Article

Duration of the Early Toarcian carbon isotope excursion deduced from spectral analysis: Consequence for its possible causes

March 2008
Earth and Planetary Science Letters 267(3):666-679

March 2008
267(3):666-679

DOI:10.1016/j.epsl.2007.12.017

Authors:

Bernard Pittet

Claude Bernard University Lyon 1

Emanuela Mattioli

Claude Bernard University Lyon 1

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The marked 3–8‰ negative carbon isotope excursion associated with the Early Toarcian oceanic anoxic event (OAE; ~ 183 myr ago) in the Early Jurassic period is thought to represent one of the most important perturbations of the C-cycle in the last 200 myr. However, the origin of this excursion remains strongly debated, primarily due to uncertainties in the estimation of its duration, which ranges from ~ 200 kyr to 1 myr. Here we present a new orbital calibration of the Early Toarcian carbon isotope excursion, based on spectral analyses of two independent datasets generated from the sedimentary record of two hemipelagic sections from Portugal (Peniche) and SW Germany (Dotternhausen), in order to better constrain the timescale and hence the origin of this excursion. These analyses reveal that orbital cycles exert a strong influence on both the calcium carbonate content in Portugal and on the greyscale of black shales in Germany, which allow us to propose a duration of ≥ 1.9 myr for the Early Toarcian and of ~ 900 kyr for the entire carbon isotope excursion. The shift towards lower carbon isotope values lasted ~ 150 kyr, and carbon isotope values remained low for ~ 450 kyr; the subsequent increase of carbon isotope values lasted ~ 300 kyr. This calibration suggests that the sustained input of isotopically light carbon at the origin of the excursion occurred over ~ 600 kyr and thus dismisses causal mechanisms implying relatively small source reservoirs such as the massive dissociation of methane hydrates. In the light of our new cyclostratigraphic timescale, the massive input of isotopically light carbon associated with the emplacement of the Karoo–Ferrar basaltic province appears as the most likely cause of the Toarcian global carbon isotope excursion. We also show that the C-isotope perturbation coincided with a transition from precession–eccentricity-dominated cycles to obliquity–eccentricity-dominated cycles, suggesting that the OAE was marked by a fundamental change in the response of the climate system, which allowed the obliquity signal, normally better recorded at high latitudes, to be a dominant forcing factor of short-term sedimentary cycles at tropical latitudes.

Astronomical timescale for the early Toarcian (Early Jurassic) Posidonia Shale and global environmental changes

Article

May 2023
PALAEOGEOGR PALAEOCL

Lower Toarcian sediments (Early Jurassic, c. 183 Ma) record the Toarcian Oceanic Anoxic Event (T-OAE), which represents a period of profound environmental changes. The event involved the widespread deposition of organic carbon-rich strata in oxygen-depleted paleo-shelf settings, global warming, and is characterized by a negative Toarcian carbon-isotope excursion (T-CIE). The T-CIE provides a global time-correlative chemostratigraphic marker; however, its age, duration and underlying causes remain uncertain in the absence of an accurate early Toarcian timescale. This article presents a cyclostratigraphic framework for the lower Toarcian Posidonia Shale Formation from the Dotternhausen Quarry (South German Basin), one of the best-studied sediment archives that record early Toarcian environmental and ecosystem changes. The Posidonia Shale in the study site was deposited in the deeper part of the epicontinental shelf sea, where sediment accumulation rates were relatively uniform and major hiatuses unlikely. These factors are rarely found in most sections, and favor obtaining accurate cyclostratigraphic results at the study site. Our astronomical timescale indicates that black shale deposition at the site lasted about ~3.2 Myr, a remarkably long period for sufficient primary productivity in stagnant bottom-water conditions. The estimated durations of the tenuicostatum and falciferum ammonite zones, and the T-CIE are ~0.9, ~1.8 and ~ 1.2 Myr, respectively. Early Toarcian global warming, accompanying the T-CIE, occurred over a period of ~400 kyr. Early Toarcian 3rd-order sea-level cycles correlate to 1.2 to 1.6 Myr orbital cycles, reflecting long-term obliquity and eccentricity periods. Orbitally-forced 3rd-order sea-level cycles, in combination with 405 kyr long-eccentricity, and potentially 100 kyr short-eccentricity cycles, controlled sea-floor oxygenation and deposition of black shales. Short-term environmental changes (e.g., redox cycles), responding to long- and short-eccentricity cycles were most pronounced during 3rd-order sea-level lowstands, when the depositional environment was most susceptible to short-term environmental instabilities. Our results highlight orbital-forcing as a key driving mechanism for early Toarcian environmental conditions.

An appraisal of the ages of Phanerozoic large igneous provinces

Article

Feb 2023
EARTH-SCI REV

Chemo- and cyclostratigraphic records of the Albian from the Tethyan Himalaya of southern Tibet, China

Article

Sep 2022
GLOBAL PLANET CHANGE

The Albian Age is characterized by frequent short-lived perturbations of the global carbon cycle, including a series of Oceanic Anoxic Events (OAEs). Carbon Isotope Excursions (CIEs) document these OAEs in detail. A strong influence of orbital forcing on oceanographic and climatic conditions in the Albian has been documented, but the relationship between orbital changes and the carbon cycle is still unclear. The Albian CIEs are well recorded globally, except in the eastern Tethys. The major forcing mechanisms of Mesozoic carbon cycling have long remained a conundrum. Here, we present bulk carbon (δ¹³Ccarb) and oxygen (δ¹⁸Ocarb) isotope data, magnetic susceptibility (MS) and total organic carbon (TOC) data from the Nirang Section in the Tethyan Himalaya of southern Tibet, China. The δ¹³Ccarb record for the first time documents the long-term secular variations in δ¹³C during the Albian in the Tethyan Himalaya in high resolution. The δ¹³C curve obtained in Nirang can be correlated well with reference sections in other basins. OAE1b and 1c were identified as troughs in the δ¹³C profile in the lowermost and upper part of the section. Cyclostratigraphic analysis of MS, δ¹³Ccarb and TOC content from the Nirang Section reveals the imprint of the orbital cycles of eccentricity (~100 kyr and ~ 405 kyr) and a long-term amplitude modulation of either obliquity or eccentricity (~1.2 Myr). The band-pass filters of the ~100 kyr periodicity in MS provide the estimation of 563 kyr for the duration of OAE 1c and 677 kyr for that of the negative shift between OAE 1b and 1c. A detected ~1.1 Myr periodicity is the most significant cycle, providing strong evidence for astronomically paced climate change in the eastern Tethys during the Albian. The band-pass filters of the ~1.1 Myr periodicity in MS and TOC display a close correspondence, suggesting that the productivity and/or preservation potential of organic matter was likely controlled by orbitally forced changes in terrigenous input. Increasing δ¹³C values correspond to higher values of MS and TOC content and vice versa, indicating that the δ¹³C values of the eastern Tethys during the Albian were influenced by the intensity of the terrigenous flux and the subsequent burial of organic matter paced by orbital change. This work implies that cyclic changes in carbon reservoirs in the low latitudes of the Southern Hemisphere during the Albian were controlled by seasonal variations in terrigenous input driven by the influence of orbital cycles on regional hydrological processes.

Estudo Organofaciológico e Biomarcadores do Evento Oceânico Anóxico do Toarciano inferior nas Bacias Lusitânica e Astúrias (Península Ibérica)

Thesis

Full-text available

Jul 2015

Bruno Rodrigues

The following work presents the organic matter (OM) recorded in the marly-limestone successions from the upper Pliensbachian – lower Toarcian (Levisoni = Serpentinum zone), of the Alcabideque (Lusitanian Basin (LB)) and Rodiles (Asturian Basin (AB) sections), in the attempt to correlate the palaeoenvironmental conditions occurred in these sectors of Iberian Peninsula in this stratigraphic interval, related to the Early Toarcian Oceanic Anoxic Event. A detailed study of palynofacies and organic geochemistry (Total Organic Carbon (TOC), sulfur and biomarkers) has been performed in 33 samples (14 in the LB and 19 in the AB). The TOC analysis shows high values in the AB section (maximum value of 5,27 wt.%) unlike what occurs in the LB section (maximum value of 0.41 wt.%). The optical analysis prove the presence of particulate organic components, mostly of continental origin (phytoclasts and continental palynomorphs) that dominate in both sections. The amorphous organic matter is the least abundant group reaching high values only in the Margaritatus Zone and around the Tenuicostatum – Serpentinum boundary in the Rodiles section of the AB. In the LB the top of the Polymorphum Zone framed in the Alcabideque section also shows the presence of high AOM values, a feature that is associated to the highest TOC values. Lastly the palynomorphs are mainly composed by sporomorphs, which in the Levisoni Zone of the LB present themselves in tetrads and agglomerates. In this same section it is also worth noting the occurrence of high percentages of dinocysts in the top of the Emaciatum and Polymorphum zones respectively. The biomarkers corroborate the OM source and provide depositional palaeoenvironmental informations that support the palynofacies data. In the Rodiles section through the Pr/n-C17 and Phy/n-C18 diagrams it is possible to prove that the near totality of the samples fall into the transition from continental OM to the mixed OM projection fields. Through the compound analysis of the CPI, Pr/n-C17, Phy/n-C18 and Pr/Phy, it is established that a disoxic-anoxic conditions occurs around the Tenuicostatum - Serpentinum boundary. In the LB section and though the n-alkane, terpanes and regular steranes ratios, the input of continental OM is confirmed, particularly in the Levisoni Zone. On the other hand the gammacerane index shows a peak at the top Polymorphum Zone, clearly associated with the increase of marine palynomorphs (dinocysts) and the maximum marine influence in the succession. Though the obtained results a clear differentiation of the OM record between the Polymorphum (=Tenuicostatum) and Levisoni (=Serpentinum) boundary zones of the two basins can be made, showing for the first time for the LB a continental sedimentary input related to the northernmost zone of the basin.

Biomarker and compound-specific isotope records across the Toarcian CIE at the Dormettingen section in SW Germany

Article

Full-text available

May 2022

The Toarcian oceanic anoxic event (T-OAE) is associated with a prominent negative carbon isotope excursion (CIE; ~ 183 million years (Myr)). About 10-m-thick organic matter-rich sediments accumulated during the T-OAE in the Southwest German Basin (SWGB). Rock–Eval, maceral and biomarker analysis were used to determine variations of environmental conditions across the CIE interval. Carbon isotope records were determined for various n -alkanes, pristane and phytane to contribute to the reconstruction of the paleo-environment and to study the factors controlling molecular δ ¹³ C values. Geochemical redox indicators provide evidence for photic zone anoxia during the Toarcian CIE, which reached its maximum after deposition of the “Unterer Stein” marker horizon. The 2α-methylhopane index suggests enhanced activity of diazotrophic cyanobacteria, which is also supported by nitrogen isotope data. This distinguishes the SWGB from other basins with Toarcian black shale. Oxygen-depleted conditions, albeit with lower intensity continued after the CIE. All investigated compounds replicate the negative CIE, but the magnitudes vary considerably. The largest shift is observed for n -C 27 (9‰) and reflects the combined effect of the global CIE and a major change in organic matter input (termination of terrigenous organic matter input). The shift for short-chain n -alkanes, pristane, and phytane, interpreted to reflect marine biomass, varies between 4.5 and 5.0‰. This is the highest value observed so far for any Toarcian section. δ ¹³ C values of pristane and phytane reach a minimum near the base of the CIE interval and increase upsection. Thus, the maximum negative isotope shift predates the strongest basin restriction by about 450 thousand years (kyr).

Schizosphaerella size and abundance variations across the Toarcian Oceanic Anoxic Event in the Sogno Core (Lombardy Basin, Southern Alps)

Article

Apr 2022
PALAEOGEOGR PALAEOCL

Abundance and size variations of nannofossil Schizosphaerella punctulata were quantified in the uppermost Pliensbachian–Lower Toarcian succession recovered with the Sogno Core (Lombardy Basin, Northern Italy). High-resolution nannofossil biostratigraphy and C-isotopic chemostratigraphy identified the Jenkyns Event within the Toarcian oceanic anoxic event (T-OAE) interval. Absolute abundances and morphometric changes of “small S. punctulata” (< 7 μm), S. punctulata (7–10 μm; 10–14 μm; > 14 μm) and “encrusted S. punctulata” (specimens with a fringing crust) show large fluctuations across the negative δ¹³C Jenkyns Event. The Schizosphaerella crisis is further characterized by a decrease in average valve size in the early–middle Jenkyns Event. The abundance fall was caused by the failure of S. punctulata specimens >7 μm and “encrusted S. punctulata” that along with the increased relative abundance of small specimens, produced the reduction of average dimensions also documented in the Lusitanian and Paris Basins, although with a diachronous inception. The average valve size from the Lombardy Basin is ~2 μm smaller than in these other basins. Hyperthermal conditions associated with excess CO2 and ocean acidification possibly forced the drastic reduction of S. punctulata abundance/size. In the pelagic succession of the Sogno Core there is a strong positive correlation between the S. punctulata (> 7 μm) absolute abundance/size and the CaCO3 content, with a negligible contribution by small specimens (< 7 μm). Encrusted specimens testify selective neomorphic processes: the diagenetic crust seems diagnostic to separate S. punctulata from Stephanodiscus astraea.

Early Jurassic long-term oceanic sulfur-cycle perturbations in the Tibetan Himalaya

Article

Nov 2021
EARTH PLANET SC LETT

The Early Jurassic is an important interval characterized by several global carbon-isotope (δ13C) perturbations. Although the δ13C records are becoming better documented during this time interval, we have a relatively poor understanding of the associated long-term environmental and climatic changes. In order to decipher these events, we here present new stable sulfur-isotope data of carbonate-associated sulfate (δ34SCAS) for the Sinemurian–Pliensbachian interval from the Wölong section in the Tibetan Himalaya that was located palaeogeographically in the southern hemisphere. An overall positive shift in δ34SCAS coincides with the negative δ13C excursion around the Sinemurian–Pliensbachian boundary, suggesting an increased ³⁴S-depleted pyrite burial rate. The ensuing overarching negative δ34SCAS shift coincides with the upper Pliensbachian positive δ13C excursion. The initial falling limb of the δ34SCAS shift suggests a transient δ34S-depleted sulfate input, but this trend was soon reversed to become positive, likely caused by a persistently enhanced ³²S-rich pyrite burial flux in the latest Pliensbachian. Modeling results show that maximum oceanic sulfate concentration likely decreased during the Sinemurian–Toarcian interval, probably due to large-scale evaporite deposition in the western Tethys and proto-Atlantic and enhanced pyrite burial in a number of marine settings. The concentration of seawater sulfate could have been high enough to maintain a homogeneous sulfur-isotope ocean in the late Sinemurian, but its persistent decrease may have initiated a spatially heterogeneous ocean after the Pliensbachian: an oceanic geochemical state that was amplified during the Toarcian Oceanic Anoxic Event.

