Negative Carbon Isotope Excursion Research Articles

Volcanically Driven Terrestrial Environmental Perturbations during the Carnian Pluvial Episode in the Eastern Tethys

AbstractThe Carnian Pluvial Episode (CPE) fingerprints global environmental perturbations and biological extinction on land and oceans and is potentially linked to the Wrangellia Large Igneous Province (LIP). However, the correlation between terrestrial environmental changes and Wrangellia volcanism in the Ordos Basin during the CPE remains poorly understood. Records of negative carbon isotopic excursions (NCIEs), mercury (Hg), Hg/TOC, and Hg enrichment factor (HgEF) from oil shales in a large‐scale terrestrial Ordos Basin in the Eastern Tethys were correlated with marine and other terrestrial successions. The three significant NCIEs in the study section were consistently correlated with those in the CPE successions of Europe, the UK, and South and North China. The U‐Pb geochronology indicates a Ladinian–Carnian age for the Chang 7 Member. A comprehensive overview of the geochronology, NCIE correlation, and previous bio‐ and chronostratigraphic frameworks shows that the Ladinian–Carnian boundary is located in the lower part of Chang 7 in the Yishicun section. HgEF may be a more reliable proxy for tracing volcanic eruptions than the Hg/TOC ratio because the accumulation rates of TOC content largely vary in terrestrial and marine successions. The records of Hg, Hg/TOC, HgEF, and NCIEs in the Ordos Basin aligned with Carnian successions worldwide and were marked by similar anomalies, indicating a global response to the Wrangellia LIP during the CPE. Anoxia, a warm‐humid climate, enhancement of detrital input, and NCIEs are synchronous with the CPE interval in the Ordos Basin, which suggests that the CPE combined with the regional Qinling Orogeny should dominate the enhanced rate of terrigenous input and paleoenvironmental evolution in the Ordos Basin.

Disparity between Toarcian Oceanic Anoxic Event and Toarcian carbon isotope excursion

AbstractThe Toarcian Oceanic Anoxic Event (T-OAE, Early Jurassic) is marked by widespread marine deoxygenation and deposition of organic carbon (OC)-rich strata. The genesis of the T-OAE is thought to be associated with environmental changes caused by the emission of 12C-enriched greenhouse gasses (CO2, CH4), manifested in a negative Toarcian carbon isotope excursion (nT-CIE). The nT-CIE is commonly used to stratigraphically define the T-OAE, and despite the complex interrelationship of the different environmental phenomena, both terms (nT-CIE and T-OAE) are commonly used interchangeable. We here demonstrate that occurrence of OC-rich strata is diachronous and not restricted to the nT-CIE, reflecting the interaction of global- and regional-scale processes. Thus, the interchangeable use of T-OAE and nT-CIE should be discarded. The nT-CIE, however, hosts the T-OAE climax, marked by the widest extent of OC-rich strata. Early Toarcian environmental changes, particularly sea level rise and rising temperatures, may have made marine areas more susceptible to develop oxygen deficient conditions, favoring OC-accumulation. Graphical abstract

Planktic foraminifer response and paleoceanographic changes across the Danian – Selandian interval in the southern Tethys (Elles section, Central Tunisia)

The Danian – Selandian interval experienced several paleoceanographic changes with two prominent carbon cycle perturbations: the Dan-C2 and Latest Danian Event (LDE). We present integrated planktic foraminifer biostratigraphy, carbon isotope stratigraphy, and paleoceanographic interpretations from the Danian-Selandian at Elles, central Tunisia. Whereas the response of the planktic foraminifers to these events has been widely explored in deep sea sites, this shallow Tethyan shelf setting is significant for evaluating the expression of these global events in proximal environments. Both the Dan-C2 and LDE events are distinguished in our δ13Cbenthic record by biostratigraphically-constrained negative carbon isotope excursions. Recognition of these reliably dated excursions enables assessment of bioevent synchroneity. The lowest occurrence of Subbotina triloculinoides was diachronous at DanC2, but the highest occurrence of Praemurica was broadly synchronous at the onset of the LDE. Subsequent lowest occurrence of Igorina albeari shows latitudinal diachroneity between ocean basins. The planktic foraminifer assemblage composition indicates mesotrophic and stratified conditions in the early Danian, warming and increasing oligotrophy during the P2 Zone, peak oligotrophy and enhanced stratification across the LDE, shallowing-induced disruption of the upper water column niches in the latest Danian, and recovery with high diversity in the early Selandian. The Dan-C2 excursion shows a small 0.3‰ negative δ13C shift coinciding with increased abundance of low oxygen tolerant biserial Chiloguembelina, suggesting slightly stressed conditions. In contrast, the larger 0.6‰ LDE excursion coincides with the extinction of Praemurica and proliferation of Morozovella, reflecting an oligotrophic surge with warming in the surface ocean.

