Relationships between the early Toarcian anoxic event and organic-walled phytoplankton in central Italy

Abstract

The integration of palynological and geochemical data from three lower Toarcian successions in central Italy reveals that the composition of organic-walled phytoplankton assemblages were strongly affected by palaeoecological conditions related to bituminous sedimentation which accompanied the global anoxic event. The marked compositional variations of dinoflagellate cysts and prasinophytes, together with geochemical variations, have been linked to changes in surface water habitats during the lower Toarcian transgression. On the basis of the relationships between total organic carbon (TOC) and marine palynomorph assemblage composition, the lower Toarcian evolution of the Umbria-Marche Basin, central Italy, has been divided into four phases. Total organic carbon values rose significantly during the early Toarcian (Lower-middle Dactylioceras tenuicostatum ammonite Zone), and this can be linked to certain dinoflagellate cyst datums, for example the temporary disappearance of Mancodinium semitabulatum and the extinction of Luehndea spinosa. The presence of Umbriadinium mediterraneense and Valvaeodinium spp. accompany these moderately high TOC values. Subsequently, TOC levels increased to over 2% and prasinophytes became abundant in the Middle-upper D. tenuicostatum ammonite Zone. Mancodinium semitabulatum reappeared when TOC values eventually decreased in the Upper D. tenuicostatum ammonite Zone. This analysis has allowed the different sunlight requirements and life strategies of the early Toarcian Tethyan dinoflagellates to be modelled. Due to the cosmopolitan nature of the early Toarcian anoxic event, the principal marine palynological signals observed have been interpreted as sequence stratigraphical and palaeoecological indices. The Transgressive Systems Tract (TST) is accompanied by an increase in dinoflagellate cyst species diversity and a decrease in abundance. The succeeding maximum flooding surface (mfs) corresponds with a prasinophyte acme. During the Highstand Systems Tract (HST), the phytoplankton shows an increase in abundance and a decrease in diversity. The range top of Luehndea spinosa appears to characterise the early Toarcian TST.