Stable isotopic (
and
) and strontium
isotopic (87Sr/86Sr ) data generated from
Ocean Drilling Program (ODP) Sites 926 and 929 on Ceara Rise
provide a detailed chemostratigraphy for the latest Oligocene
through early Miocene of the western Equatorial Atlantic. Oxygen
isotopic data based on the benthic foraminifer Cibicidoides
mundulus exhibit four distinct
excursions of more than 0.5,
including event Mi1 near the Oligocene/Miocene boundary from 23.9
to 22.9 Ma and increases at about 21.5, 18 and 16.5 Ma, probably
reflecting episodes of early Miocene Antarctic glaciation events
(Mi1a, Mi1b, and Mi2). Carbon isotopic data exhibit well-known
increases near the
Oligocene/Miocene boundary (~23.8 to 22.6 Ma) and near the
early/middle Miocene boundary (~17.5 to 16 Ma). Strontium
isotopic data reveal an unconformity in the Hole 926A sequence at
about 304 meters below sea floor (mbsf); no such unconformity is
observed at Site 929. The age of the unconformity is estimated as
17.9 to 16.3 Ma based on a magnetostratigraphic calibration of
the 87Sr/86Sr seawater curve, and as 17.4
to 15.8 Ma based on a biostratigraphic calibration. Shipboard
biostratigraphic data are more consistent with the
biostratigraphic calibration.
Similar isotopic values at Sites 926 and 929 during the latest
Oligocene through early Miocene suggests both were bathed by the
same water mass throughout this interval. However, comparison
with North Atlantic Site 608 (representing a Northern Component
Water [NCW] end-member) and South Atlantic Site 704 (representing
a Southern Component Water [SCW] end-member) reveals a switch in
south-to-north deep-water gradients between about 23 and 19 Ma. These results
confirm earlier suggestions that Atlantic deep-water circulation
featured a single SCW source near the Oligocene/Miocene boundary
and developed a second NCW source during the early Miocene.
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).
2Institute of Marine Sciences and Earth Sciences Board, University of California, Santa Cruz; Santa Cruz, CA 95064, U.S.A. flower@earthsci.ucsc.edu
3Department of Geology, University of Florida, Gainesville; Gainesville, FL 32611, U.S.A.