Carbon and oxygen-isotope stratigraphy of the Early Cretaceous carbonate platform of Pădurea Craiului (Apuseni Mountains,Romania): A chemostratigraphic correlation and paleoenvironmental tool

Stable C and O isotope records were obtained from carbonate rocks spanning the Hauterivian to Cenomanian interval collected in several sections from the carbonate platform of Pădurea Craiului (Apuseni Mountains, Romania). In the absence of some key biostratigraphic marker species, stable isotopes were applied as a tool for stratigraphic correlation and dating. The composite δ¹³C and δ¹⁸O curves for the Early Cretaceous shows variable conditions with large positive and negative excursions and provide information on past environmental changes. The Hauterivian and the Barremian limestones (Blid Formation) display lower δ¹³C values (−2.8‰ to +2.9‰) relative to the Aptian–Albian deposits (−2.6‰ to +5.4‰) (Ecleja, Valea Măgurii and Vârciorog Formations). The red detrital formation (Albian–Cenomanian) is characterized by a highly variable distribution of the δ¹³C values (−3.5‰ to +3.9‰). Based on the similarities between the C-isotope curve established in Pădurea Craiului and from other sections in the Tethyan and the Pacific regions, two major oceanic anoxic events characterized by δ¹³C positive excursions were clearly recognized. The first is the OAE1a event (Early Aptian) in the upper part of the Ecleja Formation and the Valea Măgurii Formation. The second is the OAE1b event (Late Aptian–Albian) in the upper part of the Vârciorog Formation and in the Subpiatră Member. The position of the Aptian/Albian boundary is estimated to be at the upper part of the Vârciorog Formation, immediately after the beginning of the δ¹³C positive excursion. The δ¹³C data show major negative excursions during the Barremian (Blid Formation), Early Aptian (Ecleja Formation), and Late Aptian (Vârciorog Formation). The O isotope variation pattern (−10.2‰ to −2.1‰) is consistent with progressively warming temperatures during the Early Barremian followed by a cooling period. A subsequent warming period culminated in the Early Aptian. A significant cooling phase corresponds to the Late Aptian and Early Albian and the climate cooled again during the Late Albian and into the Early Cenomanian stage. The data provide a better understanding of the Early Cretaceous sedimentation cycles in Pădurea Craiului and create a more reliable framework for regional correlations.     

Geochemistry of the late Cenomanian-early Turonian chalks of the Paris Basin: Manganese and carbon isotopes in carbonates as paleooceanographic indicators

In the North Atlantic DSDP/IPOD cores, carbon isotope data on the bulk carbonates show significant fluctuations. In sediments now exposed on land coeval fluctuations in the carbon isotope concentrations are also recorded in pelagic and epeiric facies. For instance, in the Upper Cretaceous chalks of the Paris Basin, there is a major break at the Cenomanian-Turonian boundary. At this time, the manganese content of the chalks was also at a maximum and consequently a positive relation can be demonstrated between δ¹³C and manganese concentrations. The same positive correlation is also recorded in many pelagic limestones.In the North Atlantic cores, carbon isotope events are related to the black shale facies and to global oceanic anoxic events and one can suppose that in sediments deposited on the continental margins they are also related to mildly anoxic conditions. Considering the manganese geochemistry in carbonate rocks, a high manganese content in such a reducing environment can be found in the sediments only if the Mn concentration of the interstitial solutions are abnormally high. As a high Mn content in marine pore waters is believed to originate from hydrothermal process, Mn and δ¹³C positive excursions are ultimately related to mid-oceanic ridge activity and to a closely connected phenomenon, the great transgressive pulses during which mid-depth waters may have been anoxic. Consequently, major Mn and carbon isotope events would seem to be useful tools in paleooceanographic reconstructions.     

