Orbitally driven fertility cycles in the Palaeocene pelagic sequences of the Southern Alps (Northern Italy)

Palaeocene pelagic sequences cropping out in the Southern Alps are often characterized by a cyclic alternation of marly and calcareous lithologies. In particular, two sets of limestone–marlstone couplets are well exposed in the Ponte nelle Alpi and Tabiago sections, both Danian to Thanetian in age, located in the Belluno region (Venetian Prealps) and in the Brianza region (Central Prealps), respectively. A multidisciplinary approach was adopted to determine the nature of these alternations by analyzing for each semicouplet the abundance and composition of planktonic foraminiferal faunas, the degree of bioturbation and the carbonate content. Both sections show similar general features. The marly semicouplets of Ponte nelle Alpi and Tabiago sections are poor in planktonic foraminifera, but enriched in more tolerant (for nutrients and temperature) foraminiferal forms, and they record more oxygenated conditions. A possible interpretation of the lithological rhythms suggests that the marly semicouplets were deposited in periods of high seasonality. A marked seasonality, leading to an increased circulation of water masses, induced a nutrient enrichment in surface waters and oxygenated conditions at the seafloor. The data prove that the marly semicouplets represents fertile surface waters, whereas the calcareous ones record the opposite conditions: low seasonality, sluggish circulation and fewer nutrients at the sea surface. Spectral analysis was applied to evaluate the possible periodicities that could have driven the observed cyclicity. The periodicities recorded agree with those predicted by the palaeoclimatic Milankovitch theory for the precession, obliquity and eccentricity cycles. The stronger signal associated to obliquity has also been recorded during the mid-Cretaceous, confined to some intervals. This inconstancy is probably attributable to the response of the Earth to a signal that may only be detected in the low latitudes when climatic or oceanic conditions are particularly favorable to its transmission.