Búsqueda de señal orbital en isótopos estables de carbono y oxígeno de belemnites del Jurásico Inferior de la cuenca Vasco-Cantábrica mediante el periodograma suavizado de Lomb-Scargle

Article

Full-text available

Jun 2021

Josué M. Polanco-Martínez

Los isótopos estables obtenidos de muestras fosilizadas de macrofósiles, como los belemnites, son unos de los pocos registros geológicos existentes del Jurásico Inferior, por lo cual han sido usados para reconstrucciones de temperaturas y salinidades del océano de esa época. Sin embargo, hasta la fecha no han sido utilizados para averiguar si estos paleoindicadores pudieron registrar algún tipo de evento periódico relacionado con los ciclos orbitales de la Tierra. Por esta razón y teniendo en cuenta varias de las limitaciones al utilizar estos tipos de datos, en este trabajo se presenta un novedoso estudio donde se aplica una técnica avanzada de análisis espectral conocida como el periodograma suavizado de Lomb-Scargle, la cual puede ser aplicada de modo directo a series temporales no equiespaciadas contaminadas por ruido. El objetivo de este trabajo es identificar la presencia de ciclos orbitales que se hayan podido registrar en series temporales de valores medios de δ18O y δ13C provenientes de muestras fosilizadas de belemnites recolectadas en la cuenca Vasco-Cantábrica teniendo en cuenta los errores de datación de estas muestras en la estimación del espectro. De manera especial se hace énfasis en la búsqueda de alguno de los ciclos de la excentricidad, como el de largo periodo localizado ca. 400 Ka o el de menor periodo localizado ca. 100 ka, porque se sabe que el ciclo de 400 Ka ha permanecido invariable desde el Mesozoico hasta el presente y porque ambos ciclos han sido identificados en registros estratigráficos del Mesozoico recolectados en diferentes áreas geográficas. Sin embargo, a pesar de la rigurosidad de nuestros análisis, no se encontró señal orbital de 400 Ka aunque si una localizada cerca de los 109 Ka, la cual podría estar relacionada con los ciclos de corto periodo de la excentricidad de la Tierra.

Early Jurassic (Toarcian) climate warming and correlation of continental coal-bearing basins of Siberia: problems and potential solution

Article

May 2024

The impact of environmental change is intensively studied in the marine realm, whereby only few studies focus on continental sediment archives. Precise correlation of marine and continental sediment archives is complicated, but essential to understand early Toarcian environmental dynamics. The early Toarcian global warming was accompanied by broad volcanism, major marine transgression, and oxygen deficiency in the ocean. The environmental change impacted marine and continental ecosystems. Correlation of marine and continental sections from Western Siberia is based on coeval changes in the microfauna, palynomorphs, and macroflora during the early Toarcian. In Kuznetsk, Kansk, and Irkutsk continental coal-bearing basins of the Siberian realm, the stratigraphic position of the early Toarcian climatic optimum is debated. Thermophilic palynomorphs from palynozone 6 do not correlate with macroflora composition, which does not reflect environmental changes. The Toarcian warming is well expressed in palynomorph assemblages, but not in the macroflora. However, the general evolutionary trends of the Siberian groups of spore plants and gymnosperms (Coniopteris, Ginkgoales, and Leptostrobales) allow us to correlate the palynological and palaeobotanical assemblages from the Prisayan Formation of the Irkutsk Basin to the stratigraphic scheme of Western Siberia. This study may be useful for interregional correlations between the Jurassic continental deposits of the Irkutsk, Kuznetsk, and Kansk basins.

The Toarcian Posidonia Shale at Salem (North Alpine Foreland Basin; South Germany) hydrocarbon potential and paleogeography

Article

Full-text available

Mar 2024
INT J EARTH SCI

The Posidonia Shale in the basement of the North Alpine Foreland Basin of southwestern Germany represents an important archive for environmental changes during the Toarcian oceanic anoxic event and the associated carbon isotope excursion (T-CIE). It is also an important hydrocarbon source rock. In the Salem borehole, the Posidonia Shale is~10 m thick. The lower 7.5 m (1763.5–1756.0 m) of the Posidonian Shale and the uppermost part of the underlying Amaltheenton Formation were cored and studied using a total of 62 samples. Rock–Eval, palynological, maceral, biomarker and carbon isotope data were collected to assess variations in environmental conditions and to quantify the source rock potential. In contrast to most other Toarcian sections in southwest Germany, TOC contents are high in sediments deposited during the T-CIE, but reach a peak in post-CIE sediments. Biomarker ratios suggest that this refects strong oxygen-depletion during the T-CIE (elegantulum to lower elegans subzones), but also during the falciferum Subzone, which is also refected by a prolonged dinofagellate cyst blackout. While sediments of the tenuicostatum Zone to the elegans Subzone are thinner than in neighbouring sections (e.g., Dotternhausen), sediments of the falciferum Subzone are unusually thick, suggesting that increased subsidence might have contributed to anoxia. The T-CIE interval is very thin (0.75 m). δ13C values of n-alkanes show that the maximum negative isotope shift predates the strongest basin restriction during the T-CIE and that the carbon isotope shift is recorded earlier for aquatic than for terrigenous organisms. In Salem, the Posidonia Shale is thermally mature and highly oil-prone. The residual source petroleum potential is about 0.8 tHC/m2.

Long eccentricity forcing of the Late Pliensbachian to Early Toarcian (Jurassic) terrestrial wildfire activities in the Tarim Basin, northwestern China

Article

Full-text available

Mar 2023
PALAEOGEOGR PALAEOCL

An Early Jurassic (Sinemurian–Toarcian) stratigraphic framework for the occurrence of Organic Matter Preservation Intervals (OMPIs)

Article

Oct 2021
EARTH-SCI REV

Lower Jurassic sedimentary successions in the Atlantic margin basins include several organic-rich intervals, some with source rock potential; time-equivalent units are also identified in on- and offshore areas worldwide. Despite decades of research, it is still unclear which mechanisms lead to the deposition of organic-rich sediments during the Early Jurassic. The objectives of this study are to construct a detailed temporal and geographical framework of Sinemurian–Toarcian organic matter preservation intervals (OMPIs; subdivided into local, regional, and superregional) and roughly constrain the relationship of OMPIs with the Lower Jurassic δ¹³C record. This survey combines an in-depth analysis of literature on the distribution of OM in the Sinemurian–Toarcian with new geochemical studies [total organic carbon (TOC) and organic matter pyrolysis] from selected outcrop sections from Portugal, Spain, and Morocco. Strong local control on OMPIs during most of the Sinemurian is suggested. Regionally widespread organic-rich facies are associated with the most negative δ¹³C values of the broad Sinemurian–Pliensbachian negative carbon isotopic trend recorded in organic matter (including the Sinemurian–Pliensbachian Boundary Event). Pliensbachian OMPIs are expressed in the areas bordering the proto-Atlantic Ocean and are often linked with positive δ¹³C excursions and short-lived warm intervals, but OMPIs are also defined for the Late Pliensbachian cool interval. Early Toarcian superregional OMPIs are associated with some of the most pronounced δ¹³C excursions of the Mesozoic. Toarcian maximum TOC content occurs with the positive δ¹³C (recovery) trend following the δ¹³C negative shift typically linked with the Early Toarcian Oceanic Anoxic Event (T-OAE), supporting the notion that peak carbon sequestration/ocean anoxia post-dated the main phase of carbon input into the atmosphere, as also suggested by recent modelling efforts. However, additional superregional OMPIs predate and postdate the T-OAE, indicating that conditions favouring preservation of OM (increased productivity and/or enhanced preservation) during the Early Toarcian were not restricted to the T-OAE interval. The compilation of Sinemurian–Toarcian OMPIs presented in this paper demonstrates that organic-rich intervals of regional and superregional expression in the Lower Jurassic sedimentary record are ubiquitous and may even be more numerous than in the Cretaceous. Considering the association of some of the Sinemurian, Pliensbachian, and Toarcian regional and superregional OMPIs with well-defined carbon isotopic excursions, it is here suggested that these hold the same relevance as the secondary OAEs of the Cretaceous, such as the Valanginian OAE (Weissert Event), Hauterivian OAE (Faraoni Event), and Late Aptian–Early Albian OAE (OAE 1b cluster).

Response of early Toarcian (Early Jurassic) benthic marine faunas from South-Western Europe to temperature-related stressors

Thesis

Jul 2021

Veronica Piazza

Globaler Temperaturanstieg, Ozeanversauerung und Sauerstoffmangel (temperaturbedingte Stressoren), spielen eine wichtige Rolle während Klimawandel. Die kombinierten Auswirkungen wirken sich negativ auf marine Lebensgemeinschaften und Ökosystemen aus und verschärfen die Effekte anderer Stressoren. Es gibt für den aktuellen und vergangenen Erwärmungsphasen Belege für veränderte Artenverteilung, Lebensraumverlust, Artensterben und verminderte physiologische Leistungen von Organismen. Die spezifischen Mechanismen werden diskutiert. Umweltveränderungen in der Erdvergangenheit bieten die Gelegenheit, die Dynamik von Ökosystemkrisen vor, zu untersuchen. Diese Arbeit integriert verschiedenen Disziplinen um unser Verständnis der Rolle temperaturbedingter Stressoren für marine benthische Ökosysteme zu verbessern. Das Toarcian Event (Unterjura) wurde aufgrund seiner geologischen, geochemischen und paläontologischen Überlieferung ausgewählt. Hochaufgelöste quantitative taxonomische, paläoökologische und geochemische Daten von benthischen Makroinvertebraten wurden analysiert. Ziel ist es, die Mechanismen ökologischer Veränderungen unter Temperaturstress zu bewerten, mit Schwerpunkt auf Körpergrößenmuster sowie Veränderungen in der Zusammensetzung und Struktur von Lebensgemeinschaften, und die Rolle von Umweltstressoren als Ursachen der biotischen Krise abzuschätzen. Der Temperaturanstieg führte zu dem Verlust an biologischer Vielfalt und zu der Verringerung der Körpergröße der Organismen vor und während der biotischen Krise. Die Struktur der Gemeinschaften wurde in Bezug auf die taxonomische und ökologische Zusammensetzung neu organisiert. Die Reaktion auf Umweltstress war innerhalb und zwischen den taxonomischen Gruppen unterschiedlich, wobei Brachiopoden stark betroffen waren. Diese Ergebnisse verbessern unser Verständnis der biotischen Reaktionen auf temperaturbedingte Stressoren und können zur Vorhersage Veränderungen Ökosysteme bei den aktuellen Erwärmungstrends beitragen.

Vegetation response to climate change during an Early Jurassic hyperthermal event (Jenkyns Event) from Northern China (Ordos Basin)

Article

Apr 2024
PALAEOGEOGR PALAEOCL

The Early Jurassic Jenkyns Event (or Toarcian Oceanic Anoxic Event T-OAE) was an episode of global warming and C-cycle perturbation that affected both marine and terrestrial ecosystems, but the interplay between climate change and vegetation is not established in detail from sections outside of Europe. Here, abundance changes in spore-pollen assemblages from the lacustrine Anya succession in the Ordos Basin (North China) reveal a unique record of vegetation dynamics during the Jenkyns Event. Plant communities responded to the event with biodiversity losses and the reorganization of gymnosperm-dominated forests. Community-level shifts are observed from the Pliensbachian–Toarcian boundary, but the onset of the negative carbon excursion (NCIE) that marks the event is coeval with the most significant turnover: a switch from a high-diversity vegetation with conifers, seed ferns, cycads, bennettites and ferns to drought-adapted low-diversity flora with Cheirolepidiaceae. The demise of forests and lowland mire biomes resulted in deforestation with increased weathering and soil erosion that exacerbated the terrestrial ecosystem crisis already under stress from rising temperatures. Terrestrial recovery was initiated before the end of the Jenkyns Event with the resurgence of pioneer ferns and lycopsids that colonized disturbed habitats. Plant assemblages signal aridification at the onset of the event with frequent climatic oscillations and extreme weather patterns during the event itself. The main NCIE phase was preceded by a short-lived cooling phase in the earliest Toarcian. In the aftermath of the NCIE, Cheirolepidiaceae forests declined and a more stable biome developed with seed ferns and various conifers. This was contemporaneous with delta development and shallowing of the lake surrounded by lowland mires with ferns, clubmosses and horsetails. Comparison of floral patterns across the Jenkyns Event show that, although Cheirolepidiaceae dominated the event globally, there were differences in vegetation response between coastal and inland areas, and recovery patterns might differ regionally.

Influence of the Late Ordovician‐Early Silurian Paleoenvironment and Related Geological Processes on the Organic Matter Accumulation and Carbon Isotope Excursion

Article

Full-text available

Jul 2023

Although previous studies have shown that the paleoenvironment and geological processes contributed to the organic matter accumulation (OMA) and carbon isotope excursions (CIEs) during the late Ordovician–early Silurian, the dominated controlling factor for the OMA and the origins of CIEs still remains unclear due to complex interaction between various paleoenvironmental factors and geological processes. Therefore, based on the elemental geochemistry of the Wufeng–Longmaxi Formation shales in the upper Yangtze Platform, we analyzed the late Ordovician–early Silurian paleoenvironment and related geological processes, and further explored the origin of the OMA and CIEs. As a result, the Wufeng–Longmaxi Formation shale was divided into four stages. During Stage 1 (late Katian, ∼447.62–444.50 Ma), local tectonic and volcanic activities controlled the paleoproductivity and redox conditions, facilitating the OMA. By contrast, the productivity of the surface water and the anoxic bottom water were mainly controlled by the global climate after the Hirnantian glaciation, which contributed to the OMA during Stage 2 (early Rhuddanian, ∼444.50–441.00 Ma). The decreasing sea level and rapid uplifting of Xuefeng and Qianzhong Uplifts resulted in the organic matter depletion during Stages 3 (late Rhuddanian, ∼441.00–440.80 Ma) and 4 (Aeronian, ∼440.80–439.21 Ma). Besides, the release of ¹²C–enriched carbon reservoirs triggered by volcanic activities and ¹³C–enrichment caused by the OMA regulated the carbon cycling: the negative CIE may be the result of light carbon emissions, such as the reactivated organic matter and mantle derived carbon, and the Hirnantian CIE event is jointly controlled by the weakened carbon emission effect and the OMA.

Micromorphs: Response of the Ammonite Fauna during the Toarcian Oceanic Anoxic Event (T-OAE) in the Es-Saffeh Mountains (Tiaret, Western Algeria)

Conference Paper

Full-text available

Dec 2022

The Jurassic structural high of Sasso di Pale(Umbria-Marche Basin, Italy): How a small Apennine structurerecorded Early to Middle Jurassic global perturbations

Book

Full-text available

Jun 2022

Review of earliest Toarcian geological evolution in the East African Coastal Margin: Paleogeography, stratigraphy, and facies implications

Article

Feb 2023
J AFR EARTH SCI

In the past decades, lower Toarcian strata have gained attention on a global paleogeographical scale but these strata have been poorly studied along the East African margin. This study presents a comprehensive review of the Toarcian geological evolution of the East African Coastal Margin focusing mainly on the Mandawa Basin in Tanzania, the Morondava and Majunga Basins in Madagascar, the Lamu and Mandera Basins in Kenya, and the Luuq Mandera and Ahl-Mado Basins in Somalia. We discuss controversies in published reports regarding facies, stratigraphy, and pathways of marine ingressions during the Toarcian transgressive episode that led to the extension of a Neotethys sea arm (the Somali Ocean) into the East African Margin prior to the main phase of Gondwana drifting and the opening of the Indian Ocean. The earliest Toarcian transgressive phase was accompanied by rifting between East Africa and Madagascar, which most likely rejuvenated Karoo rifting, drove marine influxes and inundated existing depressional troughs in the Ahl-Mado, Luuq Mandera, SW Majunga, and Mandawa Basins. This resulted in the formation of the Arab-Indo-Madagascar embayment, a NNE-SSW striking epicontinental sea (Somali Ocean), which was linked to the Tethys Sea southward via the horn of Africa route, as well as in widespread deposition of Toarcian marine to coastal-lagoonal to continental sediments. The occurrence of major facies changes from predominantly continental to shallow/marginal marine facies and the widespread distribution of an Arab-Indo-Madagascan Bouleiceras ammonite association confirms the connection between the East African coastal sedimentary terrain and the Tethys Sea during the Toarcian. Influxes of marine waters from Tethys Sea were limited in extent to the rift-graben regions and reactivated Karoo faults system. Restricted marine deposition within a limited epicontinental sea likely ended during Late Aalenian-Bajocian times, when establishment of fully marine conditions began due to opening of the Indian Ocean.

