Black Shale Horizons Research Articles

Anatomy of the late Famennian Dasberg event in a deep shelf of southern Euramerica: Oxygenation and productivity in a restricted basin during a progressive long-term cooling

The late Famennian Dasberg event is described as a series of global hypoxic and transgressive events associated with global faunal turnover. The event, recorded on a deep shelf of the Rhenohercynian basin from southern Euramerica, was investigated in the Holy Cross Mountains of Poland using integrated high-resolution geochemical, mineralogical, and palynological studies. The data revealed the progressive restriction of the intrashelf basin resulting from intense regional block tectonics likely connected with the Late Devonian Variscan tectonic activity. This led to weak chemocline ventilation, the development of anoxic conditions, and the deposition of two organic-rich Dasberg black shale (DBS) horizons. The DBS was deposited in an environment characterized by the constant contribution of detrital components from a common source area. A slight change in terrestrial input may have been driven by modest bathymetric changes associated with the tectonics and stronger winds delivering charcoal and terrestrial components (i.e., miospores and phytoclasts). A supply of nutrients from land and delivery of crucial biolimiting elements (i.e., nitrogen and phosphorus) from deeper waters stimulated primary productivity, as recorded in phytoplankton blooms. The δ13Corganic values in the DBS reflect the incorporation of primary biomass from mainly marine photoautotrophs into sedimentary organic matter. Episodic delivery of toxic sulphides to the photic zone was detected by small-sized framboids and biomarkers, which record the appearance of green sulphur bacteria that photosynthesized in euxinic water column. The activity of phototrophic sulphide-oxidizing bacteria could have led to hyper-enrichment of Zn (715–1002 ppm) in the Lower DBS. The diachronous appearance of the DBS horizons in Euramerica and Gondwana, and regionally marked extinction of benthic fauna, suggest that anoxia developed in restricted Black Sea–like basins formed by intensive tectonic activity and continental plate convergence.

Pedostratigraphic age model for Devonian red beds of NW China

Detrital zircon geochronology has been used to determine provenance, but also gives maximum depositional ages that can constrain other forms of geochronology, such as magnetostratigraphy and pedostratigraphy. Pedostratigraphy and magnetostratigraphy correlate a series of global events by spacing or intensity in a fashion comparable with a commercial barcode. The Late Devonian Shaliushui Formation of NW China is poorly fossiliferous, but shows spikes in magnetic susceptibility, Al/Si ratios, Rb/Sr ratios and the depth to carbonate nodules (Bk horizon) corresponding with the timing of named Late Devonian black shale horizons. By contrast, the Ti/Al ratios show only small variations, indicating little long-term change in the source terrain or in the hydraulic sorting of heavy minerals. Most of the detrital zircon ages are older than an age model from the pedostratigraphic correlation of deep calcic horizons corresponding to global atmospheric CO 2 spikes at times of black shale expansion, but one detrital zircon age converges with the pedostratigraphic depositional ages near the top of the section. Pedostratigraphy is widely used in Quaternary sequences, but can also be a basis for age models of rocks in deep time if supported by detrital zircon ages. Supplementary material: Paleosol and geochemical data, and youngest zircon ages for a section of Devonian rocks near Ciyao, Gansu, China are available at https://doi.org/10.6084/m9.figshare.c.7008192

Paleoclimatic conditions during the accumulation of the Upper Jurassic and Lower Cretaceous organic carbon-rich deposits of the Ulyanovsk-Saratov trough (eastern Russian platform)

Relevance of the work. Black shale horizons may reflect abrupt paleoclimate changes. In the northeast of the Ulyanovsk-Saratov Trough (UST), two black shale horizons widely occur, the Middle Volgian Promzino Formation and the Lower Aptian Ulyanovsk Formation. Despite years of research, the role of paleoclimate in their accumulation is still under discussion. The aim of the research was to assess the climate as a factor of the accumulation of the Middle Volgian and Lower Aptian organic carbon-rich deposits in the northeastern UST. Methods. To reconstruct paleoclimate and to evaluate its impact on the black shale deposition, a comprehensive isotope study of calcitic and aragonitic shells of several groups of benthic and pelagic organisms was performed. Besides, the lithological and geochemical features of organic carbon-rich and host deposits were studied using optical microscopy, XRD, and pyrolysis. Results. Isotope study allowed to construct benthic and pelagic palaeotemperature curves and to reconstruct paleoclimate in the UST during the formation of black shales. Based on these data and data on lithological and geochemical features of sediments, the depositional mechanisms of the Promzino and Ulyanovsk Formations were clarified. Conclusions. No causal link has been revealed between climate changes and the deposition of the Middle Volgian black shales. At the end of the Middle Volgian, a short-term cooling has been recorded, probably related to the volcanic activity of the northern Tethyan margin. The Lower Aptian black shales were formed during the hyperthermal. This hyperthermal controlled the accumulation of organic carbon-rich sediments by increasing the input of nutrients and/or providing conditions for its stratification and stagnation.

