29. MULTISPECIES PLANKTONIC FORAMINIFER STABLE ISOTOPE STRATIGRAPHY THROUGH OLIGOCENE/MIOCENE BOUNDARY CLIMATIC CYCLES, SITE 926¹

Paul N. Pearson,² Nicholas J. Shackleton,³ Graham P. Weedon,⁴ and Mike A. Hall³

ABSTRACT

The oxygen and carbon isotopic ratios of 12 multispecimen samples of different extinct species of planktonic foraminifer are assessed to establish their relative depth ranking in a sample from near the Oligocene/Miocene boundary. Of these, the following species (with their interpreted depth habitats) were selected for high-resolution isotope stratigraphy: Catapsydrax dissimilis (subthermocline intermediate water), Globoquadrina venezuelana (lower thermocline), Paragloborotalia mayeri (upper thermocline), Paragloborotalia pseudokugleri (mixed layer), and Globigerina praebulloides (mixed layer). Two cyclic stratigraphic intervals are selected within Core 154-926B-50X, which straddles the Oligocene/Miocene boundary. Very little consistent variation is seen in of the mixed-layer and upper thermocline species through the pronounced light-dark obliquity-driven lithologic cycles in the core. This indicates that the environmental variation that caused the lithologic cycles occurred in the absence of marked sea-surface temperature change, and also without significant ice-volume fluctuations. A slight but significant positive deflection is seen in the records of the deep and lower thermocline species in some of the dark bands. It is therefore hypothesized that periodic high-latitude cooling caused the influx of relatively cold bottom waters on the Ceara Rise, enhancing local dissolution and sometimes affecting subthermocline temperatures, but not affecting the upper part of the water column. There is no regular variation in the inter-species gradient. However, parallel cyclic variations in the of all species are observed, possibly with a slight phase lag with respect to the lithologic cycles and some higher frequency variation. These results point to cyclic changes in the global carbon balance during the Oligocene–Miocene transition interval.

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 Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, United Kingdom. paul.pearson@bris.ac.uk
³Gowdin Laboratory, Department of Earth Sciences, University of Cambridge, Free School Lane, Cambridge CB2 3RS, United Kingdom.
⁴School of Geological Science, University of Luton, Park Square, Luton, Bedfordshire LU1 3JU, United Kingdom.