Volcanism Driven Pliensbachian (Lower Jurassic) Terrestrial Climate and Environment Perturbations

Article

Full-text available

Jan 2022

Complex pattern of environmental changes and organic matter preservation in the NE Ordos lacustrine depositional system (China) during the T-OAE (Early Jurassic)

Article

Feb 2023
GLOBAL PLANET CHANGE

Marine bioturbation collapse during Early Jurassic deoxygenation: implications for post-extinction marine ecosystem functioning

Article

Jan 2023

Climate change is undermining the health and integrity of seafloor ecosystems, with declines in bioturbation expected to impact future ecosystem functioning. We explored changes in the nature and degree of bioturbation during Early Jurassic global warming and ocean deoxygenation. Understanding how these communities responded can help anticipate how bioturbation and ecosystem functioning might change over large spatial and temporal scales. Trace and body fossils from outcrop and core in the Cleveland Basin, UK show how healthy seafloor communities deteriorated through the Pliensbachian spinatum Zone, and macroinfaunal behaviour fluctuated across the Pliensbachian-Toarcian boundary coincident with mass extinction. Deoxygenation began above the stage boundary, and conditions deteriorated until bioturbation ceased completely (upper tenuicostatum Zone) for 0.6-2.5 Ma, longer than anywhere else in NW Tethys. The macroinfaunal record revealed new details on the progression and timing of deoxygenation, benthic recovery and fluctuations in the palaeoredox boundary. After the OAE infauna were fewer, smaller and did not mix sediments to depth, whilst the depth and diversity of bioturbation increased by the fibulatum Subzone ( bifrons Zone) the benthos had not recovered to late Pliensbachian pre-OAE state. Bioturbation collapse over large parts of the northern hemisphere, likely contributed to regional-scale changes in ecosystem functioning. Supplementary material at https://doi.org/10.6084/m9.figshare.c.6370710

Volcanism driven Pliensbachian (Lower Jurassic) terrestrial climate and environment perturbations

Article

Full-text available

Aug 2022
GLOBAL PLANET CHANGE

Initial breakup of Pangaea and opening of the Central Atlantic Ocean during the Early Jurassic resulted in widespread volcanism and was accompanied by significant changes in atmospheric composition, climate and environment of the Earth's surface system. Although profound changes in global geochemical cycles, palaeoclimate, and palaeoenvironments during the early part of the Toarcian Stage have been extensively studied and attributed to the emplacement of the Karro-Ferrar Large Igneous Province (LIP), similar changes in other parts of the Early Jurassic are less well known, especially in terrestrial settings. We studied Pliensbachian aged terrestrial strata from the Dameigou section in the Qaidam Basin of the Qinghai-Tibet Plateau to elucidate climate and environmental changes and their potential driving mechanisms. Three significant climate and environment perturbations were recognized by high-resolution mineralogical and geochemical analyses, with each characterized by peaks in kaolinite composition that indicate relatively warm and humid climates. Each kaolinite peak is accompanied by proxy evidence that indicates anoxic redox conditions, high paleoproductivity, and strong continental weathering prevailed, and coincides with negative carbon isotope excursions and peaks in Hg/TOC ratios that record three episodes of concurrent volcanism. The three episodes of volcanism are correlated with global events and suggest volcanism as the driving mechanism for major global climate and environment perturbations during the Pliensbachian. Each interval of volcanism released massive quantities of greenhouse gases and Hg into the atmosphere, leading to negative carbon isotope excursions and mercury anomalies in sedimentary strata. Global climatic warming induced by volcanism resulted in an enhanced hydrological cycle and can be correlated with more humid climatic conditions, intensified continental weathering and organic matter burial, lake expansion, and eutrophication. Such changes have important negative feedback on the global carbon cycle by increasing consumption of CO2 in lacustrine settings, which in the late Pliensbachian led to the restoration of similar climates and environments that occurred pre-volcanism in the study area.

Ecological adaptation of marine floras and faunas across the Early Jurassic Toarcian Oceanic Anoxic Event – A case study from northern Germany

Article

Aug 2022
PALAEOGEOGR PALAEOCL

We present new, high resolution bio- and chemostratigraphic data (calcareous nannofossils, ammonites, δ¹³Corg) of the lower Toarcian from an outcrop (Hondelage) in northern Germany. The three data sets, which cover lithostratigraphically the uppermost Amaltheenton-Formation (upper Pliensbachian) and the lower part of the Posidonienschiefer-Formation (lower Toarcian), allow a calibration of the different stratigraphic schemes in ultra high resolution. They further shed light on the ecologic conditions controlling the assemblage composition and evolutionary patterns of calcareous nannofossils, ammonites and gastropods. The distinctive negative δ¹³Corg excursion (negative CIE, Jenkyns Event), which is part of the early Toarcian Oceanic Anoxic Event (T-OAE), falls lithostratigraphically into the lower part of the laminated black shales of the Posidonienschiefer facies. The lower part of the Jenkyns Event is marked by the first occurrences of the calcareous nannofossil species Carinolithus superbus crassus and Diductius constans. These two events are biostratigraphically assigned to the Tiltoniceras antiquum ammonite Biohorizon (uppermost Dactylioceras tenuicostatum ammonite Zone). The top of the Jenkyns Event has been assigned to the Cleviceras exaratum ammonite Biohorizon (Harpoceras serpentinum ammonite Zone) of the lower Toarcian. The last occurrence of the nannofossil species Biscutum grande lies in the middle part of the Jenkyns Event. Calcareous nannofossils, ammonite faunas and gastropods are discussed with respect to the environmental and climatic changes, which occurred across the Pliensbachian/Toarcian boundary interval and throughout the Jenkyns Event. The extreme impoverished nannofossil and ammonite assemblages of the uppermost Pliensbachian are interpreted in the context of a cooling, primarily affecting the calcareous shelled primary producers. The onset of the Jenkyns Event in the lower Toarcian post-dates this cooling and is related to a subsequent warming. This climatic shift caused a bloom of diverse opportunistic nannofossil and ammonite taxa in the lower Toarcian. Nannofossil abundance patterns were controlled by an increased nutrient availability. The rapid evolution of the ammonite genera Tiltoniceras, Eleganticeras and Cleviceras (family Hildoceratidae), which started in the lower part of the Jenkyns Event, is potentially related to their adaptation to a shallow water dwelling habitat. The mass occurrence of phylogenetically first holoplanktonic gastropods in the lower part of the Jenkyns Event is interpreted in the context of impoverished ocean water oxygenation, which in turn triggered the evolution of a new ecological strategy.

Organic matter variation during the Toarcian Oceanic Anoxic Event in the central and northern Atlantic margins: the interplay between local constraints vs global events

Thesis

Jul 2020

Bruno Rodrigues

In many locations of the northwestern Tethyan, Boreal and Panthalassic margins, the uppermost Pliensbachian–Lower Toarcian is characterised by the occurrence of organic-rich sediments. Significant perturbations of the global carbon cycle, related with an excess of 12C in the atmospheric and ocean reservoirs, are recorded as a negative carbon isotope excursions (CIE) in carbonate, bulk OM, fossil wood, kerogen and individual organic compounds, linked to the Pliensbachian–Toarcian Event (Pl–Toa Event) and Toarcian Oceanic Anoxic Event (TOAE). Contrariwise, the uppermost Pliensbachian–Lower Toarcian successions in northern Africa and most of Iberian Peninsula basins (Central–Northern Atlantic margin) are poor in OM. It is hypothesized that OM deposition and preservation in northern Africa and most of the Iberian Peninsula basins during the Pl–Toa Event and TOAE was controlled by the interplay of local–regional constraints and global forcings, therefore resulting in unique OM assemblages and differentiated from the organic-rich northern European basins. The main objective of this thesis was the analysis of kerogen assemblages from the uppermost Pliensbachian–Lower Toarcian references sections of the Middle Atlas (northern Africa), Betic Cordillera, Lusitanian and Asturian (Iberian Peninsula) basins based on total organic carbon (TOC), palynofacies, and δ13C in kerogen (δ13CKerogen). The obtained results indicate that the studied sections generally have low TOC contents (below 1 wt.%). The TOAE interval in the Asturian Basin and a discrete level in the Peniche section (Lusitanian Basin) recorded high TOC values, reaching up 2.9 wt.% at Asturian Basin and likely signalling local episodes of OM preservation promoted by dysoxic conditions. The obtained δ13CKerogen profiles in this study presented similarities with previously published high-resolution δ13CWood and δ13COrg records. Even with small differences between the studied sections, the δ13CKerogen record presented a positive trend during the Emaciatum/Spinatum and Polymorphum/Tenuiscostatum zones, followed by the negative trend associated with the TOAE. The difference in magnitude between δ13C records from different locations probably reflects the impact of local and/or regional environmental processes over global forcing. The kerogen assemblages are characterised almost exclusively by terrestrial OM, with a high abundance of opaque phytoclasts, suggesting that the OM was sourced from an area characterised by semi-arid or arid climate. Around the base of Polymorphum/Tenuiscostatum and Levisoni/Serpentinum zones, small increments in terrestrial OM were observed and represented by non-opaque phytoclasts, and terrestrial palynomorphs (sporomorphs, tetrads, and agglomerates). These were interpreted to represent an increase in fluvial runoff associated with the palaeoenvironmental perturbations of the Pl–Toa Event and TOAE. The subsequent change to higher δ13CKerogen values during the TOAE δ13C positive trend is more abrupt in the Asturian Basin (probably reflecting a low sedimentation rate) and more gradual in the western Iberian sections. During the middle part of the Levisoni/Serpentinum Zone, the δ13C remains relatively stable with the return to more positive δ13C values and it was suggested that after the carbon cycle perturbation the climates gradually cooled, favouring kerogen assemblages similar to the ones observed before the TOAE.

Global organic carbon burial during the Toarcian oceanic anoxic event: Patterns and controls

Article

Jun 2022
EARTH-SCI REV

The early Toarcian oceanic anoxic event (T-OAE, ~183 Ma), also known as the Jenkyns Event, was one of the most important hyperthermal events of the Phanerozoic, caused by the large-scale release of carbon from massive volcanism and/or surficial carbon reservoirs. Organic-rich facies record the event in basins worldwide, but the precise controls on organic carbon burial, and the relative importance of this burial as a mechanism of sequestering excess carbon from the Toarcian atmosphere, are uncertain. In this study, we have compiled total organic carbon (TOC) data from 67 lower Toarcian sections to reconstruct the pattern of organic enrichment through the T-OAE. Shallow marine sites dominate the compilation, and rates of organic carbon burial during the T-OAE were low relative to shallow water margins at the present day. Redox was a major control on organic enrichment and burial rates, but the very high TOC (>5%) observed in hydrographically restricted anoxic-euxinic basins in northern Europe and elsewhere were largely a consequence of low sedimentation rates. Globally, organic enrichment and redox conditions were highly variable, but a majority of sites show an increase in TOC at the T-OAE relative to pre-event values. TOC increases were generally highest where deoxygenation was most severe. In anoxic-euxinic marine basins in Europe organic carbon burial rates may have increased ~500% on average during the T-OAE, potentially sequestering an extra ~791 Gt of carbon relative to the same time period immediately prior to the event. Sites outside of Europe and those that remained oxic-suboxic or were dominated by terrestrial organic matter show variable, and often negligible, changes in organic enrichment across the T-OAE. Globally, an extra ~9000 Gt of carbon may have been buried in shallow seas during the T-OAE relative to before the event. Nevertheless, a paucity of data and significant uncertainties mean that the precise amount of excess carbon sequestered during the T-OAE remains uncertain.

The Jurassic structural high of Sasso di Pale (Umbria-Marche Basin, Italy): How a small Apennine structure recorded Early to Middle Jurassic global perturbations

Chapter

May 2022

The reduced Jurassic sedimentary sequences deposited on a structural high in the Umbria-Marche Apennines, as well their relationships with adjacent expanded basinal sequences, have been reconstructed through detailed, interdisciplinary study of the Sasso di Pale and Monte Serrone areas near Foligno, Italy. The physiographic features of the basin originated in the Early Jurassic (latest early Pliensbachian), when extensional tectonic activity broke up a shallow water platform where themCalcare Massiccio had been deposited, and the area evolved from an edge-stepped structural high to a distally steepened ramp. The biostratigraphic framework of this paper is mainly based on calcareous nannofossils, which are a useful tool for dating condensed Jurassic successions. Although the sections studied have limited thickness and much lateral facies variation, the sedimentary evolution can be traced and interpreted within a wider Jurassic environmental perspective. In the upper Pliensbachian– lower Bajocian interval, local sea-level variations are compatible with the global sea-level curve. Furthermore, some of the characteristic events—such as the Pliensbachian–Toarcian crisis, the Early Toarcian Jenkyns Event, and the Middle Jurassic carbonate crisis—can be recognized. The present study shows how the reconstruction of local paleogeography can fit into a more general framework and how regional and global signals can be recognized even in a small structural high such as the one we have investigated.

Rapid light carbon releases and increased aridity linked to Karoo-Ferrar magmatism during the early Toarcian oceanic anoxic event

Article

Full-text available

Mar 2022

Large-scale release of isotopically light carbon is responsible for the carbon isotope excursion (CIE) of the Toarcian Oceanic Anoxic Event during the Lower Jurassic. Proposed sources include methane hydrate dissociation, volcanogenic outgassing of carbon dioxide and/or thermogenic methane release from the Karoo-Ferrar magmatic province (southern Africa). Distinct small-scale shifts superimposed on the long-term CIE have been interpreted as rapid methane pulses linked to astronomically forced climate changes. In the Peniche reference section (Portugal), these small-scale shifts correspond to distinct brownish marly layers featuring markedly high mercury (Hg) and magnetic mineral concentration. Total organic carbon and Hg increase are uncorrelated, which suggests input of Hg into the atmosphere, possibly released after the intrusion of the Karoo-Ferrar sills into organic-rich sediments. Enhanced magnetic properties are associated with the presence of martite, washed-in oxidized magnetite, inferred to be due to increased aridity on the continental hinterland. This study provides strong evidence for a direct link between the Karoo-Ferrar magmatism, the carbon-isotope shifts and the resulting environmental changes.

