INTRODUCTION

The change in the climatic system from an earlier 41-k.y. dominant oscillation to the stronger 100-k.y. oscillation of the late Pleistocene has been recognized based on the spectral analysis of paleoceanographic indicators such as ¹⁸O, carbonate content, the nannofossil Florisphaera profunda, and others. This event, called the "Mid-Pleistocene Revolution" (Berger et al., 1993; Berger and Loutre, 1994), occurred around 0.9 Ma in the interval corresponding to isotopic Stages 25 and 22 and is related to the first severe northern hemisphere glaciation (Pisias and Moore, 1981; Ruddiman et al., 1989; Raymo et al., 1990; Berger et al., 1993). However, it is not yet clear whether the development of the 100-k.y. oscillation was caused by a rapid and global change in the climatic system (Pisias and Moore, 1981; Berger et al., 1993) or was more progressive, taking place approximately in the interval from 1.0-1.2 to 0.6 Ma (Ruddiman et al., 1987, 1989).

In western equatorial Atlantic sites drilled during Leg 154 at Ceara Rise, the "Mid-Pleistocene Revolution" appears to be associated with an important change in Atlantic surface current dynamics as shown by a drastic change in the phase relationships between ¹⁸O and the F. profunda 41-k.y. oscillation (Bassinot et al., 1997). This abrupt change occurred over a short period of time centered roughly on 930 ka (isotope Stage 24) and appears to have resulted from modification of the circulation in the subtropical gyre during the growth of the Laurentide ice sheet.

However, Cullen and Curry (1997) did not find a step-like change in foraminifer proxies at ~930 ka, in contrast to the work of Bassinot et al. (1997).

Hole 997A (2770 m in water depth) is located on the crest of the Blake Ridge, in the mid-latitude western Atlantic Ocean (Fig. 1). We expected this site to be sensitive to variations in tropical-subtropical and temperate water masses in the Pleistocene given its location near the frontal boundary between the gyre and the Gulf Stream. Our primary goals were to (1) reconstruct hydrographic conditions of the overlying surface-water mass based on faunal variability of the planktonic species, and (2) document the timing of changes in surface circulation patterns at this temperate site in the western Atlantic as a monitor of Pliocene-Pleistocene changes in Gulf Stream intensity.