Carbon and oxygen isotope compositions of Lower-Middle Ordovician carbonate rocks in the northwestern Russian platform

Carbon and oxygen isotope compositions of Lower-Middle Ordovician carbonate rocks in the northwestern Russian Platform (eastern Ladoga Klint, Lynna River, and Babino quarry sections) are considered. In the studied section interval, average δ¹³C and δ¹⁸O values are 0 ± 0.5 and ?5 ± 0.5‰ (V-PDB), respectively. Two closely-spaced negative carbon isotope excursions with the amplitude of 2‰ are established near the Lower-Middle Ordovician boundary (between the Floian and Dapingian stages). The lower part of the Darriwilian Stage is marked by the gradual decrease in δ¹³C values to 1‰. Excursions of δ¹³C do not correlate with δ¹⁸O variations and can be considered as primary. The carbon isotope event defined at the Lower-Middle Ordovician boundary is traceable at the interregional level and represents a promising stratigraphic reference level. It may likely be explained by decrease in the relative rate of organic matter burial due to sea level fall and expansion of well-aerated shallow-water basins with a low primary production of phytoplankton.     

Lacustrine sedimentary record of early Aptian carbon cycle perturbation in western Liaoning,China

The early Aptian abrupt carbon isotope excursion in marine carbonate and sedimentary organic matter reflects a major perturbation in the global carbon cycle. However, until now almost all the evidences of this event came from marine deposits. Here we present organic-carbon isotope (δ¹³C_org) data from the non-marine Jehol Group in western Liaoning, China. The lacustrine δ¹³C_org curve is marked by a relative long-lasting δ¹³C_org minimum followed by two stages of positive δ¹³C_org excursions that are well correlated with contemporaneous marine records. The carbon isotope correlation shows that the lacustrine strata of the Jehol Group were deposited at the same time of the early Aptian Oceanic Anoxic Event (OAE1a). The relative long-lasting δ¹³C_org minimum supports the hypothesis that volcanic CO₂ emission may have played the main role in triggering the negative δ¹³C excursion and global warming at the onset of this event. In addition, the onset of δ¹³C_org minimum is concomitant with the radiation of the Jehol Biota, implying that the evolutionary radiation of the Jehol Biota may have been closely related with the increase in atmospheric CO₂ and temperature.     

Impact of methane seeps on the local carbon‐isotope record: a case study from a Late Jurassic hemipelagic section

An Oxfordian (Late Jurassic) hemipelagic succession from Beauvoisin (SE France) contains a pronounced, short‐lived negative excursion in the bulk‐carbonate carbon‐isotope record, with an amplitude of 4‰. It was shown previously that the Beauvoisin paleoenvironment was impacted by hydrocarbon seepage. New isotopic data corroborate that methane was a significant constituent of these hydrocarbons. The negative excursion was caused by transient enhanced precipitation of ¹³C‐depleted carbonate, mediated by anaerobic oxidation of methane. Despite its local diagenetic origin, the Beauvoisin excursion is similar in shape and duration to globally recognized negative C‐isotope excursions that have been related to catastrophic, massive dissociation of methane hydrate. Shape and duration of negative excursions therefore cannot be used as an argument when determining their origin if they have not been shown to represent a global perturbation of the carbon cycle.     

Ascription of the island of Ibiza to the Prebetic of Alicante (Betic Range,Spain) after correlation of Lower Cretaceous sedimentary successions