Middle Eocene Climatic Optimum (MECO) and its imprint in the continental Escanilla Formation, Spain

Abstract. The Middle Eocene Climatic Optimum (MECO) is a global warming event mainly recognized in the marine domain and described less in the terrestrial environment. Here we present a comprehensive geochemical record of the MECO from the Escanilla Formation, a fluvial sedimentary succession in the southern Pyrenees, Spain, based on a suite of sampled paleosols, fluvial stromatolites, and pedogenic nodules. Our multiproxy approach involves using carbon and oxygen stable isotope compositions to identify the regional preservation of the MECO, calculate chemical weathering intensity and mean annual precipitation, perform clumped isotopes on carbonates, and identify clay mineralogy assemblages of paleosols. Results indicate that the Middle Eocene interval in the southern Pyrenees was characterized by low weathering rates under warm and arid climatic conditions. This is further supported by the presence of smectite, palygorskite, illite, and chlorite, which suggest seasonal rainfall but under generally dry conditions resulting in weak chemical weathering. Importantly, a negative organic carbon isotopic excursion indicates the regional, terrestrial impact of the MECO, highlighting that fluvial sedimentary successions even in active foreland basins can represent particularly interesting terrestrial archives of past changes in global climate.

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.

Early Jurassic extrinsic solar system dynamics versus intrinsic Earth processes: Toarcian sedimentation and benthic life in deep-sea contourite drift facies, Cardigan Bay Basin, UK

The Cardigan Bay Basin (UK) may have functioned as a deep and narrow strait, and thereby influenced Early Jurassic oceanic circulation through the northern and southern Laurasian Seaway, and between Boreal and Peri-Tethys domains. Toarcian hemipelagic deposits of the basin in the Mochras borehole show strongly bioturbated contourite facies. Trace fossils are strongly dominated by Phycosiphon incertum (represented by four morphotypes), which was produced by opportunistic colonizers. Thalassinoides, Schaubcylindrichnus and Trichichnus are common (the latter is a deep-tier trace fossil produced by filamentous sulfide-oxidizing bacteria with a high tolerance for dysoxia), accompanied by less common Zoophycos, Planolites, Palaeophycus, Teichichnus, Rhizocorallium, Chondrites, and dwelling and resting structures, such as cf. Polykladichnus, Siphonichnus, Skolithos, Arenicolites, Monocraterion and Lockeia. Ichnological and lithological signals suggest repetitive fluctuations in benthic conditions attributed to a hierarchy of orbital cycles (precession and obliquity [4th order], short eccentricity [3rd order], long eccentricity [2nd order] and Earth–Mars secular resonance [1st order]). The Pliensbachian–Toarcian transition appears to be a significant palaeoceanographic turning point in the Cardigan Bay Basin, starting a CaCO3 decline, and with the most severe oxygen crisis of the Tenuicostatum Zone (here dysoxic but not anoxic) ending at the onset, in the early Serpentinum Zone (Exaratum Subzone), of the Toarcian negative carbon isotope excursion (To-CIE—linked with the Toarcian Oceanic Anoxic Event occurring in the lower part in the Serpentinum Zone). This trend contrasts with the prevalence of anoxia synchronous with the To-CIE in many other settings. Minor dysoxia returned to the Mochras setting in the latest Thouarsense to Dispansum zone interval. Extreme climate warming during the To-CIE may have enhanced and caused a reversal in the direction of deep marine circulation, improving oxygenation of the sea floor. Spectral analysis of binary data on ichnotaxa appearances gives high confidence in orbital signals and allows refined estimation of ammonite zones and the duration of the Toarcian (minimum ~ 9.4 Myr).

