We studied two sites cored during Ocean Drilling Program Leg 154 on the Ceara Rise in the western equatorial Atlantic: Sites 925 (3041 m water depth) and 928 (4012 m water depth). Both sites were sampled every 5 cm from about 25 to 38 meters below sea floor (mbsf; 680- to 1100-ka interval) for a high-resolution study of low-latitude climatic variations and changes in carbonate deposition associated with the first large-amplitude glaciation of the late Quaternary (isotopic Stage 22 at about 880 ka) and the subsequent development of the 100-k.y. glacial-interglacial oscillations (the mid-Pleistocene revolution). We measured carbonate and coarse fraction contents of samples from both sites. From Site 925 samples, we also measured of the planktonic species Globigerinoides ruber and the relative abundance of the calcareous nannofossil Florisphaera profunda, an index of past changes in nutricline depth (which permits us, by proxy, to reconstruct changes in thermocline dynamics).
Our results indicate that carbonate deposition was mainly controlled by changes in biogenic productivity and dilution by terrigenous inputs from the Amazon River. In the time interval studied, all of our paleoclimatic indicators show a dominant 41-k.y. cyclicity. We suggest that 41-k.y. oscillations in the F. profunda record reflect changes in the thermocline depth resulting either from north-south oscillations of the Intertropical Convergence Zone or from changes in trade-wind intensity. These changes were most probably controlled by high-latitude variations in sea-surface temperatures that modified the subtropical gyre dynamics and were responding to changes in the volume of continental ice sheets. The mid-Pleistocene revolution appears to be associated with an important change in the equatorial Atlantic surface current dynamics as shown by the drastic change in the phase relationships between 1/41-k.y.1 filtered records that takes place over a short period of time centered roughly on 930 ka. This change may result from an important modification of the subtropical gyre dynamics at the initiation of the large Laurentide ice sheet. Before the mid-Pleistocene revolution, 41-k.y. carbonate mass-accumulation rate cycles were out of phase with F. profunda cycles, indicating that biogenic productivity was most probably responding to changes in regional or global changes in nutrient content rather than to local changes in the nutricline (thermocline) dynamics. Starting at about 900 k.y., the biogenic productivity appears to be in phase with changes in the nutricline 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).
2Centre des Faibles Radioactivités (CNRS/CEA), Domaine du CNRS, 91190 Gif-sur-Yvette, France. Franck.Bassinot@cfr.cnrs-gif.fr
3Laboratoire de Géologie du Quaternaire, CNRS, CEREGE, BP 80, 13545 Aix-en-Provence, France.