Long-term Aptian marine osmium isotopic record of Ontong Java Nui activity

AbstractThe early to mid-Aptian was punctuated by episodic phases of organic-carbon burial in various oceanographic settings, which are possibly related to massive volcanism associated with the emplacement of the Ontong Java, Manihiki, and Hikurangi oceanic plateaus in the southwestern Pacific Ocean, inferred to have formed a single plateau called Ontong Java Nui. Sedimentary osmium (Os) isotopic compositions are one of the best proxies for determining the timing of voluminous submarine volcanic episodes. However, available Os isotopic records during the age are limited to a narrow interval in the earliest Aptian, which is insufficient for the reconstruction of long-term hydrothermal activity. We document the early to mid-Aptian Os isotopic record using pelagic Tethyan sediments deposited in the Poggio le Guaine (Umbria-Marche Basin, Italy) to precisely constrain the timing of massive volcanic episodes and to assess their impact on the marine environment. Our new Os isotopic data reveal three shifts to unradiogenic values, two of which correspond to black shale horizons in the lower to mid-Aptian, namely the Wezel (herein named) and Fallot Levels. These Os isotopic excursions are ascribed to massive inputs of unradiogenic Os to the ocean through hydrothermal activity. Combining the new Os isotopic record with published data from the lowermost Aptian organic-rich interval in the Gorgo a Cerbara section of the Umbria-Marche Basin, it can be inferred that Ontong Java Nui volcanic eruptions persisted for ∼5 m.y. during the early to mid-Aptian.

Geochemical Indicators of “Camouflaged” Pyroclastic Material in the Upper Jurassic–Lower Cretaceous Deposits of the Eastern Russian Platform

Based on the results of studying the lithological–geochemical features of the Upper Jurassic–Lower Cretaceous mudrocks and black shales in the Eastern Russian Platform, the continuous distribution of pyroclastic material in the whole section is established. Possible sources of acid pyroclastics were active volcanoes of the Transcaucasian ensialic island arc. The explosions of the Sanandaj–Senjan ensimatic island arc (in the Late Jurassic) and the Arctic Large Igneous Province (in the Aptian) could have been the source of basaltic pyroclastics in the black shale horizons. The highest activity of the Transcaucasian and Elburs island arcs in northwest Tethys was apparently recorded during the Albian, which determined the maximum content of felsitic material in the Middle Albian bentonite-like mud rocks.

Size variations and abundance patterns of calcareous nannofossils in mid Barremian black shales of the Boreal Realm (Lower Saxony Basin)

Abstract We present new quantitative calcareous nannofossil data from a Barremian (Early Cretaceous) black shale interval of the Boreal Realm. Thirty samples from two sections in northern Germany have been investigated. The current study aims to understand the genesis of the Barremian black shales, so far only described from the Boreal Realm, in a supraregional to global context by assessing the diversity patterns, relative abundances and biometry of calcareous nannofossils from a distinctive late early Barremian black shale horizon. For morphometric size measurements, three common taxa, Biscutum constans, Rhagodiscus asper and Watznaueria barnesiae have been investigated. The results show a decrease in size and abundance of the high fertility indicator B. constans for the main black shale horizon (Hauptblatterton), while abundances of the warm water species R. asper and Watznaueria spp. increase. Deep dwelling nannoconids only flourished during black shale deposition. This indicates warm, oligotrophic surface waters and an expanded photic zone during the formation of the Hauptblatterton black shales. Similar observations have been made for the Oceanic Anoxic Event 1d (OAE 1d, Albian) in the Vocontian Basin (Southern France) and for the OAE 2 (Cenomanian) in the Western Interior Seaway and southern England. For W. barnesiae our data confirm a high ecological tolerance and we assume a response in size if a certain limit is exceeded towards more extreme conditions. Nutrients therefore likely play a larger role than water temperature. From our results, it seems likely that nutrient availability controls both, size and abundance patterns of B. constans.

