14 pagesInternational audienceThe Oxfordian (Late Jurassic) was a time of widespread change in Jurassic marine (carbonate) sedimentation patterns. A marked positive excursion in δ13C is dated as Middle Oxfordian in age. In this study we investigate if changes in carbonate sedimentation coincided with altered carbon cycling and climate. We use C-isotope records as a proxy for the evolution of the carbon cycle and compare δ13C-trends with the evolution of sedimentation in a segment of the opening Tethys seaway. One of the studied sections is located in the Subalpine basin of France (Trescléoux and Oze), the other in the Swiss Jura mountains (Liesberg). Carbon-isotope stratigraphy of carbonate carbon locates the peak values of the Middle Oxfordian positive excursion to the antecedens and parandieri subzones of the plicatilis and transversarium ammonite zones, respectively. Causes of the excursion remain enigmatic. The excursion seems not to coincide with a global oceanic anoxic event, but regionally enhanced organic carbon accumulation during the Early and early Middle Oxfordian may be the cause of the increase in δ13C. The excursion occurs during a time of progressive climate warming, and its peak values coincide with the first calcareous sediments recurring after a period of reduced carbonate accumulation in the Early and early Middle Oxfordian. The excursion is also time-equivalent to a major reorganisation of global ocean currents: the opening Atlantic and Tethys oceans combined with rising sea level led to the formation of an efficient circumglobal seaway.We conclude that this favoured the widespread recurrence of carbonate producers by providing new habitats. As a result, Ccarb burial increased, leading to a lower Corg/Ccarb burial rate and therefore to stabilisation and decrease in δ13C in the Middle Oxfordian. Thus, the mid-Oxfordian positive excursion in carbonate C-isotopes may reflect changes in the carbon cycle that were triggered by a copious reorganisation of the ocean current system caused by major plate tectonic movements

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A new occurrence of authigenic albite was found in a Jurassic sedimentary succession of the Glarus Nappe near Walenstadt (eastern Switzerland). The euhedral shape and the chemical purity of these albites are evidence for their authigenic origin. The crystals are irregularly distributed in the sediment, highlighting the importance of the host rock composition for albite authigenesis. The crystals occur exclusively in limestones with a carbonate content higher than 80 wt-%. A diagenetic or hydrothermal origin of albite authigenesis is discussed for the studied region. Clay mineral transformation appears to be an important source of ions for albite formation. RESUME Un nouveau gisement d'albite authigène a été découvert dans une succession sédimentaire du Jurassic, situé dans la nappe de Glaris près de Walenstadt (Est de la Suisse). La pureté chimique et la forme euédrique de ces albites atteste de leur origine authigène. La répartition irrégulière des cristaux dans le sédiment souligne l'importance de la composition de roche hôte pour la formation des albites. Les cristaux sont présents uniquement dans les calcaires contenant plus de 80 %m de carbonate. La possibilité d'une origine diagéné-tique ou hydrothermale est discutée pour la région étudiée. La transformation des minéraux argileux paraît être une source importante d'ions pour la formation de l'albite.

An Oxfordian (Late Jurassic) hemipelagic succession from Beauvoisin (SE France) contains a pronounced, short‐lived negative excursion in the bulk‐carbonate carbon‐isotope record, with an amplitude of 4‰. It was shown previously that the Beauvoisin paleoenvironment was impacted by hydrocarbon seepage. New isotopic data corroborate that methane was a significant constituent of these hydrocarbons. The negative excursion was caused by transient enhanced precipitation of 13C‐depleted carbonate, mediated by anaerobic oxidation of methane. Despite its local diagenetic origin, the Beauvoisin excursion is similar in shape and duration to globally recognized negative C‐isotope excursions that have been related to catastrophic, massive dissociation of methane hydrate. Shape and duration of negative excursions therefore cannot be used as an argument when determining their origin if they have not been shown to represent a global perturbation of the carbon cycle.