The lithostratigraphic succession of the Tithonian – Albian interval of the island of Ibiza shows a great similarity with that of the Internal Prebetic Zone in the Alicante area (Betic Range), with only slight differences in age and stratigraphic distribution. This similarities are based on the correlation of the following units: i) the Punta Jondal Formation of Ibiza with the Sierra del Pozo Formation of Alicante (Tithonian – early Valanginian); ii) the Port Sant Miquel Formation (Aptian) with the Arroyo de los Anchos Formation; iii) the Torre des Molar (early Aptian), Penyal de s’Águila (late Aptian) and Cala d'en Sardina (late Aptian) members of the Port Sant Miquel Formation with the Llopis, Almadich and Seguilí formations in Alicante; and iv) the Es Cubells Formation (Tithonian – earliest Cenomanian) with several marly units of the Prebetic of Alicante.The Ibiza Tithonian – Albian sedimentary succession was deposited within a NNW–SSE trending basin related to the Tethyan domain of SE Iberia. It is organized in three sedimentary successions (named Aubarca, San José and Ibiza successions, from NNW to SSE), which were tectonically stacked towards the NNW during the Alpine inversion of the basin. These sedimentary successions were deposited within the distal regions of a carbonate platform opened towards the southeast. In the SE sector of the island, the Ibiza succession is characterized by a thick and rhythmic alternation of basinal marls and marly limestones. Northwestwards, the San José succession is characterized by the presence of inner platform carbonate deposits at the base of the succession (Tithonian – early Valanginian). Finally, the presence of shallow-water rudist-bearing limestones (Aptian) in the northwestern sector, defines the Aubarca succession. The NNW–SSE evolution of the stratigraphic architecture from the Aubarca – San José – Ibiza successions is clearly similar to the tectonostratigraphic and palaeogeographic N–S zoning previously recognized from the Sierra de Mariola – Cabezón de Oro – Fontcalent successions in the Prebetic of Alicante, respectively.Stratigraphical sequence analysis of the sedimentary successions of the island of Ibiza allows recognizing a depositional stacking pattern defined by four long-term depositional megasequences, which can also be correlated with equivalent megasequences in the Prebetic of Alicante. The three lower megasequences (Tithonian – Albian) show a transgressive–regressive evolution, revealed by the deposition of transgressive hemipelagic facies in the lower part and the development of prograding shallow-water carbonate platforms during regressions. The fourth megasequence (Albian) is not as well developed as the previous megasequences, showing siliciclastic levels instead of the shallow-water carbonate platform facies, thus suggesting a development during major sea-level fall. Nevertheless, in the Ibizan successions, high resolution sequence stratigraphy and accurate biostratigraphic scales have not yet been established; consequently, the chronostratigraphy of megasequence boundaries and the maximum flooding surfaces are less accurate than in their Prebetic counterparts.     

广西弗拉阶—法门阶之交碳同位素与分子地层对比研究

对广西 7条碳酸盐台地、斜坡和盆地相剖面的碳同位素与分子地层的对比研究表明 ,在牙形石生物地层带或偏心率轨道旋回层建立的等时地层格架内 ,跨越弗拉阶—法门阶 (F—F)之交的碳同位素组成在 1 4Ma内不具一致性 ,表现为正偏、负偏和无偏 3种模式。主要的分子化石包括正构烷烃、类异戊二烯烃、萜类、甾类 ;其母体生物源主要为海源浮游植物、浮游动物、底栖非光合作用的菌类和陆源高等植物。其中浮游植物和浮游动物是构成F—F事件期碳酸盐台地、斜坡和盆地相生物量的主体。分子地层参数及其与碳同位素的关系显示 ,F—F之交广西海域高温、高盐、缺氧、多风 ;Pr Ph与δ1 3C曲线的变化表现为负相关。地层的加积方式 (加积、进积和退积 )、堆积速率、缺氧程度、分子化石类型和丰度是影响广西F—F之交碳同位素组成变化的主要因素。退积序列、快速堆积、缺氧程度和有机质埋藏量增加通常对应δ1 3C值增加。由于分子地层参数能提供生物与环境这两大直接影响碳同位素组成的量化信息 ,因此 ,碳同位素与分子地层的对比研究对正确解释碳同位素的特征和成因具有重要意义     

Aptian palaeoclimates and identification of an OAE1a equivalent in shallow marine environments of the southern Tethyan margin: Evidence from Southern Tunisia (Bir Oum Ali section,Northern Chott Chain)