Análisis de elementos traza en braquiópodos del Jurásico Inferior del Paleomargen Sud-Ibérico (SE de España). Correlación con las señales bióticas e implicaciones ambientales en torno al Evento de Extinción Masiva del Toarciense inferior

Article

Full-text available

Dec 2021

En el Jurásico Inferior se registran diversos eventos críticos que influyeron significativamente en los ecosistemas marinos del Tethys occidental. Entre las comunidades bentónicas, en el Paleomargen Sudibérico, los braquiópodos se vieron particularmente afectados por dichos eventos. El episodio tectono-sedimentario distensivo asociado a la apertura del proto-Atlántico conllevó el colapso de las amplias plataformas someras imperantes en el Tethys hasta el Sinemuriense superior-Pliensbaquiense basal, con la consiguiente reorganización de los ecoespacios faunísticos. Posteriormente, el evento de extinción registrado en el Toarciense inferior, trajo consigo importantes alteraciones en el ciclo del carbono así como el desarrollo de condiciones anóxicas que afectaron principalmente a las comunidades bentónicas. En el dominio Subbético, la dinámica poblacional de los braquiópodos coincidió con estos importantes eventos de perturbación ambiental. Se ha analizado la impronta geoquímica registrada en conchas de braquiópodos del Subbético oriental, revelando una clara sincronía entre las oscilaciones del contenido en elementos traza, las tendencias globales en el ciclo del C y del O y la diversidad de la braquiofauna en torno a dichos eventos críticos, lo que permite validar modelos globales y regionales relacionados tanto con el evento de rifting incipiente de las plataformas someras en el Sinemuriense-Pliensbachiense, como con la crisis biótica global en torno al Toarciense inferior. En la renovación faunística verificada para el tránsito Sinemuriense-Pliensbachiense y para el evento de extinción del Toarciense, los metales traza sensibles a las condiciones redox, la concentración de REE y el contenido en Fe en las conchas de braquiópodos muestran excursiones positivas. Esta tendencia, junto a los patrones de diversidad de los braquiópodos, los bajos valores de TOC y las evidencias sedimentarias, sugieren que, en esta región, la anoxia debió representar un factor secundario como causa de estrés ambiental para la fauna bentónica. En cambio, se postula que el progresivo aumento de la temperatura jugó un papel determinante en las cuencas marginales del Tethys occidental, como se demuestra al correlacionar los principales eventos de renovación y sustitución faunística con las paleotemperaturas de las plataformas peri-ibéricas. Los cambios en la paleoproductividad, los aportes continentales y posibles contribuciones hidrotermales se relacionan asimismo con las oscilaciones de determinados elementos traza y se interpretan, por tanto, como factores coadyuvantes desencadenantes de estos bioeventos del Jurásico Inferior en el Tethys occidental.

Carbon Cycle and Ecosystem Response to the Jenkyns Event in the Early Toarcian (Jurassic)

Article

Oct 2021

The Toarcian Oceanic Anoxic Event, also known as the Jenkyns Event, was a hyperthermal episode which occurred during the early Toarcian (c. 183 Ma; Early Jurassic) and resulted in numerous collateral effects including global warming, enhanced weathering, sea-level change, carbonate crisis, marine anoxia–dysoxia, and a second-order mass extinction. This volume presents the last advances for understanding early Toarcian environmental changes through different disciplines: biostratigraphy, micropalaeontology, palaeontology, ichnology, palaeoecology, sedimentology, integrated stratigraphy, inorganic, organic and isotopic geochemistry, and cyclostratigraphy. The study of this abrupt climate change is critical for predicting future global changes, and for understanding the complex biogeochemical interactions through time between geosphere, atmosphere, hydrosphere and biosphere.

Assessing the importance of thermogenic degassing from the Karoo Large Igneous Province (LIP) in driving Toarcian carbon cycle perturbations

Article

Full-text available

Oct 2021

The emplacement of the Karoo Large Igneous Province (LIP) occurred synchronously with the Toarcian crisis (ca. 183 Ma), which is characterized by major carbon cycle perturbations. A marked increase in the atmospheric concentration of CO 2 ( p CO 2 ) attests to significant input of carbon, while negative carbon isotope excursions (CIEs) in marine and terrestrial records suggest the involvement of a ¹² C-enriched source. Here we explore the effects of pulsed carbon release from the Karoo LIP on atmospheric p CO 2 and δ ¹³ C of marine sediments, using the GEOCLIM carbon cycle model. We show that a total of 20,500 Gt C replicates the Toarcian p CO 2 and δ ¹³ C proxy data, and that thermogenic carbon (δ ¹³ C of −36 ‰) represents a plausible source for the observed negative CIEs. Importantly, an extremely isotopically depleted carbon source, such as methane clathrates, is not required in order to replicate the negative CIEs. Although exact values of individual degassing pulses represent estimates, we consider our emission scenario realistic as it incorporates the available geological knowledge of the Karoo LIP and a representative framework for Earth system processes during the Toarcian.

First record of the early Toarcian Oceanic Anoxic Event in the Hebrides Basin (UK) and implications for redox and weathering changes

Article

Oct 2021
GLOBAL PLANET CHANGE

The early Toarcian (~183 Ma) was characterized by a prominent volcanism-induced warming event associated with a massive addition of ¹²C-enriched carbon to the ocean-atmosphere system. This warming likely contributed to marked ocean deoxygenation during this time, giving the event its name: the early Toarcian oceanic anoxic event (T-OAE). Although the T-OAE has been recognized globally, clear geographic differences in the character of the event and its environmental effects have been noted. Here we present new carbon isotope, element abundance and organic geochemical data from a lower Toarcian succession on the Isle of Raasay, Scotland (Hebrides Basin, Northwest European Shelf). These data provide the first evidence of the T-OAE in Scotland. The succession is generally enriched in organic matter through the T-OAE interval, though redox-sensitive trace element data indicate that oxic-suboxic bottom water conditions prevailed, potentially interspersed with ephemeral anoxic episodes. Our elemental data contrast with evidence for persistent anoxia/euxinia in nearby basins, and emphasizes how deoxygenation was spatially variable and dependent on water depth and basin hydrography. Similarly, the data emphasize how anoxia was not a prerequisite for the deposition of organic-rich lithologies during the T-OAE. Sedimentological evidence, coupled with inorganic geochemical data, indicates increased coarse-grained detrital flux and enhanced chemical weathering during the T-OAE. Our findings support emerging evidence for a marked strengthening of hydrological cycling and increased storminess at tropical and subtropical latitudes globally in response to global warming during the T-OAE.

Did ocean warming, acidification or saturation state control coccolithophorids production through time ?

Thesis

Oct 2020

Alessandro Menini

Ongoing CO2 emissions are expected to increase global temperatures and affect ocean chemistry better known as ‘ocean acidification’. This latter is a major threat for marine calcifying biota (corals, foraminifera, coccolithophorids). However, recent works identify a particular resilience of coccolithophorids to past ocean acidification events, whereas temperature limits biocalcification. Studies on cultured coccolithophorids or surface-water samples allow understanding the response to short-term temperature, pCO2 or pH variations, but long-term species-specific resilience to environmental changes is crucial to evaluate the respective role of ocean acidification and temperature on fossil calcifying biota. We approach the long-term response of coccolithophorids to these parameters by analyzing the fossil record of two past global events showing perturbation of the carbon cycle associated to CO2 fluctuations and global warming: these are the Toarcian Oceanic Anoxic Event (T-OAE; 183 Ma) and the Paleocene-Eocene Thermal Maximum (PETM; 56 Ma). Firstly, I present new high resolution biostratigraphy for both events in order to link the bioevents to the carbon stable isotopes curve and hence to the carbon isotopes excursion defining the T-OAE and the PETM. We quantify the response of fossil cocoliths to past hyperthermal events by quantifying nannofossils accumulation rate. Both are hyperthermals, but only the former records organic matter accumulations (black shales). Besides ocean acidification, the carbonate saturation and alkalinity of the oceans have an impact on coccolithophorids; we will thus use the calcium isotope composition of the test of planktonic foraminifera to track changes in CO32- concentration across the PETM, where ocean acidification is inferred, and compare these data with analogue records for the T-OAE

The Toarcian Oceanic Anoxic Event: where do we stand?

Article

Full-text available

Aug 2021

The study of past climate changes is pivotal for understanding the complex biogeochemical interactions through time between the geosphere, atmosphere, hydrosphere and biosphere, which are critical for predicting future global changes. The Toarcian Oceanic Anoxic Event, also known as the Jenkyns Event, was a hyperthermal episode which occurred during the early Toarcian (∼183 Ma; Early Jurassic) and resulted in numerous collateral effects including global warming, enhanced weathering, sea-level change, carbonate crisis, marine anoxia-dysoxia, and biotic crisis. The IGCP-655 project of the IUGS-UNESCO has constituted an international network of researchers with different disciplinary skills who collaborated and shared conceptual advances on uncovering drivers of the environmental changes and ecosystem responses. This volume, Carbon Cycle and Ecosystem Response to the Jenkyns Event in the Early Toarcian (Jurassic), presents 16 works that investigate the early Toarcian environmental changes related to the global warming, sea-level rise, carbon cycle perturbation and second-order mass extinction through biostratigraphy, micropalaeontology, palaeontology, ichnology, palaeoecology, sedimentology, integrated stratigraphy, inorganic, organic and isotopic geochemistry, and cyclostratigraphy.

Primary versus carbonate production in the Toarcian, a case study from the Llanbedr borehole (Mochras Farm, Wales)

Article

Aug 2021

The leading hypothesis for the Toarcian oceanic anoxic event (T-OAE; ∼183Ma) and the associated negative C-isotope excursion is the massive release of ¹² C favouring greenhouse and continental weathering. The nutrient delivery to shallow-basins supported productivity and, because of O 2 -consumption by organic-matter respiration, anoxia development. However, several works showed that calcareous nannoplankton experienced a decrease during the T-OAE. Nannofossil fluxes measured in the Llanbedr borehole (Mochras Farm; Wales, UK) were the highest prior to the negative C-isotope excursion, along with high amounts of taxa indicative of nutrient-rich environments (Biscutaceae). Such conditions attest to high productivity. Fluxes show the lowest values in the core of the event, along with a size decrease of Schizosphaerella and a peak in Calyculaceae. The recovery of nannofossil fluxes and Schizosphaerella size occurred concomitant with the return of C-isotopes to more positive values. Concomitantly, deep-dwellers ( Crepidolithus crassus ) dominated, indicating a recovery of the photic-zone productivity. These observations demonstrate that the cascade of environmental responses to the initial perturbation was more complex than previously considered. In spite of elevated nutrient delivery to epicontinental basins in the early Toarcian, carbonate and primary productions of nannoplankton were depressed in the core the T-OAE likely because of prolonged thermohaline sea-water stratification. Supplementary material at https://doi.org/10.6084/m9.figshare.c.5541440

Pacing of late Pliensbachian and early Toarcian carbon cycle perturbations and environmental change in the westernmost Tethys (La Cerradura section, Subbetic Zone of the Betic Cordillera, Spain)

Article

Jun 2021

The detailed assessment of high-resolution elemental and isotopic geochemical datasets collected from the marl-limestone alternations cropping out at La Cerradura (Subbetic domain of the Betic Cordillera, Spain) and chrono- and chemostratigraphic correlation with the reference Mochras borehole (Cardigan Bay Basin, UK) unveiled valuable new insights to the understanding of late Pliensbachian-early Toarcian palaeoenvironmental dynamics at a key geographical area between the northern European seaway and the Tethys Ocean. This study shows that deposition in the study area took place under dominantly oxic water column conditions, indicated, for example, by the generalised lack of enrichment in organic matter and redox metals typically associated with anoxia and euxinia. Carbon isotope stratigraphy (δ13CTOC) allowed to recognise the spinatum (=emaciatum in the Submediterranean Province), Pliensbachian-Toarcian, and early Toarcian Oceanic Anoxic Event negative carbon isotopic excursions and the late Pliensbachian positive carbon isotopic excursion. It is here suggested that the observed periodic changes in lithology and sedimentary geochemistry occur at orbital frequencies (i.e., long and short eccentricity and, tentatively, precession), hinting at an astronomical control of the local-regional climate and environment during the Pliensbachian and Toarcian in the mid-low latitude South Iberian palaeomargin area.

A bibliometric analysis of two-decade research efforts in turning natural gas hydrates into energy

Article

Apr 2024
ENERGY

Variations of sedimentary environment under cyclical aridification and impacts on eodiagenesis of tight sandstones from the late Middle Jurassic Shaximiao Formation in Central Sichuan Basin

Article

Mar 2024
MAR PETROL GEOL

The Toarcian Oceanic Anoxic Event in a shelf environment (Eastern Tethys): Implications for weathering and redox conditions

Article

Sep 2023
SEDIMENT GEOL

The Toarcian Oceanic Anoxic Event (T-OAE) is characterized by significant environmental perturbation and climatic instabilities due to a substantial release of greenhouse gases. Several investigations of the T-OAE were conducted based on reliable and well-developed integration of multi-proxy approaches, revealing the global distribution of this event in the Panthalassic, Boreal, and Western Tethyan regions. However, a few marine shelf T-OAE records are reported from the Eastern Tethys. This study presents high-resolution organic and inorganic and isotope geochemistry, mineralogical, and sedimentological analyses from the marine Suobucha section of the Qiangtang Basin to assess the role of continental weathering and paleoceanographic conditions, including paleoredox and paleoproductivity conditions, and factors that govern organic carbon accumulation during the T-OAE in the Eastern Tethys. The carbon isotope profile of Suobucha displays a long-term negative excursion characterizing the Jenkyns Event, which is recognized in geographically widespread sedimentary archives indicating its global nature. Continental weathering was intensified at the onset of the Jenkyns Event. Increased values of fluvial detrital proxies (i.e., Si/Al and Ti/Al) and the occurrence of coarser-grained sediments (i.e., silty mudstones) at Suobucha indicated enhanced terrigenous input at the onset of the Jenkyns Event, which was driven by accelerated continental weathering at this time. The lower Toarcian interval in the marine shelf environment of the Qiangtang Basin is characterized by fully oxidizing conditions intermittent with minor phases of dysoxic setting, especially during the Jenkyns Event interval. Given that a stratified water column with anoxic bottom water occurred in the proximal and more restricted lagoonal area (i.e., the Bilong Co area), redox conditions were spatially variable and controlled by basin hydrography and water depth during the Jenkyns Event in the Qiangtang Basin. Oxygen-enriched conditions and low marine bioproductivity were responsible for the low organic carbon burial within the Suobucha section.