Diagenetic and environmental control of the clay mineralogy, organic matter and stable isotopes (C, O) of Jurassic (Pliensbachian-lowermost Toarcian) sediments of the Rodiles section (Asturian Basin, Northern Spain)

Abstract Clay mineralogical and geochemical analyses (δ18Ocarb, and paired carbonate and organic matter carbon isotope), completed by the characterization of organic matter, have been performed on the Pliensbachian sediments exposed on the Rodiles section (Asturian Basin, Northern Spain). The objectives were to precise the Pliensbachian climate evolution at a relatively low paleolatitude of the Tethyan domain by comparison with previously published high-resolution clay mineralogical and geochemical data from the NW Tethyan domain (e.g. Paris Basin and Cardigan Bay Basin, West Wales). The clay mineral assemblages are dominantly composed of illite and illite-smectite mixed-layers (I–S) associated with minor proportions of chlorite and kaolinite. Smectites are not identified what may result either to their absence at the time of sedimentation or to their progressive illitization due to the burial diagenesis. Rock-Eval data show that organic matter reaching more than 5% in black-shale horizons is thermally mature with Tmax comprised between 444 and 454 °C indicating that the section is in the oil window and that the burial temperature reached or exceeded 80–90 °C. The low δ18Obulk values comprised between −3 and −6‰ clearly results from a diagenetic influence (thermal effect due to burial) but their fluctuations compared with δ18O measured on well-preserved belemnites guards show that the overall trend is however preserved. By comparison with higher latitude sedimentary basins, the most striking feature is the very small proportions of kaolinite (generally less than 5% while the proportions of this mineral reach 40% at higher paleolatitudes), except at the Pliensbachian/Toarcian transition, where the proportion of kaolinite increases significantly. Clay minerals likely originated from the erosion of the Iberian Massif located to the South of the Asturian Basin in a semi-arid climatic belt. This may explain the scarcity of kaolinite by comparison with sedimentary basins located at higher latitude influenced by more humid climate. The increasing proportions of kaolinite by the end of the Pliensbachian suggest southward expansion of the humid belt as a consequence of cooler temperatures and the establishment of steeper latitudinal gradients of temperature likely resulting from the formation of restricted polar ice cap. The main δ13C (on both 13Cinorg and 13Corg) Pliensbachian excursions including the negative excursion of the Sinemurian/Pliensbachian Boundary Event (SPBE) and the Late Pliensbachian positive excursion are well recognized.

Storm-induced concentration of sulfurized, marine-origin, organic matter as a possible mechanism in the formation of petroleum source-rock

Abstract Black shales, though laminated, are not systematically synonymous with quiet conditions of deposition. A number of papers report about black shales yielding sedimentary structures echoing relatively high hydrodynamic conditions. Here we examine two sedimentary sequences pertaining to the Late Jurassic Argiles de Châtillon Formation of the Boulonnais area (Northernmost France), each of them including a meter-thick black-shale horizon. The two laminated black shales contain sulfurized (i.e., organic S-rich) organic matter, reaching a maximum of 9%. However, some differences set the two black shales apart. The lower one was deposited under calm, suboxic to anoxic, bottom-water conditions (the « classic » way); the upper black shale was deposited under oxic, agitated bottom-water conditions, not compatible with stable sulfidic conditions required for organic-matter sulfurization. The upper black shale experienced hydrodynamically-induced concentration of sulfurized, recalcitrant organic matter of marine origin. The sulfurized organic matter could be preserved and quantitatively accumulated owing to its non-putrescible nature, leading to the formation of a potential hydrocarbon source rock. The association of pyrite inclusions with sulfurized organic matter probably modified the hydrodynamic behavior of organic particles. The storm-induced remobilization and concentration of sulfurized organic matter implies that the “sulfurization factory” operated in proximal, shallow environments such as estuaries, mud flats or mangrove environments. Such a model is in agreement with findings from modern coastal/shelf environments (notably mangroves) and fully transposable to many other sedimentary situations of any geological time period.

