Tethyan Cenomanian pelagic rhythmic sedimentation and Pleistocene Mediterranean sapropels: is the biotic signal comparable?

摘要

A comparison between the biotic signal recorded in the Tethyan Cenomanian black shales and in the Mediterranean Pleistocene sapropels is proposed here. To this end the Antruiles section (northeastern Dolomites, Italy) has been investigated as a reference section for the Cenomanian black shales. The data obtained, based on calcareous nannofossils and foraminifera, have been compared to those from a core in the Mediterranean Sea containing a complete succession of Late Pleistocene-Holocene sapropels. Our study suggests that deposition of black shales, like sapropels, occurred when the water column was stratified, as implied by increased abundance of the Cenomanian deeper-dwellers Rotalipora cushmani and R. greenhornensis. The superficial/subsuperficial opportunist form Heterohelix reussi, which strongly increases in numbers within the black shales, could be analogous to the modern surface-dweller Globigerinoides, ruber, which is abundant in Late Pleistocene-Holocene Mediterranean sapropels. The peaks of G. ruber can be related to decreased salinity in surface water, thus the increase of H.reussi could also be related to a freshwater superficial layer. We hypothesise also that an increase in abundance of Watznaueria barnesae reflects conditions of superficial low salinity. In black shales increases in abundance of Dicarinella algeriana and Praeglobotruncana gibba, by analog to modern Neogloboquadrina dutertrei, could reflect availability of food restricted to subsuperficial waters. However, a Cretaceus equivalent of the Pleistocene nannoplankton form Florisphaera profunda, which proliferates within the Deep Chlorophyll Maximum, is not observed. In addition, the abundance of H. reussi indicates a well-expanded oxygen minimum zone, although less in extent than in Pleistocene sapropels, as shown by the occurrence within the Cenomanian black shales of a benthic fauna, even though reduced in size and impoverished. Spectral analysis applied to calcareous nannofossils identifies close relations with the Milankovitch precession and eccentricity cycles in both the black shales and the sapropels.