Cyclostratigraphy of the Lower Jurassic (Toarcian) terrestrial successions in the Sichuan Basin, southwestern China

Article

Mar 2023

Long duration of benthic ecological recovery from the early Toarcian (Lower Jurassic) mass extinction event in the Cleveland Basin, UK

Article

Full-text available

Dec 2022

The Cleveland Basin of Yorkshire, UK, hosts one of the most iconic Lower Jurassic rock successions for studying the Toarcian Oceanic Anoxic Event and the associated mass extinction, yet our understanding of the subsequent recovery is limited. This study documents for the first time the full extent and nature of benthic macrofaunal recovery from the early Toarcian mass extinction event within the Cleveland Basin. Following the extinction event benthic oxygen levels remained low, allowing for specialist low-oxygen tolerant communities to dominate. Recovery properly commences once sea floor ventilation began to improve and was first expressed by an expanded ecological tiering structure. Recovery progressed slowly thereafter with the possible return to oxygen restricted environments. As sea levels fell and sand-dominated deposition occurred again within the basin, the recovery accelerated with ecological and species richness reattaining, and furthermore exceeding, pre-extinction levels. Full recovery occurred, at the latest, ca. 7 myr after the extinction, this duration is on par with estimates of recovery rates from the largest mass extinction of the Phanerozoic (the end-Permian mass extinction event). Recovery within the Cleveland Basin was likely to have been strongly influenced by local sea levels and continuation of challenging environmental conditions after the extinction event. Supplementary material: https://doi.org/10.6084/m9.figshare.c.6332986

On the Rarity and Peculiarity of the Early Toarcian (Lower Jurassic) Leukadiellinae Ammonites—Systematic Review and Insights on the Interplay of Environmental Stress, Evolution and Biodiversity

Article

Full-text available

Nov 2022

Domenico Ridente

The subfamily Leukadiellinae Macchioni and Venturi, 2000 includes the two rare genera Leukadiella Renz, 1913 and Renziceras Arkell, 1953. Genus Leukadiella is characterised by ornamental and structural features unusual to the family Hildoceratidae Hyatt, 1867; for this reason, it has been occasionally grouped with other “odd” representatives of this family, as, for instance, Frechiella Prinz, 1904 and Paroniceras Bonarelli, 1893. While Renziceras comprises only one species, the eleven species currently recognised within Leukadiella show a noticeably discontinuous variability field; together with rarity of findings, the interpretation of this variability represents a renowned obstacle to their classification and phylogenetic reconstruction. Following an analysis of the morphological characters commonly used for defining Leukadiella species, a revised taxonomic scheme is herein proposed. Two clusters of characters are defined, based on which two morphologically distinct groups of species are distinguished, referred to as Helenae Group and Ionica Group. The genus name Leukadiella is maintained for species of the Helenae Group, which are restricted via synonymy to Leukadiella helenae Renz, 1913 and Leukadiella jeanneti Renz, 1927. The new genus name Neoleukadiella gen. nov. is proposed for species of the Ionica Group, which, by analogous limitation, are Neoleukadiella ionica Renz and Renz, 1946 and Neoleukadiella gallitellii Pinna, 1965. The reduction in the number of species from eleven to four is consistent with their rarity and limited paleogeographic distribution. The closely related Renziceras is considered the direct progenitor of Leukadiella; in turn, the Apennine genus Cingolites Sassaroli and Venturi, 2010 is proposed as transitional between Hildaites Buckman, 1921 and Renziceras. Conversely, the progenitor of Neoleukadiella remains uncertain, although a possible relationship between Leukadiella and Neoleukadiella species is tentatively traced. Finally, some challenging assumptions are made by emphasizing the role of environmental stress in controlling developmental dynamics that may drive striking phenotypic modification, of the kind observed in the Leukadiellinae species.

Control of climate event caused by volcano on organic matter enrichment in the lacustrine source rocks of the Ordos Basin, central China

Article

Jul 2022

To reveal the influence mechanism of volcano on lacustrine organic matter enrichment, this paper, taking the Yanchang Formation in the Ordos Basin as an example, systematically analyzed the geochemical characteristics of source rocks, and found that black shale was affected by climate event caused by volcano. However, organic matter enrichment is not synchronous with volcanic eruption in time, but has “hysteresis” effect. The evidence of volcanic activity mainly includes two aspects: one is large amount of volcanic ash in black shale, and the other is negative shift of organic carbon isotope. The volcanic ash was airborne, which came from the southern Qinling Orogenic Belt. Because the volcano eruption occurred on land, the negative shift of organic carbon isotope reached −2‰. The climate event caused by volcano has three effects on organic matter enrichment in black shale: first, it brought nutrients for the growth of organisms and released a large amount of greenhouse gases such as CO 2 to accelerate photosynthesis and promote the growth of organisms; second, a large amount of toxic gases such as SO 2 and H 2 S were released and O 2 was consumed and diluted in the air, forming a severe anoxic environment and accelerating the preservation of organic matter; third, it caused extremely hot weather, resulting in long-time surface runoff and other climate events, increasing input of terrestrial organic matter, and forming light components of hydrocarbons.

The Triassic–Jurassic transition – A review of environmental change at the dawn of modern life

Article

Jul 2022
EARTH-SCI REV

The Triassic–Jurassic transition, which is here broadly defined as extending from the Late Triassic through the Early Jurassic (~237 Ma to 174 Ma), was an important interval in Earth history. The end-Triassic mass extinction (ETME), at ~201 Ma, ranks among the ‘Big Five’ Phanerozoic mass extinctions. It largely completed the shift from the ‘Paleozoic Evolutionary Fauna’ to the ‘Modern Evolutionary Fauna’ that had been initiated by the end-Permian mass extinction, and may have contributed to the ‘Mesozoic Marine Revolution’ and rise of dinosaurs to dominance in terrestrial environments. In addition, the Triassic–Jurassic transition encompasses a second-order mass extinction during the early Toarcian oceanic anoxic event (T-OAE), at ~181 Ma. The ETME was triggered by Central Atlantic Magmatic Province (CAMP) magmatism, and the T-OAE by Karoo-Ferrar Large Igneous Province (KFLIP) magmatism, both associated with the stepwise disintegration of the Pangean supercontinent. These events led to major changes in continental and marine habitats, including climatic warming, ocean acidification, and widespread watermass anoxia, that produced a cascade of lethal environmental stresses. This article undertakes a review of the ETME and T-OAE mass extinctions, the large igneous province eruptions that triggered those biotic events, and the web of environmental changes that linked them together.

The palynology of the Toarcian Oceanic Anoxic Event at Dormettingen, southwest Germany, with emphasis on changes in vegetational dynamics

Article

Full-text available

Jun 2022
REV PALAEOBOT PALYNO

The Toarcian Oceanic Anoxic Event (T-OAE; ~ 183 Ma) represents an episode of marine anoxia that lasted for several hundred thousand years. Abiotic factors contributing to the formation of the T-OAE, such as global warming, changes in weathering intensity, or sea-level change, are associated with a marked change in carbon cycling. While these factors are well studied, detailed palynological data, including marine and terrestrial palynomorphs, is still missing. Here we present comprehensive palynological data from the sedimentologically and geochemically well constrained T-OAE section in Dormettingen (SW Germany). Palynological assemblages prior to the T-OAE reflect a mixed gymnosperm-pteridophyte vegetation on land. They also include unseparated spore tetrads and sporomorphs with darkened walls indicative of environmental stress. During the early stage of the T-OAE, gymnosperms decline and only pteridophytes are recorded in the palynological assemblages, terrestrial vegetation recovery commenced before the end of the T-OAE. Contrastingly, dinoflagellate cyst diversity declines significantly in the first stage of the T-OAE and is reduced to zero towards the end of the T-OAE. Our data shows that the terrestrial ecosystems reacted early to the abiotic disturbances reflected in carbon isotope data even before the T-OAE.

Astronomical Constrains on the Duration of Toarcian Global Carbon Perturbation and Lacustrine Environmental Paced by Obliquity Variations in the Sichuan Basin, China

Article

Jan 2022

Astronomical Constrains on the Duration of Toarcian Global Carbon Perturbation and Lacustrine Environmental Paced by Obliquity Variations in the Sichuan Basin, China

Article

Jan 2022

Effects of the Pliensbachian–Toarcian Boundary Event on Carbonate Productivity of a Tethyan Platform and Slope

Article

Full-text available

May 2022

We explore the effects of the Pliensbachian–Toarcian Boundary Event (P–ToBE) on tropical carbonate productivity in the interior to margin and slope of the Venetian Platform (Northern Italy). We document the P–ToBE for the first time in the shallow‐water platform margin, and we bio‐ and chemostratigraphically tie it to transgressive/regressive cycles. Following the latest Pliensbachian sea‐level drop and emersion, transgressive grainstones at the platform edge record the P–ToBE negative carbon isotope excursion (CIE) of 1–1.5‰, also found in marl/limestone couplets on the slope. Recovery of platform productivity was ephemeral, as the platform drowned right after the peak negative CIE and was covered by deep‐sea thin‐bedded micritic limestones. The end of the P–ToBE correlates with a regression and renewed recovery of carbonate productivity. The negative CIE of the subsequent Toarcian Oceanic Anoxic Event is recorded in open‐sea cherty limestones both at the marginal and interior platform. These limestones document an even wider transgression and the renewed partial drowning of the platform in the Serpentinus Zone. We investigate the causes of the carbon perturbation at the P–ToBE, using a simple carbon cycle model. The duration and magnitude of the CIE suggest a rapid release of methane in driving the CIE, perhaps related to the preceding sea‐level drop and associated cryosphere perturbation, or to thermogenic alteration of coals near the Karoo‐Ferrar Large Igneous Province (LIP). The extent of the warming and the magnitude of the P–ToBE CIE implies a contribution of volcanogenic carbon dioxide from the Karoo‐Ferrar LIP.

What controls the duration and intensity of ocean anoxic events in the Paleozoic and the Mesozoic?

Article

Sep 2021
EARTH-SCI REV

The sedimentological and geochemical expression of Mesozoic ocean anoxic events (OAEs) in the geologic record is reasonably well-characterized and the causal mechanisms driving these events are generally agreed upon. By contrast, OAE-type intervals in the Paleozoic are relatively understudied and less well constrained. Here, we review literature published on three intervals that may be termed OAEs in the Paleozoic: the Late Cambrian Steptoean Positive Carbon Isotope Excursion (SPICE) event, the Late Ordovician – Early Silurian Hirnantian Ocean Anoxic Event (HOAE), and the Late Devonian Frasnian-Famennian, or Kellwasser events. Comparing them to Mesozoic OAEs reveals that Paleozoic OAEs are typically longer in duration than Mesozoic OAEs, and it is also likely that anoxic waters were more areally extensive during these Paleozoic events. With the traditional framework for OAEs the potential for a very extended timeframe of anoxia is problematic, given that more extensive bottom-water anoxia should accelerate reoxygenation of the atmosphere-ocean by promoting more efficient phosphorous recycling in the ocean and therefore boosting primary productivity. However, benthic phosphorous retention could have been enhanced under anoxic and iron-rich (ferruginous) marine conditions. Therefore, we propose a role for ferruginous conditions with more widespread anoxia in driving longer OAEs. Ultimately, a protracted increase in atmospheric oxygen concentrations through the Phanerozoic may have been a key factor controlling the expression of OAEs through time.

The palaeoenvironmental context of Toarcian vertebrate-yielding shales of southern France (Hérault)

Article

Sep 2021

The Early Jurassic was marked by several episodes of rapid climate change and environmental perturbation. These changes culminated during the Toarcian Oceanic Anoxic Event (T-OAE), an episode of global warming that led to the widespread deposition of organic-rich shales. The Toarcian shales of NW Europe have also yielded exceptionally preserved fossils of marine vertebrates and invertebrates, but the potential links between the occurrences of these exceptionally preserved fossils and the T-OAE remain poorly investigated. Palaeontological excavations realized in Toarcian strata near Lodève (Hérault, southern France) have yielded several specimens of marine vertebrates and abundant invertebrate fauna. We have developed a multiproxy approach (ammonite biostratigraphy, X-ray diffraction-bulk mineralogy, Rock-Eval pyrolysis, stable isotopes, trace element, phosphorus and mercury contents) to place these findings in a well-defined temporal and palaeoenvironmental context, and hence constrain the factors that led to their remarkable preservation. The Jenkyns Event interval, unambiguously identified at the base of the Toarcian organic-rich shales by a 5‰ negative carbon isotope excursion, records higher mercury fluxes, which suggest a causal link with intense volcanic activity of the Karoo–Ferrar large igneous province. This interval is very condensed and unfossiliferous, and might have been deposited under abnormally low-salinity conditions. Our data show that the deposition of the vertebrate-yielding horizons post-dated the T-OAE by several hundreds of ka, and took place during a prolonged period of widespread oxygen-deficiency and elevated carbon burial. Our results indicate that the unusual richness in vertebrates of the studied site can be explained by a combination of regional factors such as warming-induced, prolonged seafloor anoxia, and more local factors, such as extreme condensation owing to reduced dilution by carbonate and detrital input.

Développement du thermomètre Δ47 appliqué sur coccolithes : de la calibration en laboratoire à l’applicabilité au registre sédimentaire

Thesis

Jul 2017

Amandine Katz

Le géothermomètre Δ47 est basé sur la relation entre l’abondance des liaisons 13C–18O des carbonates et la température de calcification. Ce proxy contourne potentiellement les limites des autres thermomètres (δ18O, Mg/Ca) pour reconstruire les paléo-températures des océans, expliquant son développement exponentiel depuis dix ans. Cette thèse teste pour la première fois le potentiel et les limites de la thermométrie Δ47 sur les coccolithes, des nannofossiles calcaires produits par des organismes calcifiants dans la zone photique. Ces biominéraux calcitiques et ubiquistes constituent une part importante de l’archive sédimentaire. Des cultures in vitro nous ont permis d’établir que trois espèces de coccolithes actuelles enregistrent la même relation Δ47 – T que la calcite inorganique, alors qu'elles présentent de très larges effets vitaux en δ18O (±5‰). Nous concluons que ces espèces de coccolithes d'importance géologique ne présentent pas d’effets vitaux en Δ47. Nous avons ensuite appliqué le Δ47 à l’étude des sédiments enregistrant l’événement d’anoxie océanique du Toarcien (–183 Ma) au cours duquel les reconstructions de températures restent encore ambigües, notamment du fait de la méconnaissance de la composition isotopique en oxygène de l’eau de mer. Sur la base des données Δ47 acquises, nous proposons des températures élevées (de l’ordre de 36°C), mais restant relativement stables sur l'intervalle d'étude. En couplant ces températures aux données de δ18O des carbonates, nous suggérons une variation importante du δ18O de l'eau de mer dans le Bassin de Paris lors de la mise en place des faciès black shales. Enfin, sur des sédiments pélagiques subactuels, l’une des espèces étudiées présente des déséquilibres isotopiques en Δ47 non observés en culture et explicables par d’autres paramètres environnementaux comme l’intensité lumineuse. Cette thèse illustre le potentiel du thermomètre Δ47 des coccolithes en différents contextes, ouvrant un vaste champ d’application de reconstruction des paléo-environnements sur le Méso-Cénozoïque

Le genre Soaresirhynchia nov. (Brachiopoda, Rhynchonellacea, Wellerellidae) dans le Toarcien du sous-bassin nord-lusitanien (Portugal)

Book

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Jan 1994

Yves Alméras

The Toarcian of the North-Lusitanian sub-basin (Portugal) has yielded many brachio- pods and specially more than 2 800 shells referred to the new genus Soaresirhynchia nov. gen. (Rhynchonellacea, Wellerellidaè). Stolmorhynchia Buckman is now well defined since the révision of its type-species, S. stolidota (Presser, 1993) and the toarcian species up to here attributed to this genus must be separated. They are now referred to Soaresirhynchia nov. gen. In addition to the type-species, Rhynchonella bouchardi Davidson, these species are Rhynchonella rustica (Dubar), R.flamandi (Dubar),Æ. clesiana (Lepsius) and R. renzi nov. sp., the paleontological révision of which is here presented (chapter II, excepted that of R. clesiana which is missing in Portugal). S. renzi is here described and fïgured for the first time. It was a nomen nudwn, used by Mouterde étal, in his papers with stratigraphy, but its définition was known only according to the collections of Choffat in Lisbon. The chapter I, Stratigraphy, gives the main localities which hâve yielded brachiopods (and specially the Soaresirhynchia nov.). It constitutes a preliminary inventory which will allow to establish a biostratigraphy of the portuguese Toarcian according to the brachiopods. The conclusions explicit the phylogeny and the paleobiogeography of Soaresirhynchia nov. gen. on the Middle European platforms and in the Western Tethys. Outwards S. bouchardi, the Soaresirhynchia species hâve evolved in the western tethyan realm only. The influence of environments controlled by the paleogeographical évolution, is evidenced also. The gaps in the Soaresirhynchia lineage, as in ail brachiopods, are due to unfavorable environmental conditions (but notexclusively). As examples of unfavorable faciès, we can quote the Early Toarcian «schistes-carton» (Normandie, Quercy), the Late Toarcian oolitic and/or crinoidic Limestones at Peniche or the Maris with Dumortierinae developed in Quercy and in the ibero-pyrenean realm. Some species were adapted to peculiar environments. For instance, S. renzi has lived in the sponge-bioconstructions characterized by a moderate hydrodynamism (Speciosum Zone of Sâo Giao) while Nannirhynchia cotteri (Choffat), with its almostentirely smooth and délicate shells, prevails in calmmarly faciès situated betwwen the bioconstructions.

