Update of the carbon and oxygen isotopic records of the Early-Late Pliensbachian (Early Jurassic, ~187Ma): Insights from the organic-rich hemipelagic series of the Lusitanian Basin (Portugal)

摘要

A high-resolution carbon and oxygen isotopic bulk carbonate record from organic-rich hemipelagic series of Early-Late Pliensbachian age (~. 187. Ma) is presented for the Lusitanian Basin (Portugal) and compared to the record of other sedimentary basins. This dataset provides an excellent basis for discussing newly raised questions concerning the hypothesis of major carbon cycle perturbations prior to the Toarcian Oceanic Anoxic Event. In addition to the recognition in the studied series of two positive carbon isotope excursions previously defined elsewhere, corresponding to the Ibex and Margaritatus (uppermost Stokesi-Gibbosus subzones) zones, a third positive excursion is identified here for the first time, at the Davoei Zone. In the Lusitanian Basin these excursions are associated with organic-rich facies intervals. The younger excursion may be related to worldwide enhanced preservation of organic matter in the Margaritatus Zone. We suggest that until these organic-rich facies are well age constrained at a wider scale and high-resolution carbon data are provided from other locations, and that worldwide anoxia is proven, this interval should be referred to as an Organic Matter Preservation Interval (Late Pliensbachian OMPI). This period is probably linked with the complex chain of events that eventually led to the Toarcian Oceanic Anoxic Event.