Alternations between siliciclastic, carbonate and evaporitic sedimentary systems, as recorded in the Aptian mixed succession of southern Tunisia, reflect profound palaeoceanographic and palaeoclimatic changes in this area of the southern Tethyan margin. The evolution from Urgonian-type carbonates (Berrani Formation, lower Aptian) at the base of the series, to intervals dominated by gypsum or detrital deposits in the remainder of the Aptian is thought to result from the interplay between climate change and tectonic activity that affected North Africa.Based on the evolution of clay mineral assemblages, the early Aptian is interpreted as having been dominated by slightly humid conditions, since smectitic minerals are observed. Near the early to late Aptian boundary, the onset of a gypsiferous sedimentation is associated with the appearance of palygorskite and sepiolite, which supports the installation of arid conditions in this area of the southern Tethyan margin. The evaporitic sedimentation may have also been promoted by the peculiar tectonic setting of the Bir Oum Ali area during the Aptian, where local subsidence may have been tectonically enhanced linked to the opening of northern and central Atlantic. Stress associated with the west and central African rift systems may have triggered the development of NW–SE, hemi-graben structures. Uplifted areas may have constituted potential new sources for clastic material that has been subsequently deposited during the late Aptian.Chemostratigraphic (δ¹³C) correlation of the Bir Oum Ali succession with other peri-Tethyan regions complements biostratigraphic findings, and indicates that a potential expression of the Oceanic Anoxic Event (OAE) 1a may be preserved in this area of Tunisia. Although the characteristic negative spike at the base of this event is not recognized in the present study, a subsequent, large positive excursion with δ¹³C values is of similar amplitude and absolute values to that reported from other peri-Tethyan regions, thus supporting the identification of isotopic segments C4–C7 of the OAE1a. The absence of the negative spike may be linked to either non preservation or non deposition: the OAE1a occurred in a global transgressive context, and since the Bir Oum Ali region was located in the innermost part of the southern Tethyan margin during most of the Aptian, stratigraphic hiatuses may have been longer than in other regions of the Tethys. This emphasizes the importance of integrating several stratigraphic disciplines (bio-, chemo- and sequence stratigraphy) when performing long-distance correlation.     

Black shales and carbon isotopes in pelagic sediments from the Tethyan Lower Jurassic

Detailed sampling and analysis of Jurassic pelagic limestones and marls from Italy, Hungary and Switzerland have enabled construction of an isotope stratigraphy across the Pliensbachian-Toarcian boundary with resolution to the zonal level. The oxygen-isotope record is unremarkable. The carbon isotopes, however, show two positive excursions: one, relatively minor, during the Pliensbachian, margaritatus Zone, subnodosus Subzone, the other, more major, during the Toarcian. early falciferum Zone, where a maximum δ¹³C value of 4·52%_PDB is attained. These intervals are known to be favoured periods of organic-rich sedimentation in diverse parts of the globe and the isotopic excursions are interpreted as a response to abnormally high rates of storage of organic carbon in the sedimentary record. A comparable phenomenon has been documented from the Cenomanian-Turonian boundary in the Cretaceous where it has been referred to the influence of an ‘Oceanic Anoxic Event’. Some Italian sections spanning this Lower Jurassic interval contain organic-rich shales in the falciferum Zone; the isotopic signatures from their included, locally manganiferous carbonate betray a considerable diagenetic overprint and they cannot therefore be incorporated in a composite isotopic curve. Carbon isotopes from the organic carbon itself are extremely negative, falling to –33δ_PDB and, in one section examined in detail, correlate with the calcium-carbonate content of the shales; they may reflect a partial change to a non-calcified planktonic biota during deposition of this lime-poor interval, possibly responding to upwelling and increased fertility of near-surface waters. The onset of upwelling may have been as early as spinatum-tenuicostatum Zone time, that is, at the Pliensbachian-Toarcian boundary.     

A large epeiric methanogenic Bambuí sea in the core of Gondwana supercontinent?

