Transient and secular changes in global carbon cycling during the early Bajocian event: Evidence for Jurassic cool climate episodes

摘要

The early Bajocian event was characterized by a protracted, ca. 4 Myr-long positive carbon isotope excursion that has been recognized in both marine and continental records. It is concomitant with a marine eutrophication event that led to regional demises of the neritic carbonate factory, as well as floral and faunal turnovers. The ultimate cause and modality of this event remains uncertain. In this study, we present a high-resolution, bios-tratigraphically calibrated, bulk organic matter carbon isotope dataset derived from the marine sedimentary record of the Central High Atlas Basin in Morocco and spanning the upper Aalenian-lower upper Bajocian. Within the context of the overall lower Bajocian positive carbon isotope excursion, this curve displays a complex trend of subordinated positive and negative shifts, which can be correlated to the high-resolution bulk carbonate carbon isotope record of southeastern France and southern Spain. The presence of a similar complex shape in these carbon isotope records over a large paleogeographic area highlights that the carbon cycle perturbation associated with the early Bajocian event was more intricate than previously depicted. Using the difference in carbon isotope fractionation between the carbonate and organic matter records as a first-order tracer of atmospheric pCO(2) variations implies that the late early Bajocian was characterized by two interludes (late Propinquans and late Humphriesianum chronozones) of low greenhouse gas concentrations. This inference is supported by high-resolution paleotemperature and global sea-level records, suggesting the transient development of polar ice caps during the Middle Jurassic.