Strontium isotope evidence for regional enhanced continental weathering during the early Toarcian in the Tethys Himalaya

The early Toarcian of the Early Jurassic was characterized by an interval of global warming, enhanced continental weathering, and an oceanic anoxic event (T-OAE, ∼183 Ma). However, the changes in continental weathering intensity in low latitudes still remain poorly constrained. Here, we present strontium isotope (87Sr/86Sr) data of the Pliensbachian–Toarcian (Pl–To) interval from the Tibetan Himalaya, which was formerly located in the tropical/subtropical zone of the southeastern Tethys. An abrupt positive shift in 87Sr/86Sr starting around the Pl–To boundary and continuing during the interval of the negative carbon-isotope excursion (CIE) suggests enhanced continental weathering and increased terrigenous influx. Stratigraphically higher, the 87Sr/86Sr ratios gradually decrease during the recovery phase of the T-OAE CIE, suggesting a decline in continental weathering. Notably, the absolute values and positive-recovery pattern of 87Sr/86Sr data from bulk carbonate (pure micrite) of this study are not consistent with that from belemnites and brachiopods of Europe. Based on strong relationship between 87Sr/86Sr and diagenetic and silicate-derived Sr proxies, the absolute 87Sr/86Sr values here could have been affected to some extent by diagenesis and regional terrestrial input, and thus greatly amplified relative to those of Europe. However, the strontium-isotope positive-recovery pattern broadly corresponds with the pattern and interpretation of other weathering proxies (δ44/40Ca and 187Os/188Os) during the T-OAE. These observations suggest that the 87Sr/86Sr trend still likely reflects the real regional signal but was superimposed by global influence, although the data were biased by diagenesis. Taken together, the 87Sr/86Sr records from Tibetan Himalaya probably provide a useful insight into the regional weathering response, superimposed by the global weathering signal, to the events of the early Toarcian.

The Barremian–lower Aptian of NE Arabia: The Kharaib and Shu'aiba formations in Wadi Rahabah and Wadi Kebdah, Ras Al Khaimah, U.A.E.

We present a high-resolution record of carbonate depositional facies, biostratigraphy, stable-isotope and elemental geochemistry of the Kharaib and Shu'aiba formations exposed in Ras Al Khaimah, northern UAE. This is the first comprehensive study of these deposits in the northeastern Arabian Peninsula, which have previously been studied extensively in Abu Dhabi, Oman, NE Saudi Arabia, and southwestern Iran, and represents the most detailed biostratigraphic and chemostratigraphic record currently available from a continuous outcrop section of the Arabian Plate. The Barremian–Aptian boundary was approximated based on orbitolinid biostratigraphy, while a subdivision of the Barremain was not possible. There is no biostratigraphic evidence that the Shu'aiba Formation in the section studied ranges into the upper Aptian. Previously well-established depositional sequences of the Barremian–lower Aptian of the Arabian Plate are discussed in the context of carbonate textures, siliciclastic influx, and biotic assemblages. There is a general trend of decreasing siliciclastic components during the Barremian and earliest Aptian, and peaks in siliciclastic influx indicate episodes of maximum accommodation. Palaeogeographic implications of our data for the distribution of intrashelf basins and their connection to Neotethys are discussed. The negative carbon-isotope excursion that globally marks the onset of OAE1a is not evident in our dataset, while the positive excursion at the end of OAE1a is present in the late early Aptian. A comparison of carbon-isotope records across the early Aptian of the Arabian Plate highlights issues of correlations that are predominantly based on chemostratography in diagenetically altered platform carbonates.

Origin of Redbeds in the Neoproterozoic Socheong Formation and Their Relation to the Dashigou Large Igneous Province

During the latest Mesoproterozoic–Early Neoproterozoic era, extensional regimes generated a number of sedimentary basins in various regions in the Sino-Korean Craton. Mantle-plume emplacements are widely recognized in the sedimentary strata as mafic dikes and sills of the Dashigou Large Igneous Province (LIP). The occurrence of Fe-rich redbeds is first reported in the Neoproterozoic Socheong Formation of the Sangwon Supergroup in the Pyeongnam Basin. Their geochemical and mineralogical characteristics indicate basin-wide Fe enrichment due to hydrothermal fluid input. The episodic yet repetitive hydrothermal injection into the basin generated short-lived anoxia, recorded as greenish-gray coloration in the ferruginous beds. This hydrothermal fluid was likely sourced from the mafic igneous activities involved in the Dashigou LIP. The redbeds can be utilized as key beds for intra- or inter-basinal stratigraphic correlation and to study the negative carbon isotope excursions that occurred in the genetically related basins in the region (e.g., the Sangwon, Xu-Huai, and Dalian basins).

