The mid-Cenomanian Event: prelude to OAE 2

Abstract

A detailed synthesis of litho- and biostratigraphic investigations of the Cenomanian pelagic limestones of the Scaglia Bianca Formation in the Umbria–Marche Basin shows that the Bonarelli Level, which is the regional sedimentary expression of the OAE 2, represents the climax of a ∼2 Myr-long cycle of black shale deposition that began in the mid-Cenomanian planktonic foraminiferal Rotalipora cushmani Zone. This long-term cycle began with a prominent short-term event, named mid-Cenomanian Event (MCE), which is associated with several changes in the biotic and abiotic records. In particular, a comparison of the available litho-, bio-, and chemostratigraphic data from the Umbria–Marche Basin allows recognition of the MCE based on: (a) a significant change in the sedimentary and tectonic regimes; (b) a reorganisation of planktonic foraminiferal assemblages; (c) a major change in benthic foraminifera; (d) a major turnover in the radiolarian assemblages; (e) a 0.7‰ positive shift in the carbon isotope values; and (f) a positive shift of carbonate Sr/Ca ratio. Moreover, the MCE is preceded (∼400 kyr) by a short-term 0.5‰ negative excursion in the oxygen isotope record. The above mentioned changes in carbon and oxygen isotope values can be confidently correlated over different basins and oceans and seem to occur in correspondence with a major sea-level fall. These lines of evidence define the MCE as an outstanding event associated with large-scale changes in the oceanic structure and palaeoclimate. The MCE and the subsequent cyclical deposition of organic-rich beds serve as a case study to better understand differences and similarities between Mesozoic black shales and Cenozoic sapropels and related palaeoceanographic changes controlled by orbital climate cycles.