CRETACEOUS OCEANIC ANOXIC EVENTS IN WESTERN CONTINENTAL GREECE

Integrated chemostratigraphy and biostratigraphy in Cretaceous pelagic carbonate successions and associated organic sediments of the Ionian basin (western Greece) show the first documentation of the Cenomanian Turonian (OAE)2 and Lower Albian (OAE)1b Oceanic Anoxic Events from western Greece. Preliminary study of the Pindos basin (western Greece) has also identified a black shale horizon which may corresponds to the Lower Albian (OAE)1b Oceanic Anoxic Event.

Vase‐shaped microfossils from the Tonian Callison Lake Formation of Yukon, Canada: taxonomy, taphonomy and stratigraphic palaeobiology

AbstractVase‐shaped microfossils (VSMs), interpreted as the remains of testate amoebae, are found in late Tonian sedimentary rocks around the world. Here we explore the taxonomy, taphonomy and stratigraphical occurrence of VSMs from the Callison Lake Formation of the Coal Creek inlier, Yukon, Canada. Found in silicified black shale horizons and stromatolitic dolostone, sedimentological data suggest these VSMs inhabited a series of marine embayments characterized by lagoonal and/or shelf interior depositional environments. The fossiliferous strata have recently been dated with Re–Os geochronology at c. 753–740 Ma, which indicates they are not only coeval with diverse VSM assemblages described in the Chuar Group of Grand Canyon, Arizona, but also provides supportive evidence for the early diversification of eukaryotic clades prior to the Sturtian age Snowball Earth event (c. 717–660 Ma). Petrographic examination of well‐preserved specimens reveal taxa comparable to those from the Chuar Group, as well as two previously undescribed species. Species overlapping with Chuar Group VSMs are Bonniea dacruchares, Bonniea pytinaia, Cycliocyrillium simplex, Cycliocyrillium torquata, Melanocyrillium hexodiadema and Palaeoarcella athanata. New taxa described here are Bonniea makrokurtos and Cycliocyrillium rootsi. Energy dispersive x‐ray spectroscopic data reveal that the Callison Lake microfossils are preserved through a variety of taphonomic pathways, including silicification, infilling, authigenic mineralization and dolomitization. We explore the utility of M. hexodiadema as a latest Tonian biostratigraphical marker and examine the role of Callison Lake sequence stratigraphy as a control on the distribution and abundance of VSMs in the Coal Creek inlier and other global sedimentary successions.

Upper Kellwasser carbon isotope excursion pre-dates the F–F boundary in the Upper Devonian Lennard Shelf carbonate system, Canning Basin, Western Australia

Abstract Here we report four high-resolution carbon isotope records in addition to trace element data for the Frasnian–Famennian (F–F) boundary interval in the Lennard Shelf carbonate system of the Canning Basin, Western Australia. This region lacks the characteristic black shale horizons associated with the global Late Devonian Kellwasser extinction events, yet still exhibits a trend in carbon isotope character similar to what has been reported from elsewhere in the world (two positive δ13C excursions with ~ 3–4‰ amplitudes). Enrichments in select trace element ratios suggest that both excursions are related to periods of oxygen deprivation and perhaps increased biological productivity. Given the continuous and stratigraphically expanded nature of Lennard Shelf sections, together with high-density sampling constrained by both conodont biostratigraphy and magnetostratigraphy, we observe that the Upper Kellwasser isotope excursion (maximum δ13C values) and associated trace element enrichments occur distinctly lower than the F–F boundary level. These results have implications for the paleoenvironmental conditions leading up to the Late Devonian Mass Extinction in terms of ocean chemistry and circulation patterns. This data set allows for a rare, detailed look at the temporal relationship between the Kellwasser events and the F–F boundary and constrains the pattern of carbon isotope perturbations at the intra-zonal scale.

Drowning of the Triassic Yangtze Platform, South China, By Tectonic Subsidence Into Toxic Deep Waters of An Anoxic Basin