Carbon isotope compositions of both sedimentary carbonate and organic matter can be used as key proxies of the global carbon cycle and of its evolution through time,as long as they are acquired from waters where the dissolved inorganic carbon(DIC)is in isotope equilibrium with the atmospheric CO2.However,in shallow water platforms and epeiric settings,the influence of local to regional parameters on carbon cycling may lead to DIG isotope variations unrelated to the global carbon cycle.This may be especially true for the terminal Neoproterozoic,when Gondwana assembly isolated waters masses from the global ocean,and extreme positive and negative carbon isotope excursions are recorded,potentially decoupled from global signals.To improve our understanding on the type of information recorded by these excursions,we investigate the pairedδ^13Ccarb andδ^13Corg evolution for an increasingly restricted late Ediacaran-Cambrian foreland system in the West Gondwana interior:the basal Bambui Group.This succession represents a 1~(st)-order sedimentary sequence and records two majorδ^13Ccarb excursions in its two lowermost lower-rank sequences.The basal cap carbonate interval at the base of the first sequence,deposited when the basin was connected to the ocean,hosts antithetical negative and positive excursions forδ^13Ccarb andδ^13Corg,respectively,resulting inΔ^13C values lower than 25‰.From the top of the basal sequence upwards,an extremely positiveδ^13Ccarb excursion is coupled toδ^13Corg,reaching values of+14‰and-14‰,respectively.This positive excursion represents a remarkable basin-wide carbon isotope feature of the Bambui Group that occurs with only minor changes inΔ^13C values,suggesting change in the DIC isotope composition.We argue that this regional isotopic excursion is related to a disconnection between the intrabasinal and the global carbon cycles.This extreme carbon isotope excursion may have been a product of a disequilibria between the basin DIC and atmospheric CO2 induced by an active methanogenesis,favored by the basin restriction.The drawdown of sulfate reservoir by microbial sulfate reduction in a poorly ventilated and dominantly anoxic basin would have triggered methanogenesis and ultimately methane escape to the atmosphere,resulting in a^13C-enriched DIC influenced by methanogenic CO2.Isolated basins in the interior of the Gondwana supercontinent may have represented a significant source of methane inputs to the atmosphere,potentially affecting both the global carbon cycle and the climate.     

Stable carbon‐isotope record of shallow‐marine evaporative epicratonic basin carbonates,Ordovician Williston Basin,North America

Secular variations in stable carbon‐isotope values of marine carbonates are used widely to correlate successions that lack high‐resolution index fossils. Various environmental processes, however, commonly may affect and alter the primary marine carbon‐isotope signal in shallow epicratonic basins. This study focuses on the marine carbon‐isotope record from the carbonate–evaporite succession of the upper Katian (Upper Ordovician) Red River Formation of the shallow epicratonic Williston Basin, USA. It documents the carbon‐isotope signal between the two major Ordovician positive shifts in δ¹³C, the early Katian Guttenberg and the Hirnantian excursions. Eight δ¹³C stages are identified based on positive excursions, shifts from positive to negative values and relatively uniform δ¹³C_carb values. A correlation between carbon‐isotope trends and the relative sea‐level changes based on gross facies stacking patterns shows no clear relation. Based on the available biostratigraphy and δ¹³C trends, the studied Williston Basin curves are tied to the isotope curves from the North American Midcontinent, Québec (Anticosti Island) and Estonia, which confirm the Late Katian age (Aphelognathus divergens Conodont Zone) of the upper Red River Formation. The differences in the δ¹³C overall trend and absolute values, coupled with the petrographic and cathodoluminescence evidence, suggest that the carbon‐isotope record has been affected by the syndepositional environmental processes in the shallow and periodically isolated Williston Basin, and stabilized by later burial diagenesis under reducing conditions and the presence of isotopically more negative fluids.     

Development of carbonate platforms on an extensional (rifted) margin: the Valanginian–Albian record of the Prebetic of Alicante (SE Spain)

