The data presented here provide the first high resolution investigation of carbon isotope and geochemical analyses derived from the Pliensbachian-Toarcian boundary, of Issouka, Middle Atlas, Morocco. The isotope data recorded in micrite reveal a stepwise negative carbon isotope excursion with values dropping to-1.8 ‰ within the Polymorphum Zone. This excursion coincides with major marine biological changes and extinctions and corresponds with European records supporting the assertion that the excursion is global in origin. The Issouka section is relatively expanded compared to other well-studied European sections. The excursion at the Pliensbachian-Toarcian boundary also shows several similarities with the negative Early Toarcian event. In contrast, carbon isotope values derived from coeval belemnites show positive values. The belemnite δ 13 C data presented here suggests spatial heterogeneity in the Early Jurassic ocean. Overturning or upwelling of a stratified water mass, is inconsistent with our data, as it requires the belemnites to have lived elsewhere and only later migrated into the Middle Atlas area where they became fossilized. The oxygen isotope values from belemnite calcite show no distinct trend across the event, indicative of either no significant change in temperatures or change in seawater δ 18 O. We suggest the introduction of any light carbon (e.g. a volcanogenic) source must have resulted in spatial variability in the δ 13 C of the dissolved inorganic carbon of seawater. Alternatively, a regional change in the source of the carbonate carrying the isotope signal, could lead to a negative shift in the δ 13 Cmicrite signature without any relation to variations in the global carbon isotope trend.

The Late Pliensbachian-Early Toarcian is a pivotal time in the Mesozoic era, marked by pronounced carbon-isotope excursions, biotic crises and major climatic and oceanographic changes. Here we present new high-resolution carbon-isotope and magnetic-susceptibility measurements from an expanded hemipelagic Late Pliensbachian-Early Toarcian section from the Middle Atlas Basin (Morocco). Our new astronomical calibration allows the construction of an orbital time scale based on the 100-kyr eccentricity cycle. The Early Toarcian Polymorphum Zone contains 10 to 10.5 repetitions of the 100-kyr eccentricity both in the carbon-isotope and the magnetic-susceptibility data, leading to an average duration of 1.00 ± 0.08 myr. We also show that the Late Pliensbachian-Early Toarcian global carbon-cycle perturbation has an average duration of 0.24 ± 0.02 myr. These durations are comparable to previous astrochronological time scales provided for this time interval in the most complete 2 sections of the Tethyan area, and longer than what has been provided in condensed sections. Anchoring this framework on published radiometric ages and astrochronological time scales, we estimate that the carbon-cycle perturbation of the Late Pliensbachian-Early Toarcian corresponds with the early phase of the Karoo and Chonke Aike large igneous provinces. Likewise, our new age constraints confirm that the Toarcian oceanic anoxic event is synchronous to the main phase of the Ferrar volcanic activity. Thus, these successive and short phases of the volcanic activity may have been at the origin of the successive phases of the mass extinctions observed in marine biotas in the Pliensbachian and Toarcian times.

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<p>The Cretaceous period was punctuated by several episodes of widespread deoxygenation of the sea floor referred to as Oceanic Anoxic Events. The OAE1b around the Aptian - Albian boundary is characterized by a series of black shales deposits, namely Jacob, Kilian, Paquier and Leenhardt levels. They are well documented in the Vocontian Basin, and their equivalent have been observed in different basins across the globe. Disagreement of more than a million of years exist about the timing of these events, leaving vast uncertainties about the causes of these recurring environmental changes. In order to better understand the relation between the climate perturbation and anoxic events during the Aptian-Albian period, we have focused on high-resolution investigations of magnetic susceptibility of Col de Pré-Guittard section, Drôme, France (GSSP of the Albian Stage; Kennedy et al., 2017). This section in the Blue Marls Formation consists of monotonous dark-grey marlstones interrupted by limestone beds and organic-rich layers. Spectral analyses were conducted on a magnetic susceptibility signal sampled every 5 cm. From this, we detected the record of the eccentricity, obliquity and precession cycles. We used the 100-kyr eccentricity cycles to construct an orbital time scale and shows that the interval starting above the Jacob level and ending above the Leenhardt level contains 21 repetitions of the 100-kyr eccentricity in the magnetic susceptibility data, leading to a duration of ca. 2.1 Myr. This duration is significantly shorter than the duration of 4 Myr provided by the current geologic time scale (Gale et al., 2020) but agrees with the U-Pb ages anchored to a δ<sup>13</sup>C<sub>org</sub> curve from the High Arctic (Herrle et al., 2015).</p><p>References:</p><p>Gale, A.S., Mutterlose, J., Batenburg, S., 2020. Chapter 27: The Cretaceous Period, in: Gradstein, F.M., Ogg, J.G., Schmitz, M.D., Ogg, G.M. (Eds.) Geologic Time Scale 2020. Elsevier BV, Amsterdam, The Netherlands, pp. 1023–1086.</p><p>Herrle, J., Schröder-Adams, C.J., Davis, W., Pugh, A.T., Galloway, J.M., Fath, J., 2015. Mid-Cretaceous High Arctic stratigraphy, climate, and Oceanic Anoxic Events. Geology 43, 403–406.</p><p>Kennedy, J.W., Gale, A.S., Huber, B.T., Petrizzo, M.R., Bown, P., Jenkyns, H.C., 2017. The Global Boundary Stratotype Section and Point (GSSP) for the base of the Albian Stage, of the Cretaceous, the Col de Pré-Guittard section, Arnayon, Drôme, France. Episodes 40, 177–188.</p>