Abstract The Yangtze Platform drowned and was buried by pelagic facies and siliciclastic turbidites in western Guizhou Province during the Late Triassic, Carnian. The uppermost platform facies of the Ladinian Yangliujing Fm. consists of peritidal cyclic carbonate. Ladinian margin facies of the Longtou Fm. consist of grainstone and lenses of coral–Tubiphytes algal boundstone indicating high-energy shoals and patch reefs. The drowning horizon is laterally variable; it is either a sharp surface or a gradational shift to dark, nodular-bedded lime mudstone and wackestone of the Zhuganpo Fm. The contact lacks phosphatized or glauconitized hardgrounds that would indicate drowning by excess nutrient flux. Uppermost platform carbonates have a tropical photozoan biota and lack siliciclastic content indicating neither climate cooling nor siliciclastic flux played a role in drowning. The Zhuganpo Fm. contains a dominantly pelagic biota of ammonoids, planktonic bivalves, pseudoplanktonic crinoids, conodonts, and the marine reptile Keichousaurus. Rare bioturbation and benthic biota in the lower part indicate dysoxic conditions with an upward shift to anoxic conditions. The overlying Wayao Fm. is a pyritic, laminated, black argillaceous lime mudstone that grades to black shale with an exclusively pelagic fauna and marine reptile lagerstattes indicating anoxic deep-marine sedimentation. Syndepositional faults played a significant role in the evolution of the western sector of the Yangtze Platform and controlled three local, backstepped accommodation cycles in the Zhenfeng area. Faults that developed during the last accommodation cycle tip out at the drowning horizon and include a flower structure upon which a pinnacle reef developed while the rest of the platform drowned. Lateral variability in the drowning horizon and thickness of the post-drowning pelagic facies suggest that differential tectonic subsidence caused the platform to sink into deep water along faults. Magnetic susceptibility and paleomagnetic-reversal correlations demonstrate that the western sector of the platform drowned while shallow-marine, mixed carbonate siliciclastic sedimentation continued in the eastern sector which later terminated in shallow water by increasing siliciclastic flux. Starved black shale horizons in the basin indicate recurrent bottom-water anoxia. Elevated trace-metal concentrations in the pelagic facies of the Zhuganpo and Wayao formations indicate dysoxic to anoxic conditions and enhanced preservation of organic matter. Tectonic subsidence likely submerged the western sector into deep, toxic waters of the basin, killing benthic carbonate production. Previous studies have implicated the Carnian pluvial event as a cause of drowning of the Yangtze Platform and the shift to siliciclastic turbidites of the Laishike Fm., but the Yangtze Platform drowned earlier than western Tethys platforms and siliciclastics were already rapidly infilling the basin in the Anisian and Ladinian, indicating that the events are probably unrelated. Siliciclastics of the Laishike Fm. that overlapped and buried the platform only after the basin was filled are better interpreted as a response to tectonics than to a shift in climate.

Freshwater discharge controlled deposition of Cenomanian–Turonian black shales on the NW European epicontinental shelf (Wunstorf, northern Germany)

Abstract. Global warming, changes in the hydrological cycle and enhanced marine primary productivity all have been invoked as having contributed to the occurrence of widespread ocean anoxia during the Cenomanian–Turonian oceanic anoxic event (OAE2; ~94 Ma), but disentangling these factors on a regional scale has remained problematic. In an attempt to separate these forcing factors, we generated palynological and organic geochemical records using a core spanning the OAE2 from Wunstorf, Lower Saxony Basin (LSB; northern Germany), which exhibits cyclic black shale–marl alternations related to the orbital precession cycle. Despite the widely varying depositional conditions complicating the interpretation of the obtained records, TEX86H indicates that sea-surface temperature (SST) evolution in the LSB during OAE2 resembles that of previously studied sites throughout the proto-North Atlantic. Cooling during the so-called Plenus Cold Event interrupted black shale deposition during the early stages of OAE2. However, TEX86 does not vary significantly across black shale–marl alternations, suggesting that temperature variations did not force the formation of the cyclic black shale horizons. Relative (i.e., with respect to marine palynomorphs) and absolute abundances of pollen and spores are elevated during phases of black shale deposition, indicative of enhanced precipitation and run-off. High abundances of cysts from inferred heterotrophic and euryhaline dinoflagellates supports high run-off, which likely introduced additional nutrients to the epicontinental shelf resulting in elevated marine primary productivity. We conclude that orbitally forced enhanced precipitation and run-off, in tandem with elevated marine primary productivity, were critical in cyclic black shale formation on the northern European epicontinental shelf and potentially for other OAE2 sections in the proto-Atlantic and Western Interior Seaway at similar latitudes as well.

