30. LATEST OLIGOCENE THROUGH EARLY MIOCENE ISOTOPIC STRATIGRAPHY AND DEEP-WATER PALEOCEANOGRAPHY OF THE WESTERN EQUATORIAL ATLANTIC: SITES 926 AND 929¹

B.P. Flower,² J.C. Zachos,² and E. Martin²

ABSTRACT

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.