest that mineralization at the studied seep sites is controlled by the mixing of seawater with minor amounts of hydrothermal fluids, and oxygen-depleted conditions favoring anaerobic microbial processes.