HIGH-RESOLUTION IMAGE ANALYSIS OF LAMINAE IN A BLACK SHALE SEQUENCE FROM ODP SITE 1260 ON THE DEMERARA RISE: EVIDENCE FOR ORBITAL AND SUBORBITAL CLIMATE CYCLES IN THE LATEST CENOMANIAN

We conducted a high-resolution image analysis of the lamination present in a 4 m interval of black shale from Hole 1260B (ODP Leg 207) on the Demerara Rise, western tropical Atlantic. Gray scale digital image spectral 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. Study of thin sections showed that the basic pattern of the millimeter scale laminae is couplets of alternating dark organic rich layers and white biogenic carbonate layers. The composition of the dark organic rich layer appears to include the remains from some kind of microbial organisms that have a chain-like structure – perhaps Cyanobacteria. From combination of the gray scale and thin section analyses, three notable cyclic events can be recognized in the black shale interval: (1) meter scale cyclic events implying ~40 ky time intervals , (2) centimeter scale cyclic events indicating 1~2 ky time intervals, and (3) millimeter scale cycles indicating 300~700 yr time spans. In addition, two other cyclic events indicating 20 ky and 100~250 yr time intervals are weakly expressed. Results in recent reports of high resolution studies of cyclicity in other black shale sequences (Friedrich et al., 2005; Kuhnt et al., 2005) are similar to our findings and suggest that these cycles record fundamental features of mid-Cretaceous climate.