Marine palynology of the Oceanic Anoxic Event 3 (OAE3, Coniacian – Santonian) at Tarfaya, Morocco, NW Africa – transition from preservation to production controlled accumulation of marine organic carbon

The paper presented is the first comprehensive, fully quantitative, high resolution study of marine palynology from an OAE3 black-shale environment. It is based on 175 m core spanning the upper Turonian to lower Santonian at Tarfaya, Morocco, NW Africa, which has been sampled from centimetre to 3 m intervals. The results are integrated and discussed with lithology and geochemistry data to (1) distinguish between potential changes in production and preservation of total organic carbon (TOC) accumulation and (2) constrain the stratigraphic position of the Oceanic Anoxic Event 3 (OAE3).The succession is characterized by increased total organic carbon (TOC), varying between 1% and 19% (average about 6%). Distinct black-shale horizons of variable thickness appear episodically throughout the succession, with higher frequency in the late Turonian. Higher TOC contents do not strictly correlate to lithologic black-shales or peaks of a specific taxon of organic-walled algae. The palynomorph spectrum is strongly dominated by organic-walled algae, with the ratio of terrigenous sporomorphs to organic-walled algae (t/m index) varying between zero and 0.05 (average 0.01). The dominance of algal organic matter is corroborated by the prevalence of Type I kerogen identified using Rock-Eval pyrolysis. Dinocyst diversity is low, with the absolute taxa number varying from 7 to 27 between single samples. The peridinioid/gonyaulacoid ratio of dinocysts (p/g ratio) shows strong fluctuations, varying between 1 and 283 (average of about 100).The upper Turonian interval is dominated by Bosedinia spp., a dinocyst taxon formerly described as abundant only in lacustrine sediments from the Oligocene and Miocene of SE Asia. This dominance is episodically modified by the increase of the warm-temperate waters dinocysts fraction, here mainly represented by the genera Alterbidinium, Isabelidinium and Spinidinium. Within the Coniacian-Santonian, black-shale horizons are limited in number and are concentrated within the upper Coniacian to lower Santonian interval. The dinocysts show alternating, prominent peak abundances of Palaeohystrichophora spp. and the warm-temperate water dinocysts fraction, here mainly represented by the genera Trithyrodinium and Chatangiella. However, a final episode of increased proportions of Bosedinia spp. is confined to a 5 m thick black-shale horizon closely spanning the Coniacian-Santonian boundary.Changes in the ratio of total sulphur to total organic carbon (TS/TOC) reflect fluctuating oxygen contents of bottom waters throughout the late Turonian to Santonian. These are significantly parallelled by the alternation of dinocysts assemblages suggestive of enhanced upwelling and water column stratification respectively, probably reflecting changes in the mode of TOC accumulation. Accordingly, preservation largely prevails during the late Turonian interval and changes towards increased production within the Coniacian-Santonian. However, a final preservation-event is probably represented by the black-shale horizon closely spanning the Coniacian-Santonian boundary (top Dicarinella concavata foraminifera zone), which may reflect an episodic shutdown of a major upwelling cell. It is thus proposed, that the “culmination” of the OAE3 at Tarfaya may represent intermittent preservation of TOC within an otherwise high productivity environment related to a global cooling trend.

The expression of the Hangenberg Event (latest Devonian) in a relatively shallow-marine succession (Pomeranian Basin, Poland): the results of a multi-proxy investigation

AbstractThe uppermost Famennian – lowermost Tournaisian interval has been analysed in detail using biostratigraphy, sedimentology, magnetic susceptibility and geochemistry in a reference section of the relatively shallow carbonate ramp environment within the Pomeranian Basin. High-resolution biostratigraphic study, based on miospores, allows recognition of the standard western Europeanlepidophyta–nitidus(LN) andverrucosus–incohatus(VI) zones, as well as theConvolutispora majorZone, a local Pomeranian equivalent of the European standardhibernicus–distinctus(HD) Zone. The sedimentary succession and specific phenomena recognized close to the Devonian/Carboniferous boundary, such as fluctuations in water column euxinia, wildfire evidence, relative sea-level changes and perturbations of the carbon cycle reflected by positive carbon excursions, display a pattern partly similar to that observed in many areas in Europe during the Hangenberg Event, although the Hangenberg Black Shale horizon is not developed here. These important microscale environmental perturbations were observed not only within the Famennian LN miospore Zone but in a wide interval between the LN and the lowermost localConvolutispora majormiospore zones ( = lower part of HD standard miospore Zone). It is still uncertain whether the recognized event(s) were connected solely with the Hangenberg Event, which was possibly complex and multi-phased as is sometimes suggested, or whether they represent a succession of regionally limited, post-Hangenberg events. This question needs to be further investigated on broader stratigraphic and geographical scales.

