Lamina-scale analysis of sedimentary components in Cretaceous black shales by chemical compositional mapping: Implications for paleoenvironmental changes during the Oceanic Anoxic Events

Abstract

Relative abundances and distribution patterns of mineral components in finely laminated “Livello Bonarelli (LB)” black shale from the Cenomanian/Turonian boundary, Cretaceous, were investigated by 10 μm-resolution compositional mapping using an electron probe microanalyzer. Sedimentary components such as biogenic silica (Si bio), calcium phosphate, calcium carbonate, and pyrite were reconstructed by image processing of the compositional maps. Total organic carbon (C org) content was also determined at a sampling interval of 1.5 mm. The depth profiles of these sedimentary components, covering ∼60% of the total 104-cm LB sequence, indicate the highly variable nature of their deposition. Especially, the C org contents vary widely, ranging from <0.1% to 26%. Intervals enriched in C org are clearly discernible in the LB, and correspond to “dark” colored intervals. Repeated occurrences of C org-rich and Si bio-rich intervals are one of the most prominent features in the LB. These features could be explained partly by dilution of C org by Si bio, but largely by the presence of two distinct paleoenvironmental settings: one involves predominant deposition of organic matter and the other is dominated by Si bio deposition. The thickness of individual intervals varies widely, mostly ranging from 1.5 to 35 mm, representing oceanic changes on the order of 10 3–10 4 years. Furthermore, we found 100-μm-scale laminations of fine-grained pyrite, aluminosilicate, and calcium carbonate, which when superimposed on the alternation of C org-rich and Si bio-rich intervals, suggests 10 2-year scale fluctuations in oceanic environments.