Oxygen and carbon isotope records measured in epibenthic foraminifers (Cibicides wuellerstorfi, Cibicides spp., and Nuttalides umbonifera) are presented for Sites 925 (3041 m water depth), 927 (3315 m), and 929 (4356 m), recovered during Ocean Drilling Program (ODP) Leg 154 at the northeast slope of the Ceara Rise, western equatorial Atlantic. These records comprise the time interval from 2.6 to 0 Ma. Dating is based on tuning variations of the magnetic susceptibility records to orbital parameters using the 1,1 solution of Laskar.
The isotope records of all sites are exceptionally coherent and show a cyclicity dominated by a 41-k.y. cycle during the late Pliocene and the early Pleistocene and a 100-k.y. cyclicity during the late Pleistocene. Carbon isotope records show a varying depth gradient during the studied period. For interglacials, a difference of 0.3 reflects the modern pattern of admixing of Lower Circumpolar Deep Water into the overlying North Atlantic Deep Water (NADW). During glacials, carbon isotope values of shallow and deep sites differ by up to 1.0, but converge since 1.6 Ma, and especially since 1.0 Ma.
The evolution of Atlantic to Pacific carbon isotope gradients is examined using a record of the western equatorial Pacific ODP Site 806B (2520 m). From 2.6 to 1.6 Ma glacial reductions in NADW are less than those observed between 1.6 and 0 Ma. During the mid-Pleistocene (0.70.3 Ma), glacial carbon isotope values of even the shallower Atlantic cores are indistinguishable from Pacific values from approximately the same depth.
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).
2Fachbereich Geowissenschaften, Universität Bremen, 28334 Bremen, Federal Republic of Germany. firstname.lastname@example.org
3Department of Geology, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, U.S.A.