Carbon isotope stratigraphy and carbonate production during the Early–Middle Jurassic: examples from the Umbria–Marche–Sabina Apennines (central Italy)

The carbon isotopic signature of carbonates depends on secular variations of organic carbon and carbonate carbon production/burial rates. A decrease in carbonate productivity makes the organic/carbonate carbon ratio unstable up to the point that even minor variations in the organic carbon reservoirs can provoke carbon isotopic shifts. The δ13C positive shifts of the middle Carixian (early Pliensbachian) and the early Bajocian recorded in the Umbria–Marche–Sabina domain represent a good example of this mechanism. Both sedimentology and lithostratigraphy of pelagic platform–basin carbonate systems in this area show that important changes in the source of carbonates correspond to the observed isotopic shifts. The middle Carixian event is in fact well correlatable to the drastic reduction of benthic carbonate production on rift-related intrabasinal highs, which then became pelagic carbonate platforms. The early Bajocian event is concomitant with the beginning of a long hiatus on the pelagic carbonate platforms and with a drop of the biodiversity of calcareous organisms followed by the onset of biosiliceous sedimentation in basins.