Depositional sequences in deep-shelf environments formed through carbonate-mud import from the shallow platform (Late Oxfordian, German Swabian Alb and eastern Swiss Jura)

Article

Full-text available

Jan 1998

Upper Oxfordian deposits in southern Germany exhibit limestone-marl alternations typical of deep-shelf depositional environments. These deposits contain varying amounts of brachiopods, echinoderms, foraminifera (mainly represented by Spirillina, Lenticulina, Usbekistania, Bigenerina, Glomospira and Reophax), cephalopods, sponges and associated encrusters, and scarce bivalves, ostracods and gastropods. Fragments of reworked microbialites (tuberoids, Tubiphytes, nubecularians, bryozoans, serpulids, and Terebella), and glauconite also occur in variable quantities. In one section of the proximal shelf area (eastern Swiss Jura) there is interfingering of facies dominated by platform-derived elements (ooids, oncoids, coral fragments, peloids, bivalves, ostracods, gastropods) and of facies related to a more parautochthonous, distal sedimentation (Rhaxella and other sponge-spicules, brachiopods, Lenticulina, Spirillina). The complete record of each ammonite zone as well as the recognition of different ammonite horizons suggest that no important sedimentary gap is present. For the deep-shelf deposits of southern Germany, statistical analysis shows that the higher the total percentage of particles is in a sample, the more frequent are glauconite, bioturbation, nodularization, cephalopods, sponges, and microbial crusts. Wackestone and packstone samples thus generally correspond to lower sedimentation rates than mudstones that reflect a high carbonate-mud sedimentation rate. The carbonate mud is thought to be exported from the shallow platform because scarce nannofossils and/or insignificant bioerosion in sponge reefs exclude the possibility of relating carbonate-mud variations to changes in autochthonous productivity. Variation in carbonate-mud exportation from the platform towards the deep shelf has thus been implied from the relative abundance of particulate elements (fauna, tuberoids) which are considered as mainly autochthonous or parautochthonous. Consequently, changes in carbonate-mud content in the studied deep-shelf settings may be related to carbonate production on the shallow platform and to the export dynamics from the platform to deeper sedimentary environments. Variation in carbonate sedimentation rate has been used to interpret depositional sequences in terms of sequence stratigraphy and cyclostratigraphy: constrained by a detailed bio- and chronostratigraphical framework, a correlation of these sequences is proposed between the sections in southern Germany and the eastern Swiss Jura.

Correlaciones bioestratigráficas en el Toarciense de la Península Ibérica

Article

Full-text available

Jan 1989

Carbon Isotope Excursion in Atmospheric CO2 at the Cretaceous-Tertiary Boundary: Evidence from Terrestrial Sediments

Article

Full-text available

Aug 2000

A -1.5‰ to -2‰ carbon isotope excursion immediately above the clay layer that defines the Cretaceous-Tertiary (K/T) boundary has been reported in marine sediments world wide. This paper reports a similar -1.5‰ to -2.8‰ carbon isotope excursion recorded by C3 land plants from three temporally-controlled, stratigraphically-constrained terrestrial sections in the Western Interior of North America (Garfield County, Montana, and Slope County, North Dakota). Carbon isotope measurements of bulk sedimentary organic carbon were well-correlated with those of isolated plant cuticle, suggesting that the terrestrial organic carbon signature in these sediments parallels that of plant cuticle. Carbon isotope signatures were also independent of rock type and depositional environment, showing that the carbon isotope signature of plants, although altered, is not biased taphonomically. Because the signature in terrestrial facies records the isotope composition of paleoatmospheric CO 2, this record-combined with that from marine sections-offers additional insight into changes in carbon cycling underlying the K/T negative carbon isotope excursion. For example, radiometric age determinations from the Hell Creek Road locality in Montana bracket the atmospheric carbon isotopic recovery between 65.00 ± 0.05 Ma and 65.16 ± 0.04 Ma. This reflects a more rapid recovery for the terrestrial biosphere than for that of the marine realm, perhaps due to lower extinction rates in land plants than in marine primary producers.

The timing of paleoenvironmental change and cause-and-effect relationships during the Early Jurassic mass extinction in Europe

Article

Full-text available

Dec 2005

Paul B Wignall

The early Jurassic marine mass extinction is one of several crises thought to coincide with anoxia, transgression and warming caused by catastrophic release of gas hydrates. However, high-resolution study of expanded sections in Yorkshire, England, reveal that only the first of these factors is truly coincidental with extinction. The well known transgression and large, negative carbon isotope perturbation , attributed to massive release of methane from gas hydrates, occur after these events. The anoxic event is developed diachronously in the European area, with anoxia developing and fading away earlier in the Mediterranean region compared with the NW European record. However, for a brief interval (in the mid semicelatum Subzone), anoxia was simultaneously developed throughout the European region and it is this time that is marked by extinction of both benthic and pelagic marine fossils. The sea-level curve is also more complex than hitherto assumed, with a minor regression occurring late in the semicelatum Subzone, shortly after the extinction. The Toarcian crisis occurred during a phase of global warming, but the postulated release of methane from gas hydrates is too late to be implicated in the extinction mechanism as indeed is a recently reported cooling event from within the warming trend.

The Karoo large igneous province: Brevity, origin, and relation with mass extinction questioned by new 40Ar/39Ar age data: REPLY: REPLY

Article

Full-text available

Jan 2007
GEOLOGY

The peak activities of continental flood basalts are currently considered as huge and brief (˜1 m.y.) magmatic events, with strong implications for geodynamics and biotic turnover. New 40Ar/39Ar dates on the Karoo flood basalts (southern Africa) show a longer duration of magmatism (˜8 m.y., with 6 m.y. for the main volume) with an apparent south-to-north migration, along with briefer distinctive pulses inside the province. This suggests that the Karoo province does not fit the general plume model invoked for most continental flood basalts (including the Karoo) and may explain the absence of a major contemporaneous mass extinction.

Glacial/interglacial sedimentation rates and turbidite frequency in the Bahamas

Article

Full-text available

Jan 1985
GEOLOGY

The southern Tongue of the Ocean is a 1300-m-deep, flat-floored basin in the Bahamas that receives large amounts of sediment from the carbonate platforms surrounding it on three sides. We have examined five 8 13-m-long piston cores and determined bulk sedimentation rates, turbidite frequency, and turbidite accumulation rates for the past two glacial and interglacial periods. The mean of bulk sedimentation rates is four to six times higher in interglacial periods; average accumulation rates of recognizable turbidites are higher by a factor of 21 to 45, and interglacial turbidite frequency is higher by a factor of 6 to 14. Sediment composition indicates that increased interglacial rates are due to higher accumulation of platform-derived material. Additional data from other Bahamian basins as well as published material from the Caribbean strongly suggest that highstand shedding is a general trend in pure carbonate depositional systems. Carbonate platforms without a siliciclastic component export more material during highstands of sea level when the platform tops are flooded and produce sediment. The response of carbonate platforms to Quaternary sea-level cycles is opposed to that of siliciclastic ocean margins, where sediment is stored on the inner shelf during highstands and passed on to continental rises and abyssal plains during lowstands of sea level.

Distribution and evolution of carbon and nitrogen in Earth

Article

Full-text available

Jun 1993
EARTH PLANET SC LETT

In this paper we discuss the distribution, geochemical cycle, and evolution of CO2 and N2 in Earth's degassed mantle, atmosphere, and continental crust. We estimate the present distribution of CO2 and N2 in Earth's degassed mantle based on amounts of He and Ar in the degassed mantle and observed outgassing ratios of CO2/He and N2/Ar at midocean ridges. Estimated CO2 in present degassed mantle is (18 +9 or -6) x 10 exp 22 mol, representing (72 +/- 10) percent of total degassable CO2, an amount far higher than fractions previously inferred for noble gases. This strongly suggests that most CO2 has been recycled from Earth's surface into the degassed mantle through subduction. For N2, the estimated amount in the present mantle is about 2.5 x 10 exp 19 mol, representing about 12 percent of total degassable N2. Recycling of N2 back to the mantle is also inferred, but on a much smaller scale. A simple model for the outgassing and recycling of CO2 and N2 in Earth is presented. Model results tentatively point to a higher CO2 content on Earth's surface in the Archean and Proterozoic than at present.

On the origin of the 100-kyr cycle in the astronomical forcing

Article

Full-text available

Dec 2005

Investigations during the last 25 years have demonstrated that the astronomically related 19-, 23-, and 41-kyr quasiperiodicities actually occur in long records of the Quaternary climate. However, the same investigations also identified the largest climatic cycle as being about 100 kyr long. As the 100-kyr variations in standing insolation due to eccentricity change are too small, they cannot be the direct cause of the ice ages. This is the reason why most of the modeling studies attempting to explain the relation between the astronomical forcing and climatic change have focused on this 100-kyr cycle. In this paper, we will show the astronomical origin of the periods at about 100 kyr that characterize the long-term variations of eccentricity, of its first derivative, of the frequency modulation of obliquity, and of the inclination of the Earth's orbit on the invariable plane of reference. Five independent values are found between 95 and 107 kyr, and a wavelet signature is suggested to test the possible relationships between the astronomical and climatic variables. Proxy records from deep-sea cores and European Programme for Ice Coring in Antarctica ice core and modeling results from the Louvain-la-Neuve two-dimensional model are used for illustration.

Synchrony between Early Jurassic extinction, oceanic anoxic event, and the Karoo-Ferrar Flood basalt volcanism

Article

Full-text available

Aug 2000
GEOLOGY

A well-known second-order mass extinction took place during the Pliensbachian and Toarcian Stages of the Early Jurassic. First recognized as a minor Pliensbachian peak in the global extinction rate, it has alternatively been interpreted as a regional response to the early Toarcian oceanic anoxic event. Detailed studies established it as a global long-term event spanning five successive ammonoid zones. Here we present a revised time scale based on high-precision U-Pb ages resolved to the zone level, which suggests that elevated extinction rates were sustained for about 4 m.y. and peak extinction occurred at 183 Ma. Recent isotopic dating of flood basalts from the southern Gondwanan Karoo and Ferrar provinces documents a culmination in volcanic activity ca. 183 Ma. The onset of volcanism is recorded as an inflection and start of a rapid rise of the seawater 87Sr/86Sr curve. The synchrony of voluminous flood basalt eruptions and biotic crises, as already noted for three of the major mass extinctions, permits a causal relationship, which in this case may be mediated by widespread oceanic anoxia.

Highstand Shedding of Carbonate Platforms

Article

Full-text available

Jan 1994

The term "highstand shedding" means that a depositional system sheds most sediment into the adjacent basin during highstands of sea level. In the Quaternary, highstand shedding has been observed around all rimmed carbonate platforms studied to date. Highstand shedding in the distant past is more difficult to prove for want of well-documented sea-level curves and limited stratigraphic resolution. Nonetheless, several detailed studies strongly suggest highstand shedding of ancient platforms, too. The reasons for carbonate highstand shedding are twofold: (1) the productive area increases manyfold when the flat platform top is flooded during highstands, and (2) highstand platform sediments rapidly lithify during sea-level falls when the sea floors are winnowed due to lowering of wave base or when sediments become exposed to fresh water. The causes of highstand shedding also determine the limitations of this principle. Highstand shedding is most pronounced on low-latitude, rimmed platforms. It is reduced on ramps and in cool-water carbonate systems; furthermore, the effect is damped when the lowstand wedge can expand into a full-size platform during long-lasting lowstands of sea level. Highstand shedding ceases when the platform is drowned during a sea-level rise. All in all, the principle of highstand shedding of carbonate platforms is as robust as the principle of lowstand shedding of siliciclastic systems. Both are caused by fundamental attributes of the respective depositional system and both have their limitations and exceptions.

Macintosh Program performs time-series analysis

Article

Full-text available

Sep 1996
Eos Trans. AGU

A Macintosh computer program that can perform many time-series analysis procedures is now available on the Internet free of charge. Although AnalySeries was originally designed for paleoclimatic time series, it can be useful for most fields of Earth sciences. The program's graphical user interface allows easy access even for people unfamiliar with computer calculations. Previous versions of the program are already used by hundreds of scientists worldwide.

Volcanic earthquakes at Pavlof Volcano correlated with the solid earth tide

Article

Full-text available

Nov 1981

Volcanic earthquake swarms at Pavlof Volcano, Alaska, correlate significantly with solid earth tidal stress rate for periods just before and just after explosive eruptions. The correlation changes sign systematically during the course of the 1974 minor eruption sequence; pre-eruptive earthquake swarms occur during increasing tidal compression, while post-eruptive swarms occur during increasing tidal extension.

Litostratigrafia das séries margo-calcárias do Jurássico Inferior da Bacia Lusitânica (Portugal)

Article

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Jan 2002

Early Jurassic carbonate evolution in the Lusitanian Basin (Portugal): facies, sequence stratigraphy and cyclicity

Chapter

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Jan 2004

Early Jurassic mass extinction: A global long-term event

Article

Full-text available

Jun 1995
GEOLOGY

The end-Pliensbachian extinction event (187 Ma) has been interpreted either as one of 10 global periodically recurring mass extinctions of the past 250 m.y. or as a minor localized European event. Elevated levels of family extinction spanned five ammonite zones during the late Pliensbachian and the early Toarcian, an interval of ˜7.5 m.y., and were distributed unequally in the Boreal, Tethyan, and Austral realms. Detailed sampling of invertebrate macrofaunas through complete expanded sequences in northwest Europe shows that most species extinctions occurred in the early Toarcian, following a regional anoxic event. The Early Jurassic mass-extinction event took place over a long time scale, and it was global in extent.

Dissociation of oceanic methane hydrate as a cause of the carbon isotope excursion at the end of the Paleocene

Article

Full-text available

Dec 1995

Isotopic records across the "Latest Paleocene Thermal Maximum' (LPTM) indicate that bottom water temperature increased by more than 4°C during a brief time interval (<104 years) of the latest Paleocene (~55.6 Ma). There also was a coeval -2 to -3‰ excursion in the δ13C of the ocean/atmosphere inorganic carbon reservoir. A rapid δ13C shift of this magnitude is difficult to explain within the context of conventional hypotheses for changing the mean carbon isotope composition of the ocean and atmosphere. However, a direct consequence of warming bottom water temperature from 11 to 15°C over 104 years would be a significant change in sediment thermal gradients and dissociation of oceanic CH4 hydrate at locations with intermediate water depths. In terms of the present-day oceanic CH4 hydrate reservoir, thermal dissociation of oceanic CH4 hydrate during the LPTM could have released greater than 1.1 to 2.1 × 1018 g of carbon with a δ13C of approximately -60‰. -from Authors

Site 632: Pliocene-Pleistocene sedimentation cycles in a Bahamian basin. In: Proceedings of the Ocean Drilling Program, Scientific Results 101 (Eds Austin, J.A. Jr, Schlager, W

Article

Jan 1988

Mid Toarcian

Article

Jan 1993

A Carbonate Submarine-Fan Sequence from the Jurassic of Portugal

Article

Jan 1984

The sequence is predominantly composed of shallow-water carbonate grains but can be interpreted using the current siliclastic submarine-fan models. Basin-plain to outer-fan, braided-channel midfan, and large channel-fill sequences can be recognzed.-from Authors

Environmental Changes During Oil Shale Deposition as Deduced from Stable Isotope Ratios

Chapter

Jan 1982

W. Küspert

In sections through Lower Toarcian oil shales, parallel variations in the 13-C/12-C ratios of carbonate and organic matter point to pronounced changes in the oxygen level of the water column. Fauna and flora, fossil preservation, paleogeography, and certain sedimentological features support this interpretation and justify the distinction of three lithologicisotopic facies types.Carbon isotopes might be generally useful in tracing the extent of “stagnation” during deposition of bituminous sediments. Although much lower in δ 13 C than comparable Recent marine plankton, the bulk of the organic matter (as well as the oil shale carbonate) in the Posidonia Shales is ultimately derived from phytoplankton which inhabited the oxygenated surface layer of the ocean. Diagenetic processes such as oxidation, cementation, dolomitisation, isotope exchange, impregnation and migration have led to partial redistribution of carbon and oxygen isotopes within the sediment-pore water system.