Depositional conditions and petroleum potential of the Middle Triassic Passhatten Member (Bravaisberget Formation), Spitsbergen

Abstract The Passhatten Member (Anisian-Ladinian) is the most westward exposure of the Middle Triassic sedimentary sequence of Spitsbergen. The member has an average organic carbon of 2.21 wt %. The sediments were deposited in a shallow shelf environment under conditions of high biological productivity stimulated by a well−developed upwelling system and an enhanced nutrient supply from land areas. The high biological productivity caused a high supply of organic particles to the shelf bottom. Decomposition of organic matter initiated oxygen deficiency in the bottom waters; however, bottom water dynamics on the shallow shelf temporarily replenished the oxygen. Consequently, the Passhatten Mb section is bioturbated, even in thick black shale horizons and consists of alternately spaced lithological layers with variable organic carbon content. The organic matter is dominated by type II kerogen with a mixture of strongly altered marine and/or land derived organic matter. Calculated initial hydrogen index values suggest oil−prone organic matter similar to kerogen I and II types. The organic matter is in the upper intermediate stage of thermo−cata− lytic alteration, close to the cata- and metagenetic boundary. Maturity indicators including Rock−Eval, Maximum Temperature, Organic Matter Transformation Ratio, Residual Car− bon content, as well as the volume of methane generated suggest mature to overmature organic matter. Methane potential retained in the black shales sequence is significant. Unexpelled gas is estimated at 395 mcf/ac−ft for the examined section.

The neglected history of Oceanic Anoxic Event 1b: insights and new data from the Poggio le Guaine section (Umbria–Marche Basin)

The upper Aptian to lower Albian interval (~114–109 Ma) represents a crucial period during Earth’s history, with a major evolution in the nature of mid–Cretaceous tectonics, sea level, climate, and marine plankton communities. Interestingly, it also includes multiple prominent black shale horizons that are the sedimentary expression of oceanic anoxic event (OAE) 1b. However, due to a set of geological, stratigraphic, and taxonomic challenges, difficulties constraining this OAE may sometimes result in inaccurate correlation. At Poggio le Guaine (central Italy), a continuous and undisturbed section of central western Tethyan pelagic sediments deposited during the latest Aptian and earliest Albian, complete with black shale horizons that record the OAE1b carbon cycle perturbations, has provided a unique opportunity to address the shortcomings referred to above. High–resolution geochemical proxies (CaCO3, TOC, and δ13C), along with planktonic foraminifera and calcareous nannofossils, were used to establish an integrated and robust global stratigraphic framework in order to: (1) precisely correlate the prominent black shale horizons of OAE1b in different marine environments (the Vocontian Basin in southeast France, Deep Sea Drilling Project Site 545 and Ocean Drilling Program Hole 1049C in the eastern and western North Atlantic, respectively); and (2) provide an effective tool to reconstruct, in high resolution, the paleobiological, paleoceanographic and paleoclimatic changes across the upper Aptian to lower Albian interval. The Poggio le Guaine section stands out as a valuable reference section for OAE1b, its constituent subevents, and the δ13C record for the uppermost Aptian to lower Albian interval. The exceptional chemo- and biochronostratigraphic control allow direct comparison to the proposed candidate for the GSSP for the base of the Albian Stage at the Col de Pré-Guittard section in France.

