Nannoplankton productivity and diagenesis in the rhythmically bedded Toarcian-Aalenian Fiuminata section (Umbria-Marche Apennine, central Italy)

Abstract

Pelagic rhythmic lithotypes crop out in the Fiuminata section (central Italy), middle Toarcian-Aalenian in age, and provide a good example of cyclic sedimentation within the Calcari e Marne a Posidonia Unit. The whole succession is made up of a nutty-yellowish rhythmic sequence of marlstone and marly limestone beds, with similar thicknesses. Calcareous nannofossil quantitative analyses give a good biostratigraphic resolution of the studied interval, and also reveal a different assemblage composition of the carbonate-rich lithotypes with respect to the carbonate-poor ones. Schizosphaerella spp., which is the dominant taxon in the assemblage, shows higher abundances in the limestones; conversely, coccoliths are more abundant and diverse in the marls. Such data emphasize a relationship between palaeoenvironmental conditions in the water column, nannoplankton productivity and the depositional pattern of the sedimentary basin. Diagenetic overprint can also account for the different assemblage composition in marlstones and limestones. The effects of diagenesis on the original thanatocoenosis have been detected through a detailed analysis of nannofossil preservation. This study points out that diagenesis has enhanced, without obliterating, a primary palaeoenvironmental signal. Nannoplankton productivity varied in response to differing conditions in the water column: Schizosphaerella spp. seems to be more productive concomitant with the deposition of carbonate-rich sediments, with bioturbation and abundant radiolarians. Probably, while these sediments were deposited, a more vigorous vertical mixing of the water column occurred and trophic resources were good. A stable and diversified community of coccolithophorids was established in times of clay deposition, when the surface waters were stratified due to their density and nutrients were available both at the sea bottom and in the water column, due to the runoff. The differing nannoplankton assemblage is tentatively interpreted in terms of a response to Milankovitch orbital-climatic cycles.