Orbitally-paced climate change during the Carnian Pluvial Episode

Global climate change has profound implications for human survival and prosperity. The Carnian (early Late Triassic, ∼233 Ma) was a time of global environmental perturbations, climatic change, and biotic turnover, commonly known as the Carnian Pluvial Episode (CPE). However, the understanding of possible triggering mechanisms of the CPE is still partially untraveled due to the lack of a high-resolution chronostratigraphic framework. Here we present an astrochronology of episodic negative carbon isotope excursions (NCIEs) observed in the shallow marine sediments of the Bagong Formation in the Qiangtang Basin (Tibetan Plateau, China) to explore the role of orbital forcing during the CPE. This study marks the first identification of five episodic NCIEs in the Carnian strata of the Bagong Formation. These NCIEs correspond with five distinct periods of increased detrital influx. The coupling relationship between each phase of NCIE and related terrigenous input indicates that episodic NCIEs may be influenced by pulses of active continental weathering. Mercury isotope is contemporaneous with the active volcanism that triggered the onset of the NCIEs; however, the impact of volcanism in the Qiangtang Basin was weak and almost non-existent at this time. An anchored floating astronomical timescale (ATS) for the episodic NCIEs is established, revealing evidence for a 405 kyr eccentricity in high-resolution gamma ray data series using time series analysis. A ∼13.16 Myr-long ATS of the Bagong Formation is developed by astronomical tuning of gamma ray logs to the stable 405-kyr long-eccentricity cycles. This floating ATS takes the U-Pb zircon as its anchor point and establishes an anchored floating ATS of the Bagong Formation from 233.16 ± 1.37 Ma to 220.4 ± 1.1 Ma. The innovative astrochronology approach, employing a sedimentary noise model, has successfully reconstructed sea-level changes during the Late Triassic period. These findings align well with the documented global sea-level fluctuations of the same era. The antiphase relationship of the filtered ∼1.2 Myr cycles between the sedimentary noise model sea-level curve and the obliquity modulation cycles demonstrates that the ∼1.2 Myr modulation cycles may be the main driver of sea level changes during the Late Triassic. The ∼1.2 Myr obliquity modulation maxima correlate well with the high sea level, episodic NCIEs, global warming, and marine life crisis, suggesting that obliquity forcing could have played a prominent role during the CPE. Our results reveal that the orbital forcing enhanced the hydrological cycle during the CPE, which provides a broader perspective of the CPE-related to the astronomical forcing.

Four volcanically driven climatic perturbations led to enhanced continental weathering during the Late Triassic Carnian Pluvial Episode

The arid climate of the Late Triassic was interrupted by a particularly humid episode known as the Carnian Pluvial Episode (CPE; ca. 234–232 million years ago). The CPE is often linked to eruptions in the Wrangellia Large Igneous Province (LIP), and is assumed to have led to global warming, enhanced weathering, water deoxygenation, and biotic changes. However, direct evidence for a temporal link between volcanic activity and chemical weathering has not yet been established due to the lack of comprehensive records across the CPE. In this study, geochemical and mineralogical analyses are applied to a lacustrine stratigraphic succession of the Jiyuan Basin (North China) that captures the CPE in high resolution. We identify four distinct pulses of enhanced continental chemical weathering characterized by elevated Chemical Index of Alteration values and kaolinite contents. These peaks in continental weathering coincide with Hg/TOC enrichments and negative organic carbon isotope excursions that mark four short (∼400 kyr) but intense pulses of Wrangellia LIP volcanism. In combination with signs of increased humidity, our findings provide direct and independent evidence that Wrangellia LIP eruptions significantly altered CPE chemical weathering rates in response to global warming and wetting. The lake experienced eutrophication and water deoxygenation after each volcanic pulse but the swift recovery of carbon isotopes suggests that the system rapidly returned to conditions prior to the volcanic perturbation. Organic carbon burial facilitated by widespread dysoxic and anoxic waters, and CO2 consumption via enhanced weathering likely played crucial roles in the rapid climatic recovery after each volcanic pulse.

