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.
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