High-resolution image analysis of laminae (organic-rich with calcareous nannofossils) in a black shale sequence: probability of orbital and suborbital climate cycles in the latest Cenomanian

Abstract

We conducted a high-resolution image analysis of the lamination present in a core interval of black shale from Hole 1260B (ODP Leg 207) on the Demerara Rise in the western tropical Atlantic. Gray scale digital image analysis results revealed environmental cyclic patterns that could be related to orbital and suborbital changes during the latest Cenomanian (94 Ma). High-resolution nannofossil biostratigraphy provided the detailed chronological information needed to constrain the time intervals of each cyclic event. A study of thin sections showed that the basic pattern of the millimeter-scale laminae was couplets of alternating dark organic-rich layers and white biogenic test layers. The composition of the dark organic-rich layer appeared to include the remains from microbial organisms with a chain-like structure. From the combination of the gray scale and thin section analyses, three well-expressed cyclic patterns could be recognized in the black shale interval, namely (1) meter-scale cyclic events (average interval of approximately 413 mm), (2) centimeter-scale cyclic events (approximately 10 – 30 mm intervals), and (3) millimeter-scale cycles (average interval of approximately 5 mm). These cyclic events could imply approximately 40 ky, 1 – 3 ky, and 300 – 700 yr time spans, respectively, on the basis of biostratigraphy. In addition, two other cyclic patterns indicating approximately 20 ky and 60 – 150 yr time intervals were weakly expressed.