Life through an Ediacaran glaciation: Shale- and diamictite-hosted organic-walled microfossil assemblages from the late Neoproterozoic of the Tanafjorden area, northern Norway

New organic-walled microfossil (OWM) assemblages are reported from upper Neoproterozoic glacial and interglacial siliciclastic deposits in Finnmark, northern Norway. A nearly continuous sedimentary succession of the Vestertana Group contains two glaciogenic units, the Smalfjorden and Mortensnes formations, interpreted as end-Cryogenian Marinoan and Ediacaran glaciations, respectively. We investigated the OWM record in the Nyborg, Mortensnes, and Stáhpogieddi formations to assess the impact of a glacial interval on the diversity of microscopic eukaryotes. A modified acid-extraction technique was applied to recover OWM from the diamictite matrix. The upper Nyborg Formation contains morphologically complex Doushantuo-Pertatataka acritarchs (DPA), restricting the age of the Nyborg Formation to early-mid Ediacaran. DPA occur below the dolostones that record a negative carbon isotope excursion correlated with the Shuram anomaly and below a glacial diamictite. A decline in species richness and compositional change is observed in the Mortensnes glacial assemblage. DPA are replaced by bacterial filaments and cell aggregates. The overlying Indreelva Member, Stáhpogieddi Formation contains Ediacara-type biota and palaeopascichnids, but only a depauperate OWM assemblage of leiosphaerids and flask-shaped microfossils characteristic of the late Ediacaran.The succession of assemblages in the Vestertana Group demonstrates a turnover from large eukaryotic OWM to a microbial community in the glacial interval, to a low diversity post-glacial assemblage during the rise of macroscopic life. We compared the Vestertana record to global DPA occurrences. Although one DPA assemblage zone postdates the Shuram excursion, no DPA occur above Ediacaran glacial diamictites in successions where those deposits are present. Considering this, and the community changes in the Vestertana succession, we suggest that DPA were affected by the onset of an Ediacaran glaciation. Lastly, we combined the biostratigraphic markers in the Vestertana Group to constrain the age of the Mortensnes diamictite.

Enhanced Denudation of the Emeishan Large Igneous Province and Precipitation Forcing in the Late Permian

AbstractHydroclimate is an important factor controlling landscape evolution. But establishing the impact of hydroclimate is complicated by the influences of other processes and is especially hard to prove for those in deep time from geological record. During the late Permian, voluminous basaltic sediments were derived from the erosion of the Emeishan large igneous province in western South China. They provide a unique record critical in understanding the responses of tropical basaltic erosion to hydroclimate change without impacts of orogenic uplift, lithologic variation, vegetational difference and glacial‐interglacial change. Sampled successions define a negative carbon isotope excursion capable of making regional and global stratigraphic correlations in the middle Wuchiapingian interval corresponding to the final phase of the late Paleozoic ice age. This interval is associated with a petrofacies shift and a decreasing source weathering intensity, a downward shift of erosion loci, and a reduced coastal water paleosalinity. Applying present‐day temperature dependence pattern of basaltic weathering and using land surface temperatures approximated from nearby paleo‐seawaters, denudation rates were calculated for the Emeishan basaltic province and shows an increase from ∼71 to ∼107 m/Ma. This erosional acceleration is temporally correlated with a decrease in paleosalinity and likely linked to enhanced freshwater discharge in the middle Wuchiapingian. Scaling and landscape erosion modeling suggest ∼80%–130% increase in catchment precipitation could have driven this acceleration in denudation. Our work provides a positive test for the hydroclimate forcing on landscape erosion in deep time and underlines the mechanistic linkage of sediments with erosion and climate change.