Within the upper Valanginian to upper Albian deposits of the easternmost part of the Prebetic Zone of the Betic Cordillera (Iberian Peninsula), seven lithostratigraphic formations made up of shallow-water carbonate and carbonate-siliciclastic sediments and of outer-platform hemipelagic sediments have been recognized. These formations were deposited in the most distal part of a platform that developed on the Southern Iberian Continental Palaeomargin. The geodynamic context was a margin affected by extensional or transtensional faults that produced tilted blocks. The interval studied records three major second-order transgressive-regressive facies cycles: (I) A late Valanginian to earliest Aptian cycle, mostly represented by hemipelagic and condensed sedimentation, with the development of a tectonically controlled high without sedimentation that separated two sectors with different sedimentary evolution and that ended with an episode of shallow-water carbonate platform development; (II) An earliest to latest Aptian cycle, with a transgressive phase represented by a retrogradational shallow-water carbonate platform capped by a drowning event leading to hemipelagic sedimentation, which was affected by an anoxic event (OAE 1a); the regressive phase is represented by progradation and aggradation of shallow-water carbonate deposits. Finally (III) a latest Aptian to early-late Albian cycle that records the expansion of mixed platform deposits in the entire area, ending with a phase of shallow-water carbonate platform development. Extensional tectonics leading to spatial and temporal changes in subsidence patterns is envisaged as the main control on sedimentation at a local scale, resulting in notable lateral changes in thickness as the main signature. Tectonics exerted a strong control on the distribution of sedimentary environments only during Cycle I. At a higher order, sea-level fluctuations are responsible for sequential organization, and environmental factors determined shallow-water carbonate platform development and demise, as well as oceanic anoxic events. The relevant continuity of the stratigraphic record in the distal part of the Prebetic platform has led to the recognition of events related to cycle boundaries, which result mainly from a combination of tectonics and sea-level changes.     

Carbon-isotope stratigraphy in shallow-water carbonates: implications for Cretaceous black-shale deposition

A Barremian to Albian succession on Mount Kanala, part of a Tethyan isolated carbonate platform, was investigated for its δ¹³C variations. The limestone sequence is composed of a series of peritidal shallowing-upward cycles with clear petrographic evidence for strong early diagenetic overprinting related to repeated subaerial exposure. Despite significant impact of diagenesis, the observed changes in δ¹³C can be very well correlated with deep-water sections from different ocean basins and shallow water carbonate platforms in the Middle East. This lends further support to the applicability of δ¹³C variations for stratigraphic purposes in shallow-water limestones. Using the δ¹³C signal, time resolution in Lower Cretaceous platform carbonates can be significantly increased, independent of bio-zonations often hampered by ecological variability.
Cyclostratigraphic analysis of the Aptian part of the section shows that strong positive excursions of the cumulative departure from mean cycle thickness of the peritidal shallowing-upward cycles coincide with global positive δ¹³C excursions. This, and the fact that positive shifts in the δ¹³C record are preserved within shallow water limestones, provide evidence that black-shale accumulation in the ocean basins occurred during sea-level rise and flooding of platform tops. Integration of carbon-isotope-, cyclo- and sequence-stratigraphic results from different carbonate platforms indicate that strong positive global δ¹³C shifts and concurrent organic-carbon burial during black-shale deposition are ultimately caused by rapid rises of eustatic sea level. Hence, the rate of change of eustatic sea level is considered to play a crucial role in black-shale accumulation in the global ocean basins during the Cretaceous.     

Carbon and strontium isotope stratigraphy of the Upper Cretaceous (Cenomanian-Campanian) shallow-water carbonates of southern Italy: Chronostratigraphic calibration of larger foraminifera biostratigraphy

Shallow-water carbonates are invaluable archives of past global change. They hold the record of how neritic biologic communities reacted to palaeoenvironmental changes. However, attempts to decipher these geological archives are often severely hampered by the low stratigraphic resolution attained by biostratigraphy. This is particularly the case for the Upper Cretaceous carbonate platforms of the central Tethyan realm: their biostratigraphy suffers from very low resolution and poor correlation with the standard biochronologic scales based on ammonites, planktic foraminifers and calcareous nannoplankton.In this paper we show how this problem can be tackled by integrating biostratigraphy with isotope stratigraphy. We present a detailed record of the benthic foraminiferal biostratigraphy and carbon and strontium isotope stratigraphy of three upper Cenomanian-middle Campanian sections belonging to the Apennine Carbonate Platform of southern Italy. For the upper Cenomanian-Turonian interval, the carbon isotope curves of the studied sections are easily correlated to the reference curve of the English Chalk. The correlation is facilitated by the matching of the prominent positive excursion corresponding to the Oceanic Anoxic Event 2. For the Coniacian-middle Campanian interval, the correlation is mainly based on strontium isotope stratigraphy. We use the ⁸⁷Sr/⁸⁶Sr ratios of the low-Mg calcite of well preserved rudist shells to obtain accurate chronostratigraphic ages for many levels of the three studied sections. The ages obtained by Sr isotope stratigraphy are then used to better constrain the matching of the carbon isotope curves.From the high-resolution chronostratigraphic age-model stablished by isotope stratigraphy, we derive the chronostratigraphic calibration of benthic foraminiferal biostratigraphic events. For the first time the benthic foraminiferal biozones of the Apennine Carbonate Platform can be accurately correlated to the standard ammonite biozonation. This result is of great relevance because the biostratigraphic schemes of other carbonate platforms in the central and southern Tethyan realm are largely based on the same biostratigraphic events.     