Milankovitch-type cycles recorded in Toarcian black shales from the Belluno Trough (Southern Alps Italy)

Article

Jan 1995

Mikrofauna, Biostratigraphie und Fazies im Unteren Toarcium Südwestdeutschlands und Vergleiche mit benachbarten Gebieten

Article

Jan 1985

Wolfgang Riegraf

Response of Lower Toarcian (Lower Jurassic) macrobenthos of the Iberian Peninsula to sea level changes and mass extinction

Article

Jan 2005

M.E. Gahr

Le Lias de Peniche

Article

Jan 1955

R. Mouterde

Environmental changes during oil shale deposition as deduced from stable isotope ratios.

Article

Jan 1982

W. Kuspert

Cyclic fluctuation of carbonate production through the Jurassic along a Burgundy-Ardèche cross-section, eastern France [Variation cyclique de la production carbonatée au Jurassique le long d'un transect Bourgogne-Ardèche, est-France]

Article

Jan 1996

Relative carbonate accumulation rates are calculated through the Jurassic from 2-D restored depositional geometries of a shelf-basin cross-section. Rates show periodic cycles through time (12 and 19 Ma), with a difference of one order of magnitude. Maximum rates are close in value whatever the location, extent and nature of the carbonate factory. The results suggest that a carbonate platform drowning can actually be induced by a major fall in carbonate production only. The long-term cycles in carbonate productivity are believed to have been under the control of an ocean-related parametre that is the marine water CO2 contents.

Effects of an oceanic anoxic event on the stable carbon isotopic composition of early Toarcian carbon

Article

Jan 2000

S. Schouten

The Early Toarcian transgression is marked by the occurrence of organic carbon-rich shales in large parts of western Europe and in other parts of the world as well. Based on the positive carbon isotope excursion of pelagic limestones in the middle to upper part of the falciferum Zone in several of the Tethyan sections, the widespread occurrence of the Early Toarcian shales was explained by an Oceanic Anoxic Event (OAE) (Jenkyns, 1988; Jenkyns and Clayton, 1997). The rapid burial of large amounts of organic carbon, which is rich in 12 C, would have led to a relative enrichment in 13 C of the global carbon reservoir and hence to an increase in 13 C of the limestones. In Southwest Germany (and a number of other locations) both organic and inorganic carbon display a negative isotope excursion in the upper part of the tenuicostatum Zone and the lower part of the falciferum Zone synchronous to black shale deposition and predating the positive isotopic excursion (Kuspert, 1982, 1983). It is presently unclear what the exact cause for this widespread negative isotopic excursion is (Jenkyns and Clayton, 1997). The objective of our study was to examine if the negative carbon isotope excursion of organic matter (OM) in the Southwest German Toarcian shales can be attributed to compositional changes of the OM, or if the excursion is related to variations in 13 C(CO2)aq in the photic zone. To this end, we analyzed the molecular and stable carbon isotope composition of OM in ten samples spanning the Early Toarcian (TOC 2.5-10.5 percent; HI 350-700). Biomarker distributions and kero- gen pyrolysates differ only slightly among the samples, and 13 C values of primary production markers follow closely those of associated carbonate and kerogen ( 13 CCARB/TOC). This strongly suggests that differences in 13 CTOC are related to differences in 13 Co f (CO 2)aq in the photic zone supporting Kuspert's model of advection of isotopically light, organic matter-derived CO2 from anoxic bottom waters. Evidence for euxinic conditions extending into the photic zone was found by the presence of derivatives of carotenoids, such as isorenieratane, from anoxygenic phototrophic sulfur bacteria. Their concentrations are ex- tremely low in black shales from the tenuicostatum Zone but are significant in the falciferum and bifrons zone with a maximum in the exaratum Subzone suggesting that photic zone euxinia (PZE) was a common feature of the Early Toarcian sea in Southwest Germany. Our data, therefore, support the Kuspert model of advec- tion of isotopically light, organic matter-derived CO2 from anoxic bottom waters to the upper part of the photic zone resulting in a negative isotope spike. The enrichment in 13 C in the middle of the falciferum Zone, which followed the negative spike, may be attributed to the burial of isotopically light organic matter through worldwide black shale deposition.

Sedimentary Environments: Processes, Facies, Stratigraphy

Article

Jan 1996

H G Reading

A Practical Guide to Wavelet Analysis

Article

Jan 1998
B AM METEOROL SOC

A practical step-by-step guide to wavelet analysis is given, with examples taken from time series of the El NiñoSouthem Oscillation (ENSO). The guide includes a comparison to the windowed Fourier transform, the choice of an appropriate wavelet basis function, edge effects due to finite-length time series, and the relationship between wavelet scale and Fourier frequency. New statistical significance tests for wavelet power spectra are developed by deriving theoretical wavelet spectra for white and red noise processes and using these to establish significance levels and confidence intervals. It is shown that smoothing in time or scale can be used to increase the confidence of the wavelet spectrum. Empirical formulas are given for the effect of smoothing on significance levels and confidence intervals. Extensions to wavelet analysis such as filtering, the power Hovmöller, cross-wavelet spectra, and coherence are described. The statistical significance tests are used to give a quantitative measure of changes in ENSO variance on interdecadal timescales. Using new datasets that extend back to 1871, the Niño3 sea surface temperature and the Southern Oscillation index show significantly higher power during 1880-1920 and 1960-90, and lower power during 1920-60, as well as a possible 15-yr modulation of variance. The power Hovmöller of sea level pressure shows significant variations in 2-8-yr wavelet power in both longitude and time.

Strategies for assessing Early-Middle (Pliensbachian-Aalenian) Jurassic cyclochronologies

Article

Aug 1999

There are a number of fundamental problems in assessing the astronomically forced cyclostratigraphy of the Jurassic Period. First, Jurassic geochronology is not well constrained, due to a general scarcity of radiometric dates, inferior precision of the existing ones, and large inaccuracies in stratigraphic constraints. These problems are particularly troublesome in the Early to Middle Jurassic cyclic carbonates of the Colle di Sogno section in the Lombardy Pre-Alps, Italy. Second, Jurassic sedimentary formations have undergone significant diagenesis, and again, the Colle di Sogno carbonates, with their location on the southern edge of the Alpine fold belt, are no exception. This prevents the recovery of the primary geochemical proxies needed to evaluate the relationship between cyclic sedimentation and orbital forcing. Finally, even when a strong case can be made for orbitally forced cycles, a direct calibration between the Jurassic and existing theoretical orbital solutions is not yet possible. However, at Colle di Sogno new nannofossil biostratigraphy provides the opportunity to make significant headway in recognizing Jurassic signals consistent with orbital theory, and to propose cyclochronological estimates for a portion of the Pliensbachian, and for the combined Toarcian-Aalenian Stages. We evaluate the presence of orbital signals in the sequence through the use of frequency modulation analysis of the cycles, as well as by spectral analysis of lithologic rank time-series reconstructions. The results show that the Pliensbachian Domaro Limestone contains a dual-component precession-like signal that undergoes frequency variation indicative of modulation by orbital eccentricity. The signal subsequently evolves into a single-component signal with frequency perturbations similar to those of the obliquity variation in the overlying Toarcian-Aalenian Sogno Formation. This switch in response to the fundamental orbital modes occurs during the deposition of the Lower Toarcian black shale, a time of significant global environmental change. The biostratigraphy at Colle di Sogno has well-defined Pliensbachian-Toarcian and Toarcian-Aalenian boundaries, allowing a cyclochronologic minimum estimate of 11.37 +/- 0.05 (1 sigma) Ma for the duration of the Toarcian-Aalenian interval (not accounting for stratigraphic and taxonomic noise). This compares well with the 13.1 +/- 2.8 (1 sigma) Ma estimated for this same stratigraphic interval using the current radiometric geochronology. The underlying precession-forced Pliensbachian Domaro Limestone limestone-shale couplets suggest a time of formation for the sequence of least 5 Ma, with an age of onset in the jamesoni or polymorphus subzone.

Comment on “Non-uniqueness and interpretation of the seawater 87Sr/86Sr curve” by Dave Waltham and Darren R. Gröcke (GCA, 70, 2006, 384–394)

Article

Jul 2007
GEOCHIM COSMOCHIM AC

Numerical evaluation of mechanisms driving Early Jurassic changes in global carbon cycling

Article

Mar 2007
GEOLOGY

The Early Jurassic (early Toarcian, ca. 183 Ma) carbon cycle perturbation is characterized by aabout -5 parts per thousand {delta} {sup 13}C excursion in the exogenic carbon reservoirs, a 1000 ppm rise in atmospheric CO{sub 2}, and a 6-7 degrees warming. Two proposed explanations for this presumed global carbon cycle perturbation are the liberation of massive amounts of isotopically light CH4 from (1) Gondwanan coals by heating during the intrusive eruption of the Karoo-Ferrar large igneous province (LIP) or (2) the thermal dissociation of gas hydrates. Carbon cycle modeling indicates that the release of CH4 from Gondwanan coals synchronous with the eruption of the Karoo-Ferrar LIP fails to reproduce the magnitude or timing of the CO{sub 2} and {delta} {sup 13}C excursions. However, sensitivity analyses constrained by a marine cyclostratigraphically dated {delta}{sup 13}C record indicate that both features of geologic record can be explained with the huge input of about 15,340-24,750 Gt C over about 220 k.y., a result possibly pointing to the involvement of hydrothermal vent complexes in the Karoo Basin. The simulated release of > 6000 Gt C from gas hydrates also reproduces aspects of the early Toarcian rock record, but the large mass involved raises fundamental questions about its formation, storage, and release.

Multiple volcanic episodes of flood basalts generated by thermochemical mantle plumes

Article

Dec 2005

Recent geochronogical data commonly reveals more than one pulse of major eruptions of several flood basalts, spanning time scales of tens of millions of years. The long duration between eruptions suggests a deep and possibly common mechanism for this episodicity. We have shown that the entrainment by a thermal plume of a dense basal layer can generate secondary instabilities due to the interaction between thermal and compositional buoyancy forces. The time separation between the arrival at the base of the lithosphere of the original plume head and replenishment by subsequent instabilities ranges from less than 10 million to more than 100 million years. The dense layer contributes to strong lateral heterogeneity and intense mixing in the plume head and secondary instabilities that travel up the low viscosity plume conduit. Under certain conditions the plumes may remain as nearly steady features in the deep mantle for more than 70 million years, possibly providing an explanation for the low seismic velocity provinces in the lowermost mantle. The dynamics of mantle plumes can be significantly more complicated than that of purely thermal plumes. The intrinsic density contrast of the entrained material will also cause a much reduced topographic effect of the arrival of the plume below the lithosphere compared to that predicted from thermal plumes.

Terrestrial and Marine Mass Extinction at the Triassic-Jurassic Boundary Synchronized With Initiation of Massive Volcanism

Article

Dec 2001

The mass extinction at the Triassic-Jurassic (Tr-J) boundary at 200 Ma ranks amongst the five most extreme in the Phanerozoic and occurred approximately at the same time as one of the largest volcanic episodes known from the geological record, that which characterized the Central Atlantic Magmatic Province (CAMP). Interpretations of climate change across the boundary are contradictory, whilst changes in the carbon cycle are poorly constrained. Here we present new organic carbon isotope data that demonstrate that changes in flora and fauna from both terrestrial and marine environments occurred synchronously with a transient light-carbon-isotope excursion and that this happened significantly earlier than the conventionally established marine Tr-J boundary. A second negative carbon-isotope excursion dominated the shallow-marine and atmospheric reservoirs for at least 600 k.y. These data suggest that a major perturbation occurred in the global carbon cycle at the Tr-J boundary which resulted in a significant increase in atmospheric pCO2 in less than a million years. Our results indicate synchroneity between the carbon-isotope excursion, the extinction event, the eruption of the first CAMP lavas, suggesting a causal link between loss of terrestrial and marine taxa and the very earliest eruptive phases.

Strategies for assessing Early{Middle (Pliensbachian{Aalenian) Jurassic cyclochronologies

Article

J. P ark

There are a number of fundamental problems in assessing the astronomically forced cyclostratigraphy of the Jurassic Period. First, Jurassic geochronology is not well constrained, due to a general scarcity of radiometric dates, inferior precision of the existing ones, and large inaccuracies in stratigraphic constraints. These problems are particularly troublesome in the Early to Middle Jurassic cyclic carbonates of the Colle di Sogno section in the Lombardy Pre-Alps, Italy. Second, Jurassic sedi- mentary formations have undergone signicant diagenesis, and again, the Colle di Sogno carbonates, with their location on the southern edge of the Alpine fold belt, are no exception. This prevents the recovery of the primary geochemical proxies needed to evaluate the relationship between cyclic sedimentation and orbital forcing. Finally, even when a strong case can be made for orbitally forced cycles, a direct calibration between the Jurassic and existing theoretical orbital solutions is not yet possible. However, at Colle di Sogno new nannofossil biostratigraphy provides the opportunity to make signicant headway in recognizing Jurassic signals consistent with orbital theory, and to propose cyclochronological estimates for a portion of the Pliensbachian, and for the combined Toarcian{Aalenian Stages. We evaluate the presence of orbital signals in the sequence through the use of frequency modulation analysis of the cycles, as well as by spectral analysis of lithologic rank time-series reconstructions. The results show that the Pliensbachian Domaro Limestone contains a dual-component precession-like signal that undergoes frequency variation indicative of modulation by orbital eccentricity. The signal subsequently evolves into a single- component signal with frequency perturbations similar to those of the obliquity vari- ation in the overlying Toarcian{Aalenian Sogno Formation. This switch in response to the fundamental orbital modes occurs during the deposition of the Lower Toarcian black shale, a time of signicant global environmental change. The biostratigraphy at Colle di Sogno has well-dened Pliensbachian{Toarcian and Toarcian{Aalenian boundaries, allowing a cyclochronologic minimum estimate of 11:37 0:05 (1 )M a for the duration of the Toarcian{Aalenian interval (not accounting for stratigraphic and taxonomic noise). This compares well with the 13:1 2: 8( 1) Ma estimated for this same stratigraphic interval using the current radiometric geochronology. The underlying precession-forced Pliensbachian Domaro Limestone limestone{shale cou- plets suggest a time of formation for the sequence of least 5 Ma, with an age of onset in the jamesoni or polymorphus subzone.

