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 OligoceneMiocene 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, Queens 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.