New Data on the Expression of the Early Eocene Hyperthermals in Northern Bulgaria: Riben Section Revisited

We present new lithologic, nannofossil biostratigraphic and benthic carbon and oxygen isotope records for a marly-rich, 11 m thick Paleocene-Eocene section, now exposed at an old quarry at Riben Village, northern Bulgaria. The occurrence of NP9, NP10 and NP11 nannofossil zones is evidenced by detailed high-resolution nannofossil biostratigraphy. The carbon dissolution interval is marked by the abrupt drop of CaCO3 content and deposition of a clay interval at the onset of the Paleocene-Eocene Thermal Maximum (PETM). Stable isotope analyses of δ13C and δ18O were performed on benthic-foraminiferal tests. An unusual two-fold carbon isotope excursion (CIE) in the PETM interval is documented (total negative shift of 2.89‰). Moreover, three paired negative excursions: two in the Paleocene within the PETM interval, upper NP9 zone, and one within the early Eocene, NP11 zone are identified. The most prominent negative carbon isotope excursion (4.90‰ negative shift) is related with ETM3 (or “X” event), 52.84 Ma ago.
 Integration of nannofossil biostratigraphy, benthic stable isotope data sets and lithology unravel two separate early Eocene hyperthermal events: the PETM (= ETM1) and the ETM3 (= “X” event), recorded at Riben section.

Multiple carbon isotope excursions during the Carnian (Late Triassic) pluvial event: implications for global and regional perturbation

The Late Triassic Carnian pluvial event (CPE) was an interval marked by global climatic and environmental changes that occurred simultaneously with enhancement of the hydrological cycle. This event is characterized by multiple negative carbon isotope excursions (NCIEs). However, the driving mechanism behind these multiple NCIEs remains elusive because each of the NCIEs had different magnitudes in different geological settings. In this study, we present a high-resolution record of carbonate carbon isotope ( δ 13 C carb ) and major and trace element data from Well QZ-8 in the Qiangtang Basin, eastern Tethys. The carbon isotope profile from this well displays a similar trend to those for contemporaneous strata in the NW Tethys and South China. This trend is characterized by a distinct negative excursion during the CPE, supporting a global event. Interestingly, our results reveal five NCIEs for the first time in the marine sedimentary succession. Furthermore, each of these NCIEs corresponds well to changes in Ti/Al, Sr/Al and Sr/Ba, suggesting a regional effect of the hydrological cycle on carbon isotope excursions. This study emphasizes that each of the NCIEs was influenced by regional hydrological cycles although the long carbon isotope excursion during the CPE was driven by the global carbon cycle. Supplementary material: Tables showing major element, isotope and trace element data and element ratios, and a supplementary figure are available at https://doi.org/10.6084/m9.figshare.c.6805266

Fossil‐Bearing Concretions of the Araripe Basin Accumulated During Oceanic Anoxic Event 1b

AbstractFossils from the Araripe Basin (northeastern Brazil) are known for their remarkable preservation of vertebrates and invertebrates, even including soft tissues. They occur in carbonate concretions within organic carbon‐rich strata assigned to the Romualdo Formation. Here we present integrated stable isotope, elemental and microfossil records from the Sítio Sobradinho outcrop, Araripe Basin, northeastern Brazil. Our results imply that black shales hosting fossil‐bearing carbonate concretions within the lower Romualdo Formation were deposited during Oceanic Anoxic Event (OAE) 1b (Kilian sub‐event). Our high‐resolution multi‐proxy approach allows identifying four phases of environmental evolution. After a pre‐event phase, an early phase (onset of the negative carbon isotope excursion—nCIE) of water column stratification and reduced oxygenation likely preconditioned the system for organic carbon burial and preservation. A second phase (peak nCIE) was characterized by an intensified hydrological cycle and continental runoff, as well as increased influx of terrestrial organic matter. High input of continent‐derived nutrients might have enhanced biological productivity in the epicontinental sea, ultimately leading to increased organic carbon fluxes and burial, as well as carbonate dissolution at the seafloor. All together, these paleoenvironmental conditions resulted in expansion of an oxygen minimum zone (OMZ), favoring taphonomic processes that led to the excellent preservation of diverse macro‐ and microfossils. The nCIE recovery phase was characterized by reduced nutrient supply and organic carbon burial. Organic carbon sequestration in such paleoenvironments likely contributed to the recovery (increase) of stable carbon isotope (δ13C) records in the deep ocean during the Kilian sub‐event of OAE 1b.

