Early jurassic carbon-isotope perturbations in a shallow-water succession from the tethys himalaya, southern hemisphere

The Early Jurassic was characterized by extreme carbon-cycle perturbations that are associated with abrupt environmental and climatic change. However, the evidence mainly derives from sections in the western Tethys and northern Europe: localities situated in the northern hemisphere. This paper presents new records of biostratigraphical (large benthic foraminiferal), sedimentological and carbonate carbon-isotope (δ13Ccarb) data from the Tibetan Kioto Platform formed in the southeastern Tethys (southern hemisphere) during the Sinemurian–earliest Toarcian interval. Six foraminiferal zones have been recognized: late Sinemurian Textulariopsis sinemuriensis, Pliensbachian Planisepta compressa, Bosniella oenensis, Cyclor-bitopsella tibetica and Streptocyclammina liasica, and earliest Toarcian Siphovalvulina sp. A. Based on biostratigraphy, δ13Ccarb data allow correlation with coeval records from the western Tethys and northern Europe by the identification of both negative and positive δ13C excursions. The negative excursions characterize the Sinemurian–Pliensbachian boundary event (SPBE) and the margaritatus–spinatum zone boundary event (MSBE); the positive δ13C excursion characterizes the margaritatus zone event (ME). Facies evolution in the Early Jurassic indicates that the establishment of carbonate sedimentation on the Kioto Platform occurred in the context of a global sea-level rise partly coincident with the SPBE and that, in common with other coeval platforms, carbonate production following the negative shift was predominantly made up of skeletal carbonates. Furthermore, the spread of the Lithiotis Fauna on the Kioto Platform followed the rebound of isotopic values after the SPBE. This phenomenon has been observed in the western Tethys and suggests that the global biocalcification event represented by the flourishing of the Lithiotis Fauna may have occurred synchronously across the Tethys, possibly reflecting the creation of more favourable marine conditions after the SPBE. Biostratigraphical data indicate that certain index larger benthic foraminifera became extinct around the onset level of the MSBE, likely due to the deleterious impact of this event. However, as in more northerly localities, the Lithiotis Fauna persisted during the late Pliensbachian in the shallow-water platforms of the Tethys until its disappearance in the early Toarcian.