Black shale deposition during Toarcian super-greenhouse driven by sea level

Abstract. One of the most elusive aspects of the Toarcian oceanic anoxic event (T-OAE) is the paradox between carbon isotopes that indicate intense global primary productivity and organic carbon burial at a global scale, and the delayed expression of anoxia in Europe. During the earliest Toarcian, no black shales were deposited in the European epicontinental seaways, and most organic carbon enrichment of the sediments postdated the end of the overarching positive trend in the carbon isotopes that characterises the T-OAE. In the present study, we have attempted to establish a sequence stratigraphic framework for Early Toarcian deposits recovered from a core drilled in the Paris Basin using a combination of mineralogical (quartz and clay relative abundance) and geochemical (Si, Zr, Ti and Al) measurements. Combined with the evolution in redox sensitive elements (Fe, V and Mo), the data suggest that expression of anoxia was hampered in European epicontinental seas during most of the T-OAE (defined by the positive carbon isotope trend) due to insufficient water depth that prevented stratification of the water column. Only the first stratigraphic occurrence of black shales in Europe corresponds to the "global" event. This interval is characterised by >10% Total Organic Carbon (TOC) content that contains relatively low concentration of molybdenum compared to subsequent black shale horizons. Additionally, this first black shale occurrence is coeval with the record of the major negative Carbon Isotope Excursion (CIE), likely corresponding to a period of transient greenhouse intensification likely due to massive injection of carbon into the atmosphere–ocean system. As a response to enhanced weathering and riverine run-off, increased fresh water supply to the basin may have promoted the development of full anoxic conditions through haline stratification of the water column. In contrast, post T-OAE black shales during the serpentinum and bifrons Zones were restricted to epicontinental seas (higher Mo to TOC ratios) during a period of relative high sea level, and carbon isotopes returning to pre-T-OAE values. Comparing palaeoredox proxies with the inferred sequence stratigraphy for Sancerre suggests that episodes of short-term organic carbon enrichment were primarily driven by third-order sea level changes. These black shales exhibit remarkably well-expressed higher-frequency cyclicities in the oxygen availability in the water column whose nature has still to be determined through cyclostratigraphic analysis.

Mo–Cr isotope evidence for a reducing Archean atmosphere in 3.46–2.76 Ga black shales from the Pilbara, Western Australia

The distribution of redox-sensitive elements and their stable isotope variations in marine sediments has been employed to track the possible oxygenation of the atmosphere at, and before, the Great Oxidation Event (GOE; 2.4–2.2Ga). Contrasting datasets have been used to advocate for and against the rise of free oxygen in the atmosphere prior to 2.4Ga based on various geochemical tracers, and evidence for a partially oxidized environment remains elusive.Herein, we present stable isotope datasets for Mo and Cr, in conjunction with major and trace elements from four black shale horizons spanning a time interval from 3.46 to 2.76Ga in order to ascertain atmospheric oxygen levels through the Archean. The Mo, Cr and U elemental signatures within all sedimentary units are dominated by continental input. Both Mo isotopic values and Mo and U elemental signatures suggest anoxic conditions with no indication of redox cycling of these elements. This contradicts previous interpretations of an oxidized atmosphere based on published sulfur isotopic data (Ohmoto et al., 2006) and oxidized hematite occurrences at ~3.45Ga (Hoashi et al., 2009).Cr concentration patterns within the sample suites are controlled by continental provenance composition rather than by an authigenic Cr seawater contribution. This interpretation is supported by the relatively homogeneous Cr isotopic compositions across all sample suites and is consistent with high temperature controlled Cr isotopic ratios. Identification of redox cycling processes within detrital dominated sediments with Cr isotopes is likely to be less sensitive compared to Mo isotopes, due to the much smaller authigenic/crustal enrichment factors for Cr.

Environmental hazards of arsenic associated with black shales: a review on geochemistry, enrichment and leaching mechanism

Black shales are high organic matter-rich dark coloured mudstones those are often deposited during ‘oceanic anoxia events’. Most of the black shale horizons are rich in arsenic far above their average crustal abundance and are susceptible to weathering eventually leaching high As contents to the surrounding environment causing As enrichment in soil and water which adversely affect the living beings. Numerous arsenic contaminations are being reported from black shale hosted areas globally, hence, making extremely crucial to understand the processes of enrichment, leaching and broader prospective of environmental hazards. Few studies have shown arsenic concentrations as high as 6,000 mg/kg within black shales causing groundwater enrichment up to hundreds mg/L. Arsenic is commonly attached to sulphide mineral structure and partly to organic matter and clay contents during deposition and diagenetic processes. Majority of sulphide bound arsenic becomes available to oxidative dissolution processes in presence of atmospheric oxygen and water which is further triggered by certain microbial community such as Acidophilus ferrooxidans hence, enhancing arsenic release. Physical weathering processes carry the arsenic-rich shale constituents to the depositional site where it is dissolved subsequently. Chemical diffusion and mechanical transport are two prime processes transporting arsenic from black shale horizons to the water bodies or soil columns, while air pollutions are caused by combustions of organic matter-rich coaly shales.