The eroded Late Jurassic Kurbnesh carbonate platform in the Mirdita Ophiolite Zone of Albania and its bearing on the Jurassic orogeny of the Neotethys realm

Several Late Jurassic (Kimmeridgian?-Tithonian) to Early Cretaceous (Late Berriasian-Valanginian) shallow-water carbonate clasts of different facies are contained in mass-flow deposits in a pelagic sequence in the Kurbnesh area of central Albania. These clasts are used to reconstruct shallow-water carbonate platforms, which formed on top of the radiolaritic-ophiolitic wildflysch (ophiolitic mélange) of the Mirdita Zone. Stratigraphic interpretation of the platform carbonates was compiled on basis of calcareous algae, benthic foraminifera, and calpionellids. From biostratigraphic data and microfacies analysis, the Neocomian clasts can be directly correlated with autochthonous platform carbonates of the western part of the Munella carbonate platform, which at least reaches up to the Late Aptian. A Late Jurassic precursor platform (Kurbnesh carbonate platform; nomen novum) was completely eroded until the Valanginian and is only documented by the clasts described here. It was deposited on top of the Mirdita Ophiolite Zone nappe stack, which formed during the Middle to Late Jurassic Kimmeridian orogeny. Thrusting and imbrications as well as the formation of the syntectonic wildflysch (mélange) therefore occurred much earlier than previously assumed. Our results constrain the Kimmeridian orogeny, which was controlled by the closure of the Neotethys Ocean, and show excellent correlation with the Eastalpine-Dinaric- Hellenic orogenic system.     

Carbon-isotope and microfaunal stratigraphy bounding the Lower Aptian Oceanic Anoxic Event 1a in northeastern Tunisia

Using an integrated multidisciplinary approach the upper part of the Lower Cretaceous northeastern Tunisian Jebel Ammar sedimentary succession was examined in detail. The method applied included lithologic and microfacies analyses, micropalaeontology, sedimentology, variations in organic matter (OM) content and carbonate carbon stable isotope (δ¹³C) record. A major result was the identification in this sector of Tunisia of the Early Aptian (Bedoulian) OAE1a event in a biostratigraphically well-calibrated context, its location keyed to planktonic foraminiferal zones and isotopic stages.The most striking feature to the Jebel Ammar Aptian sequence is the presence of a 25 m interval of black limestones and marly limestones, of which the microfacies shows that these darker beds consist of wackestones with the presence of abundant radiolarians, a fair number of diversified planktonic and relatively rare benthic foraminifers, together an indication of a pelagic palaeoenvironment. The foraminiferal marker Schackoina (Leupoldina) cabri (still very rare at the beginning of its range) first occurs about six metres above the base of this interval, but becomes much more abundant in its uppermost part, together with the radially- elongate-chambered praehedbergellids. This “acme” of S. (L.) cabri is nearly contemporary with a radiolarian bloom. Rock Eval analyses show TOC values up to 4.59% and a Tmax ranging between 441 and 513 °C, which indicates an overmature OM. The δ¹³C isotope curve shows an evolution similar to those recognized worldwide. The lower part of the darker beds includes a marked shift in isotope values from −2.40 to +3.02 ‰/PDB. This increase is assumed to be equivalent to the isotopic C4 stage of Menegatti et al. (1998, Paleoceanography, 13, 530–545). The signature of the middle and upper part of the isotopic curve is tentatively interpreted as comprising the C5–C7 stages. The first occurrence of S. (L.) cabri is located in the upper part of C4 and its acme as well as the radiolarian bloom is situated within the span of the C7 stage. These results are fairly consistent, though with minor discrepancies, with what has been published from several parts of the North Tethyan margin and more particularly the Lower Aptian type area of southeastern France, where S. (L.) cabri first occurs at the C3/C4 stage boundary with an acme that begins in the lower part of C7.     