Ecosystem recovery after the Early Jurassic T-OAE in the Châabet El Attaris section of the Tunisian Atlas

The flooding of the Pliensbachian shelf of the North-South Axis of Central Tunisia during the early Toarcian favoured the development of organic-rich facies in the most subsiding areas, such as that represented by the Châabet El Attaris section. The lower Toarcian strata in this section is represented by organic matter-rich sediments (dark laminated marls with low carbonate content) and the negative carbon isotope excursion (CIE) of the Jenkyns Event. The relatively high TOC values, increase of redox sensitive elements, and the absence of benthic organisms confirm the adverse conditions for life at the sea floor (suboxic to anoxic conditions) during the polymorphum Biozone and lower part of the levisoni Biozone. This indicates that the Jenkyns Event is expressed in this setting as the so-called Toarcian Oceanic Anoxic Event (T-OAE). The diversity and abundance of the calcareous nannoplankton was very low during the early Toarcian.After the T-OAE, the decrease in TOC and the low values of the geochemical redox proxies indicate a re‑oxygenation of the bottom waters, parallel with an increase of the carbonate content of the facies. After the negative CIE, the record of benthic macroinvertebrates (such as brachiopods, bivalves, ophiuroids and echinoids) and trace fossils (Zoophycos, Thalassinoides, Planolites, Chondrites, and locally Trichichnus) confirms the re‑oxygenation of the sea floor with increased diversity and abundance from the upper levisoni Biozone and bifrons Biozone. Benthic foraminifera colonization of the sea floor is evidenced by assemblages dominated by infaunal taxa, specially opportunist forms, such as Lenticulina, and progressive increase of diversity and abundance that confirms oxic pore-waters from the levisoni-bifrons biozone boundary. Planktic organisms attest to a progressive recovery of environmental conditions in the photic zone with increasing diversity and abundance of calcareous nannoplankton and radiolarians from the uppermost part of the levisoni Biozone and the bifrons Biozone. A first pulse after the negative CIE is represented by placoliths of the genus Lotharingius (shallow dweller) interpreted as a renewed input of nutrients to surface waters. The subsequent dominance of muroliths, especially of Crepidolithus crassus (a deep dweller), suggests a successive restoration of the deep-photic zone communities. These observations point towards enhanced oxygen availability in deep waters and pore waters due to improved basin ventilation.Taphonomic features of foraminifera pointing to dissolution in the bifrons Biozone are related to a lowered pH below the redox boundary within the sediment. In the same way, the presence of ferruginous moulds of microfossils (gastropods, bivalves, brachiopods, foraminifera, sponge spicules and radiolarians) with framboidal texture, actually preserved as goethite but originally pyrite, confirm the presence of reducing conditions within the sediment below the redox boundary. The record of Trichichnus confirms the reducing conditions in the deepest infaunal environments during the bifrons Biozone and is consistent with the development of pyrite moulds of microfossils early in the diagenesis.

Origin, stratigraphic and evolutionary significance of K-bentonites in the upper Ediacaran of South China

