Origin of rhythmic Albian black shales (Piobbico core, central Italy): Calcareous nannofossil quantitative and statistical analyses and paleoceanographic reconstructions

Abstract

The mid-Cretaceous (∼120-90 Ma) was a time of excess atmospheric CO2, greenhouse climate, and widespread O2 deficiency in the ocean. The Albian is punctuated by brief, intermittent episodes of anoxia/dysoxia, recorded as cyclic black shales in the western Tethys and Atlantic oceans. The Albian section of the Piobbico core (central Italy) contains 30 m (∼10 Ma) of rhythmic black shales that were sampled at a high resolution and examined for calcareous nannofossil assemblages and C and O stable isotopes. Unlike oceanic anoxic events, productivity was not the primary factor controlling the deposition of Albian rhythmic black shales. It is suggested that during warm humid climatic cycles, higher temperatures and/or increased precipitation and runoff produced density stratification at a regional scale. Recurrent lowered salinity in the early Albian and wanning in the late Albian are credited with causing development of a pycnocline, resulting in slower rates of deep-water renewal and consequent anoxia. Copyright 2009 by the American Geophysical Union.