Chemical model for the origin of minor limestone-shale cycles by anaerobic methane oxidation

Article

Jan 1988
GEOLOGY

Robert Raiswell

Chemical and isotopic studies of Jurassic carbonate concretions indicate an origin by anaerobic methane oxidation. Concretion growth occurred within the top 1 m of sediment in a thin zone where methane was consumed to stimulate a late, rejuvenated phase of sulfate reduction. Similar chemical and isotopic characteristics are shown by diagenetic carbonates in several limestone-shale sequences, and an origin by anaerobic methane oxidation is proposed for these sequences also. The evolution of isolated concretions into diagenetic limestones depends on the extent of, and depth variations in, carbonate supersaturation arising from anaerobic methane oxidation and the persistence of a reduced sedimentation rate or a depositional hiatus. Diagenetic limestones formed in this way can often be recognized by their association with a later pyrite phase that is more abundant and has more positive delta34S values than the adjacent shales.

Biodiversity and biogeography of Middle-Late Liassic ammonoids: implications for the Early Toarcian mass extinction

Article

Dec 2002
Geobios

In Western Tethyan areas, the Toarcian stage begins with two important evolutionary events in ammonite faunas: (1) the disruption of Tethyan–Boreal provinciality; (2) a biological crisis linked with the oceanic anoxic event OAE. The analysis of these events has been addressed by constructing curves of ammonoid diversity (species richness, origination and extinction rates) in the Late Pliensbachian (= Domerian)–Early Toarcian interval in selected localities. Two diversity drops are recognized. The first one is recorded at the end of the Dactylioceras mirabile subzone and reflects the disruption of Tethyan–Boreal provinciality, through the progressive extinction of the Boreal endemic family Amaltheidae that occupied the north-western European seas during the whole Pliensbachian on the one hand, and the extinction of Late Domerian Ammonitina endemic to the Mediterranean areas on the other hand. The Early Toarcian homogeneization of Mediterranean and north-western European ammonoid faunas was reached via elimination of both Boreal and Mediterranean endemics with differential rates of extinction in the two palaeogeographic domains and the subsequent geographical expansion of Tethyan-derived ammonoids. The second, dramatic drop in ammonite diversity in the upper part of the Dactylioceras semicelatum subzone coincided with the onset of OAE. It also affected epioceanic ammonoid clades like Phyllocerataceae and Lytocerataceae. These two drops are interpreted as two distinct extinctions and not as episodes of a single, stepwise event. Complex relations between ammonoid diversity and sea-level changes are suggested by trends in endemism, which may be reversed during either a single transgression or a single regression.

Myths and legends in the correlation between the Boreal and Tethyan Realms. Implications on the dating of the Early Toarcian mass extinctions and the Oceanic Anoxic Event

Article

Dec 2002
Geobios

FRANCESCO MACCHIONI

This study constitutes a revision of the biostratigraphy of Early Toarcian age ammonites coming from different successions in the NW Europe and Mediterranean areas, complemented by new, important data from the Migiana di M. Malbe section (Perugia, Italy). The research has demonstrated that the problems of correlation between some Zones of the Standard and Mediterranean biostratigraphic scales in this period are doubtful. On the negative side, procedural problems and defects have emerged that greatly detract from the validity of some boundaries which were, almost unanimously, held to be synchronous. Undervaluation of the problems brought to light in this study has had repercussions both on the construction of the eustatic curves and on the dating of the Oceanic Anoxic Event (OAE), and consequently on the evolutionary models for the various areas of the Tethys and of NW Europe. The various datings obtained by different writers have led to the belief that the anoxic conditions became established diachronically in the two areas. However, ammonite dating of the black shale-like facies, made for the first time in the Umbria–Marche Apennines, shows instead that, at least between this area and Germany, the diachronism was only partial. In both areas, the establishment of “anoxic” conditions took place in the Dactylioceras semicelatum Subzone. Problems linked to ammonite faunal provinciality and evolution, complicated by tectono-eustatic phenomena that produce hiatuses in NW Europe, are at the root of the lack of continuity in tracing the Pliensbachian–Toarcian boundary between the NW Europe area and the rest of the world.

Stable isotopes of carbon from basaltic rocks and their possible relation to atmospheric isotope excursions

Article

Nov 2006
LITHOS

H. J. Hansen

Repeated short-time negative excursions from the general trend of organic carbon isotopes occur at, and shortly after, the Permo-Triassic and Cretaceous–Tertiary mass extinctions. The cause of these excursions is unexplained since an extinction event can only explain one excursion at each boundary. At the P/T boundary one negative excursion occurs shortly before the boundary proper followed by one at the boundary, followed by several excursions that can be correlated in both marine and terrestrial environments over larger geographical distances. Also at the K/T boundary negative excursions in organic carbon, which can be correlated, occur after the boundary in both marine and terrestrial settings.

Distinct brief major events in the Karoo large igneous province clarified by new 40Ar/ 39Ar ages on the Lesotho basalts

Article

Oct 2007
LITHOS

Recent mineral separate ages obtained on the Karoo large igneous province (southern Africa) suggest that the province was built by several distinct magmatic pulses over a rather long period on the order of 5–6 Ma concerning the main erupted volume [Jourdan, F., Féraud, G., Bertrand, H., Kampunzu, A.B., Tshoso, G., Watkeys, M.K., Le Gall., B., 2005. The Karoo large igneous province: Brevity, origin, and relation with mass extinction questioned by new 40Ar/39Ar age data, Geology 33, 745–748]. Although this apparently atypical province is dated in more detail compared to many other large igneous provinces, volumetrically important areas still lack sufficient high-quality data. The timing of the Karoo province is crucial as this event is correlated with the breakup activity of the Gondwana supercontinent. The Lesotho basalts represent a major lava sequence of the province, but have not yet been precisely dated by systematic analysis of mineral separates. We analyzed plagioclase separates from five lava flows encompassing the complete 1.4-km-thick Lesotho sequence from top to bottom using the 40Ar/39Ar method. We obtained five plateau and mini-plateau ages statistically indistinguishable and ranging from 182.3 ± 1.6 to 181.0 ± 2.0 Ma (2σ). We derived an apparent maximum duration for this event of ∼ 0.8 Ma by neglecting correlated errors embedded in the age uncertainties.

Can biomass burning produce a globally significant carbon-isotope excursion in the sedimentary record?

Article

Oct 2006
EARTH PLANET SC LETT

Negative carbon-isotope excursions have been comprehensively studied in the stratigraphic record but the discussion of causal mechanisms has largely overlooked the potential role of biomass burning. The carbon-isotopic ratios (δ13C) of vegetation, soil organic matter and peat are significantly lower than atmospheric carbon dioxide (CO2), and thereby provide a source of low 13C CO2 when combusted. In this study, the potential role of biomass burning to generate negative carbon isotope excursions associated with greenhouse climates is modeled. Results indicate that major peat combustion sustained for 1000 yr increases atmospheric CO2 from 2.5× present atmospheric levels (PAL) to 4.6× PAL, and yields a pronounced negative δ13C excursion in the atmosphere (∼2.4‰), vegetation (∼2.4‰) and the surface ocean (∼1.2‰), but not for the deep ocean (∼0.9‰). Release of CO2 initiates a short-term warming of the atmosphere (up to 14.4 °C, with a duration of 1628 yr), which is consistent with the magnitude and length of an observed Toarcian excursion event. These results indicate that peat combustion is a plausible mechanism for driving negative δ13C excursions in the rock record, even during times of elevated pCO2.

Erratum: The Posidonia Shale (Lower Toarcian) of SW-Germany: An oxygen-depleted ecosystem controlled by sea level and palaeoclimate (Palaeogeography, Palaeoclimatology, Palaeoecology (2001) 165 (27-52) pii: S0031-018200001528)

Article

May 2001
PALAEOGEOGR PALAEOCL

The Lower Toarcian Posidonia Shale is famous for its excellently preserved fossils and its high amount of organic matter (up to 16%). Both quality of preservation and accumulation of organic matter have been explained by permanent anoxic bottom water conditions. High-resolution geochemical, sedimentological and palaeoecological investigations of various sections of the Posidonia Shale in SW-Germany, however, indicate that oxygen availability was variable and ranged from short oxygenated periods to longer-term anoxia. The benthic macrofauna consists of nine fossil communities and was used, in combination with geochemical data, to reconstruct a time-averaged oxygen curve. Anoxic conditions prevailed during the deposition of the Toarcian black shales; they were, however, punctuated by various short periods (weeks to years) with oxygenated bottom water conditions. Sedimentological (e.g. distinctiveness of microlamination, siliciclastic content) and geochemical parameters (e.g. organic matter content, isotopic signatures: δ18O and δ13C, molecular redox parameters: pristane/phytane ratio, arylisoprenoids) exhibit a remarkable covariation and seem to be controlled by sea level fluctuations. Maximum oxygen depletion and an extreme negative shift of δ13Corg values (−34‰) occurred during the early falciferum-zone. This is explained by the recycling of 12C-enriched carbon derived from remineralization of organic matter on and within the substrate during low sea level stand and a highly elevated redox boundary including photic zone anoxia. The subsequent transgression permitted enhanced water exchange with the Tethyan Ocean and caused improvement of living conditions at the end of the falciferum-zone.

Dissociation of oceanic methane hydrate as a cause of the carbon isotope excursion at the end of the

Article

The Global Significance of a Deep-sea Isotopic Event During the Toarcian Oceanic Anoxic Event Recorded in Japan

Article

Jan 2003

The record of the Toarcian oceanic anoxic event (OAE) is considered global in nature due to the occurrence of organic carbon-rich sediments worldwide, although its expression in the carbon isotopic record (marine carbonate and organic matter and terrestrial plants) has only been recorded in European localities. We have investigated the carbon-isotope record of marine organic matter in deep-sea cherts of the Inuyama area in Japan. This stratigraphic chert sequence records the Toarcian OAE through a transition from red to black to red-green cherts and radiolarian biostratigraphy indicates a Pliensbachian-Toarcian age range. A major negative carbon-isotope excursion ( ˜60/00 ) is recorded just after the Pliensbachain/Toarcian boundary based on radiolarian biostratigraphy, where delta 13Corg fluctuate and peak at a value of ˜-320/00 that is concurrent with the highest TOC values ( ˜3.5 wt %). delta 13Corg values gradually return to pre-excursion values but continue to rise to more positive values of ˜-210/00 just prior to the Toarcian Radiolarian Event. After this point carbon-isotope values return to pre-Toarcian values of ˜-250/00 . This dataset confirms that the Toarcian OAE did affect the entire global carbon reservoir and was not isotopically expressed only in Europe, shallow seas or continental margin settings. The nature and cause of the excursion has been proposed to be caused by the upwelling of oxidized organic matter, a massive dissociation of continental margin CH4-gas hydrates and/or increased volcanism all leading to a surface-water productivity event. Based on sedimentation rates the onset of the negative delta 13Corg excursion occurs in less than ˜100kyr, which is certainly within the realm of a CH4-gas hydrate event, and the total duration of the negative excursion is ˜500kyr. The duration of the negative and positive delta 13Corg excursion inclusive is ˜2Myr. Although a definitive answer as to the driving mechanism for the negative and positive excursion and high rates of carbon burial in the deep-sea is still a matter of debate, a model of the carbon-isotope curve suggests that both CH4-gas hydrate and volcanism are involved in driving the global Toarcian carbon burial event. Due to the apparent synchroneity of biostratigraphy between shallow and deep-water sequences, upwelling of 12C-rich organic matter could not explain the negative carbon-isotope event on a global scale.

Spatial and temporal distribution of calcareous nannofossils along a proximal–distal transect in the Lower Jurassic of the Umbria–Marche Basin (Central Italy)

Article

Mar 2004
PALAEOGEOGR PALAEOCL

Three Upper Pliensbachian/Lower Toarcian successions of the Umbria–Marche Basin were analysed for sedimentology and calcareous nannofossil content across a 120-km-long transect from proximal to distal areas with respect to the shallow Latium–Abruzzi Platform. The uppermost part of the Corniola Unit (Uppermost Pliensbachian) and the Marne di Monte Serrone Formation (p.p., Lower Toarcian; Polymorphum to Levisoni Ammonite Zones) that contains organic matter-rich levels were studied in the three localities. The analysis of the sedimentary structures in the studied successions indicates that the basin was a shallow-water system, above storm-wave base, during the Late Pliensbachian/Early Toarcian. The stratigraphic evolution of the sedimentary structures recorded in the studied localities shows that the Upper Pliensbachian corresponded to a regressive sedimentary interval, whereas a complete transgressive–regressive relative sea-level cycle occurred during the Lower Toarcian Polymorphum Zone. A further transgression occurred in the lowermost Levisoni Zone (Lower Toarcian).

Pliensbachian/Toarcian boundary: The proposed GSSP of Peniche (Portugal)

Article

Jan 2006

S. Elmi

The Peniche section (Portugal) is cosidered as a potential stratotype (GSSP) for the Pliensbachian- Toarcian boundary and it is analysed on the background of the available data on the ammonite successions from other Tethyan and NW European areas.

Evidence for major environmental perturbation prior to and during the Toarcian (Early Jurassic) Oceanic Anoxic Event from the Lusitanian Basin, Portugal

Article

Jun 2008

The timing and causal relationships between the pronounced negative C isotope excursion and paleoenvironmental perturbations associated with the Toarcian oceanic anoxic event (Early Jurassic) remain unclear, particularly because biotic crises and carbonate production decrease appear to have been initiated earlier than the main C isotope anomaly. Here we present a new quantification of Late Pliensbachian-Early Toarcian calcareous nannofossils abundance and size from the Peniche reference section (Portugal) together with O and C isotope records of well-preserved brachiopod shells from the same section. The brachiopod shell delta 13C curve parallels that of bulk carbonate and records two pronounced negative isotopic excursions, close to the Pliensbachian-Toarcian boundary (~-20/00) and during the Toarcian oceanic anoxic event (~-3.50/00). Our results indicate that both C isotope negative excursions were characteristic of benthic and shallow-water environments, suggesting that these two carbon cycle perturbations affected all epioceanic reservoirs. Coeval shifts toward lower values of brachiopod oxygen isotope compositions and closely correlated northward migrations of Mediterranean ammonite fauna suggest that both events coincided with major rises in seawater temperatures, probably as a result of increased CO2 levels and enhanced greenhouse conditions. CO2-induced changes in seawater chemistry likely affected the calcification potential of both neritic and pelagic systems, as evidenced by synchronous drops of platform-derived carbonate accumulation and drastic reductions in size (>3 mum) of the main pelagic carbonate producer Schizosphaerella. We suggest that the Early Toarcian paleoenvironmental crisis occurred in two distinct episodes that were most likely related to two successive phases of intense volcanic degassing in the Karoo-Ferrar province.

Facies analysis and sequential evolution of the Toarcian-Lower Aalenian series in the Lusitanian Basin (Portugal)

Article

Jan 1997

Luís Vítor Duarte

Carbon geodynamic cycle

Article

Nov 1982

The systematic use of coupled isotopic tracers has improved our understanding of many problems such as the geochemical structure and evolution of the mantle. However, some fundamental questions remain1–6; for example, does sediment injection occur in the mantle4,5,7, modifying composition and combining the internal and external geochemical cycles? Recently 3He data from the South Atlantic have been used to suggest that the sediment injection phenomenon may be important on a local scale8. Here, using carbon data only, we show that sediment injection in the mantle must occur. This conclusion was reached after comparison between the integrated flux coming from the mantle and the budget of the exogeneous reservoir of carbon.

Duration of the Early Toarcian carbon isotope excursion deduced from spectral analysis: Consequence for its possible causes

Abstract

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