Identical carbon isotope trends of carbonate and organic carbon and their environmental significance from the Changhsingian (end-Permian), Meishan,South China

High resolution carbon isotope analyses of carbonate and organic carbon from Meishan, South China showed that the variation of δ13Ccarb is marked by three large positive excursions during the Changhsingian (end-Permian). Carbon isotope stratigraphy during this stage shows three cyclic intervals in δ13Ccarb, with two cycles corresponding to the lower (Paleofusulinid minima Zone) and one corresponding to the upper Changhsingian (P. sinensis Zone). The large positive δ13Ccarb excursions indicate episodes of enhanced burial of isotopically light or-ganic carbon, presumably in response to deep-water anoxia episodically extending into shallow water with the rise of sea level. The organic carbon during the Changhsingian is distinguished into two groups, and the δ13Corg of each group parallels (separately) the more detailed profile of δ13Ccarb, strongly showing that the values of fractionation Δ13Ccab-org remain relatively constant, with only two intervals with anomaly. The enhanced fractionation Δ13Ccab-org with large negative δ13Corg excursions apparently indicates significant inputs from sulfide-oxidizing bacteria and green sulfur bacteria, notably at bed 24 just predating mass extinction. Our evidence appears to support that the ex-tended euxinic water is possible for the main pulse of mass extinction at the end-Permian.     

Ocean current intensification during the Cretaceous oceanic anoxic event 2 – evidence from the northern Tethys

Mesozoic Oceanic Anoxic Events (OAEs) are expressions of major physical oceanographic changes at times of perturbation of the global carbon cycle. A northern Tethyan record of OAE2 is preserved in expanded Cenomanian–Turonian pelagic limestone sections (Seewen Formation) in Eastern Switzerland. The new carbonate carbon‐isotope stratigraphy extracted from these limestones demonstrates that the OAE2 is condensed in all the studied successions and only the onset of the δ¹³C excursion (5.0‰) is present. The condensed interval is characterized by dissolution features, which are filled by a glauconite quartz sandstone. This bed is overlain by a well‐sorted sandstone with intercalated limestone pebbles (Götzis Member), which can be compared with palimpsest sands forming today along current‐swept shelves. The wide distribution of this thin sandstone layer within OAE2 indicates that an intense, erosive, east‐west trending shelf current was active during the highest sea level and most extreme carbon‐cycle perturbation of the OAE2.     

湖北宜昌樟村坪埃迪卡拉系陡山沱组C同位素变化及成因

以宜昌樟村坪地区万家沟和白鹭垭2个陡山沱组剖面为研究对象,开展了黄陵背斜北缘浅水沉积区埃迪卡拉系陡山沱组C稳定同位素研究。建立了黄陵背斜北缘陡山沱组C稳定同位素曲线,并识别出4次负漂移(ZN1~ZN4),3次正漂移事件(ZP1~ZP3)。其中3次负漂移可以全球对比,1次负漂移(ZN3)在峡东地区可以显著识别。确认了ZN4与DOUNCE事件的对应关系,且该区缺失峡东典型剖面陡山沱组四段。认为ZN1是全球性甲烷渗漏事件的反映;ZN2是区域性海平面下降的反映,与全球WANCE事件有很好的对应关系,而对应年代值要远比Gaskeris冰期时间早;ZN3应当是区域性洋流上升带来贫13C沉积物引起的;ZN4则可以非常好地用埃迪卡拉纪海洋有机碳库氧化来解释。