Several K-bentonite beds have been discovered in the upper part of the Ediacaran system in South China. Existing researches mainly focus on the zircon U-Pb geochronology of the K-bentonites. However, the nature of the primary magmas of the K-bentonites, the tectonic setting of their source volcanoes, the significance of their geochemical correlation, and their potential implications to evolution of late Ediacaran life have not been unraveled. Here, whole-rock and zircon geochemical studies have been carried out on the K-bentonites in the top of the Miaohe Member from the Jiuqunao section, and in the basal Liuchapo Formation and the lower Dengying Formation from the Fanglong section. The major and trace element contents of the K-bentonites and the trace element contents of volcanic zircons in the K-bentonites suggest that the nature of the primary magmas of the K-bentonites from the Fanglong and Jiuqunao sections is trachyandesitic. The oxygen isotope compositions of the volcanic zircons in the K-bentonites suggest that the source magmas of the K-bentonites from the two sections are mainly from the crust. The whole-rock Y-Nb, Ta/Yb-Th/Yb, Yb-Th/Ta, and Ta/Hf-Th/Hf diagrams, and the volcanic zircon Hf-U/Yb, Y-U/Yb, Th/U-Nb/Hf, and Th/Nb-Hf/Th diagrams of the K-bentonites show that the tectonic setting of source volcanoes of the K-bentonites was a convergent environment, which may be the result of the assembly of Gondwana during late Ediacaran. The geochemical results suggest that the K-bentonites in the basal Liuchapo Formation at the Fanglong section are correlative with the K-bentonite in the top of the Miaohe Member at the Jiuqunao section. Therefore, the geochemical correlation of the K-bentonites indicate that the age of 550.5 ± 1.0 Ma for the K-bentonite in the top of the Miaohe Member may be too young to be used as the end time of the Doushantuo negative carbon isotope excursion (DOUNCE), and that the end time of this event should be older than the age 557 ± 3 Ma of the K-bentonite in the lower part of the Dengying Formation at the Fanglong section. Thus, the termination of the DOUNCE and the Shuram carbon isotope excursion (terminated at 567.3 ± 3.0 Ma) may be isochronous. As volcanic ashfall events are known to supply nutrients to the oceans, we hypothesise that the widespread volcanism of the late Ediacaran may have enhanced oceanic productivity and play a role in the evolution of life at this time.

The Paleocene-Eocene transition in the Gulf of Guinea: Evidence of the PETM in the Douala basin, Cameroon

The Paleocene-Eocene Thermal Maximum (PETM) was identified for the first time in two sections (Bongue and Dibamba) from the Douala sub-basin located in the Gulf of Guinea, Cameroon. This discovery was based on a multi-disciplinary approach including biostratigraphy and occurrence of benthic and planktic foraminifera and ostracods, major and trace element contents, mercury, organic carbon isotope (δ13Corg values), total organic carbon (TOC), whole-rock and clay mineralogy. A combination of lithology, microfossil assemblage, and carbon isotope data indicate zone P5 and the top of the Paleocene enabling the definition of the Paleocene-Eocene boundary (PEB). A negative carbon-isotope excursion (CIE) spanning from the uppermost Paleocene deposits to the earliest Eocene sediments (PETM interval) shows a shift in δ13Corg values of 1.5‰ in Bongue and 3.0‰ in Dibamba. In both sections, this interval is affected by widespread acidification, as revealed by carbonate dissolution and microfossil preservation (i.e., species are dwarfed, broken, thin shelled, and with holes). The very low carbonate content and the scarcity of microfauna indicate the severity of acidification during the PETM, especially in the early Eocene where only one species was identified (Igorina broedermanni). Mercury anomalies, TOC contents, and trace element concentration ratios, point to volcanic activity linked to the Cameroon Volcanic Line (CVL) intrusive magma, and a decrease in productivity prior to the PETM. In addition to climate change, our geochemical and mineralogical data support the hypothesis that other environmental perturbations such as an increase in productivity and detrital input, as well as a decrease in bottom water oxygenation occurred during the PETM in the Douala sub-basin.

A possible Norian–Rhaetian boundary in the high-latitude continental Junggar Basin indicated by the δ 13 C org record

Abstract In contrast to the Triassic–Jurassic boundary, there is no consensus on the definition and age of the Norian–Rhaetian boundary (NRB), which hampers the global correlation of Rhaetian strata and thus interpretation of the patterns and driving mechanism of the end-Triassic mass extinction. Recent works show that a significant negative carbon isotope excursion (N-CIE) occurs in the NRB interval ( c. 205.7 Ma), which probably provides a physical marker for the NRB. However, no such records have so far been reported from high-latitude continental deposits. Here we present high-resolution bulk organic carbon isotope data from the Upper Triassic continuous lacustrine-to-fluvial deposits in the Haojiagou section of the high-latitude continental Junggar Basin in northwestern China. A significant N-CIE is shown at an age of c. 205 Ma, which is most probably the expression of the NRB N-CIE in the Junggar Basin owing to their very close occurrence times. The NRB is thus probably along the N-CIE in the Haojiagou section, and a short Rhaetian stage is then supported. The Late Triassic climate in the high-latitude Junggar Basin was consistent with the global changes in climate, characterized by a long-sustained cooler and humid climate interrupted by the late Norian and latest Rhaetian warm events.