Environmental changes from greenhouse earth to icehouse earth through a study of chalk/black shale sequence

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A chalk/black shale sequence exposed in the Appenine Mis. in Italy was investigated in order to reveal the basic marine environmental setting in the Cretaceous warm period. Cenomanian/Turonian black shale (Bonalleli horizon) showed the following characteristics.1. The source of organic matter is bacterial origin (Cyanobacteria and photosynthesic bacteria).2. Large excursion of sulfur isotope suggests strong ocean stratification with anoxic bottom water. About 30% of total sulfur from the global ocean was fixed as pyrite. The ocean surface had to be highly oligotrophic.3. Occasional instability of stratified ocean and upwelling caused events of nutrient supply to the surface that caused radiolarian blooming.