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 obliquityeccentricity-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.

International audienceData indicate that the morphologies of the three microbialite associations are controlled primarily by accommodation, hydrodynamics, bathymetry and grain supply. This study suggests that microbial constructions are controlled by changes between trapping and binding versus precipitation processes in variable hydrodynamic conditions. Due to the presence of numerous metazoans associated with microbialites throughout the Smithian increase inaccommodation and Spathian decrease in accommodation, the commonly assumed anachronistic character of the Early Triassic microbialites and thetraditional view of prolonged deleterious conditions during the Early Triassic time interval is questioned

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In the aftermath of the end-Permian mass extinction, Early Triassic sediments record some of the largest Phanerozoic carbon isotopic excursions. Among them, a global Smithian-negative carbonate carbon isotope excursion has been identified, followed by an abrupt increase across the Smithian-Spathian boundary (SSB; ~250.8 Myr ago). This chemostratigraphic evolution is associated with palaeontological evidence that indicate a major collapse of terrestrial and marine ecosystems during the Late Smithian. It is commonly assumed that Smithian and Spathian isotopic variations are intimately linked to major perturbations in the exogenic carbon reservoir. We present paired carbon isotopes measurements from the Thaynes Group (Utah, USA) to evaluate the extent to which the Early Triassic isotopic perturbations reflect changes in the exogenic carbon cycle. The δ(13) Ccarb variations obtained here reproduce the known Smithian δ(13) Ccarb -negative excursion. However, the δ(13) C signal of the bulk organic matter is invariant across the SSB and variations in the δ(34) S signal of sedimentary sulphides are interpreted here to reflect the intensity of sediment remobilization. We argue that Middle to Late Smithian δ(13) Ccarb signal in the shallow marine environments of the Thaynes Group does not reflect secular evolution of the exogenic carbon cycle but rather physicochemical conditions at the sediment-water interface leading to authigenic carbonate formation during early diagenetic processes.

The exhumation history of basement areas is poorly constrained because of large gaps in the sedimentary record. Indirect methods including low temperature thermochronology may be used to estimate exhumation but these require an inverse modeling procedure to interpret the data. Solutions from such modeling are not always satisfactory as they may be too broad or may conflict with independent geological data. This study shows that the input of geological constraints is necessary to obtain a valuable and refined exhumation history and to identify the presence of a former sedimentary cover presently completely eroded. Apatite fission-track (AFT) data have been acquired on the northern part of the Ardenne Massif close to the Variscan front and in the southern Brabant, in particular for the Visean ash-beds. Apatite fission-track ages for surface samples range between 140 ± 13 and 261 ± 33 Ma and confined tracks lengths are ranging between 12.6 ± 0.2 and 13.8 ± 0.2 μm. Thermal inversion has been realized assuming that (1) samples were close to the surface (20–40 °C) during Triassic times, this is supported by remnants of detrital Upper Permian–Triassic sediments preserved in the south of the Ardenne and in the east (border of the Roer Graben and Malmédy Graben), and (2) terrestrial conditions prevailed during the Early Cretaceous for the Ardenne Massif, as indicated by radiometric ages on paleoweathering products. Inversion of the AFT data characterizes higher temperatures than surface temperatures during most of the Jurassic. Temperature range is wide but is compatible with the deposition on the northern Ardenne of a significant sedimentary cover, which has been later eroded during the Late Jurassic and/or the Early Cretaceous. Despite the presence of small outliers of Late Cretaceous (Hautes Fagnes area), no evidence is recorded by the fission-track data for the deposition of a significant chalk cover as highlighted in different parts of western Europe. These results question the existence of the London-Brabant Massif as a permanent positive structure during the Mesozoic.

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