On the Ceara Rise, complete Oligocene sections were recovered at Sites 925 and 929, and part of the Oligocene was recovered at Sites 926 and 928 during Ocean Drilling Program Leg 154. At all the sites, high-resolution shipboard measurements of magnetic susceptibility (MS) and light reflectance record persistent decimeter- to meter-scale cyclicity in the pelagic sediments. The cyclicity primarily relates to variations in the calcium carbonate contents. Spectral analysis shows that the cycles are regular in thickness, implying a regular fluctuation in the environment of deposition. Dating, constrained by the current chronometric scale and biostratigraphic datum levels, indicates that this cyclicity records the 40,000-yr obliquity orbital-climatic signal.
High-resolution lithostratigraphic correlation between sites is difficult because of the independent variations in sedimentation rate at each site, combined with the occurrence of slumps and carbonate turbidites, as well as a fault at Site 925. The spectral results were used to constrain the wavelength of the 40,000-yr cyclicity and hence infer pelagic sedimentation rates. For each site the reflectance data, omitting the results from the redeposited sediment, were replotted in cumulative time relative to the end of the Oligocene. Correlation of the data between sites, once plotted on the independent cumulative time scales, allowed the development of a “composite” interval time scale for the Ceara Rise sediments. This spans the mid- to latest Oligocene (i.e., planktonic foraminiferal Biozones P20-P22 and nannofossil Biozones CP18-CP19b). The duration of the earliest Oligocene was estimated using data from Site 925 only. The time scale for the mid- to late Oligocene, which spans approximately 6 m.y., permits detailed correlation of other parameters such as isotopic data between sites on the Ceara Rise. The new biozone duration estimates and the temporal relationships between the planktonic foraminifer and nannofossil biozones differ substantially from published time scales.
1Shackleton, N.J., Curry, W.B., Richter, C., and Bralower, T.J. (Eds.), 1997. Proc. ODP, Sci. Results, 154: College Station, TX (Ocean Drilling Program).
2Department of Geology, University of Luton, Park Square, Luton, Bedfordshire LU1 3JU, United Kingdom. graham.weedon@luton.ac.uk
3Godwin Laboratory, University of Cambridge, Free School Lane, Cambridge, Cambridgeshire, United Kingdom.
4Department of Geology, Wills Memorial Building, University of Bristol, Queen’s Road, Bristol BS8 1RJ, United Kingdom.
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