Beginning ~2.0 Ma, foraminifer assemblages are dominated by northern gyre margin taxa such as G. inflata and include rare warm species, suggesting cold conditions near the base of the Olduvai Subchron (Fig. 12). We suggest that weakening of the western boundary current caused less warm water to flow into the western boundary of the subtropical gyre and result in a shallower thermocline and generally cooler conditions. In the early Matuyama, the warm subtropical gyre assemblage became dominant, presumably because of the general warming and northward movement of the warm part of the subtropical gyre system (Fig. 12). This warming could have been produced by intensification of the gyre circulation due to a stronger trade-wind system (Fig. 13; warm time).
In the early Jaramillo Subchron, the subtropical gyre assemblage gave way to a gyre margin assemblage at Site 997 (Fig. 10, Fig. 12), suggesting expansion of the mixing zone between the Gulf Stream to the west and the subtropical gyre to the east. Bassinot et al. (1997) have hypothesized that the switch from 41-k.y. to 100-k.y. cyclicity during the "Mid-Pleistocene Revolution" (~930 ka) was associated with a southward shift of the Intertropical Convergence Zone (ITCZ) or a southward shift in the North Equatorial Counter Current. Their data suggest that this change in tropical hydrography produced a shallower thermocline at Ceara Rise Site 927 in the tropical Atlantic Ocean. Other work at Ceara Rise has demonstrated that thermocline taxa dominated in glacial stages and mixed-layer taxa dominated in interglacials during the mid-Pleistocene between 1000 and 500 ka (Cullen and Curry, 1997). These authors did not observe major changes in assemblages at the 930 k.y. onset of the 100-k.y. cycle, but they did observe a general decrease in mixed-layer species during the Jaramillo and the early Brunhes as in our data from Hole 997A. These observations suggest that the southern part of the subtropical gyre became less well stratified than it had been in the lower Matuyama Chron. The rapid change in Hole 997A from the warm subtropical gyre (mixed layer) assemblage to the cooler gyre margin/slope-water assemblages at the top of the Jaramillo Subchron suggests that the "Mid-Pleistocene Revolution" was accompanied by an expansion of the frontal zone between the western boundary current and the subtropical gyre, along with greater upwelling along this frontal boundary.
During the late Jaramillo and upper Matuyama, benthic foraminifers characteristic of low oxygen conditions became common (samples between 33.29 and 40.79 mbsf; Samples 164-997A-5H-2, 39-41 cm, to 5H-7, 39-41 cm) and sediments also contain abundant dark-gray to black colored, cylindrical pyrite tubes that are 1-2 mm in length and 100-200 µm in diameter (Fig. 3, Fig. 12). In addition, diatoms became abundant (up to 35% of the fossil assemblage), whereas they are usually below 10% of the sediments throughout the remainder of the studied interval (Fig. 3; Shipboard Scientific Party, 1996). All these features suggest lower oxygen contents of intermediate waters and more upwelling over the Blake-Bahama Outer Ridge. Our observations are consistent with weak upper North Atlantic Deep Water ventilation during the late Jaramillo Subchron and the upper Matuyama Chron.
The combination of weak intermediate water ventilation, evidence for an increase in Gulf Stream species, and the re-establishment of the gyre margin assemblage suggests that the waters over Site 997 marked a dynamic frontal system where faunas converged from the Gulf Stream, as well as warm and cold core rings spun off the Gulf Stream and both gyre margin water and the gyre water (Fig. 13, Transition time). The increase of both diatoms and the gyre margin faunas indicate high surface productivity, associated with stronger surface overturning. The "Mid-Pleistocene Revolution," therefore, marked a transition from warmer subtropical gyre conditions to strong warm-cold oscillations at the location of Site 997 between 1.0 and 0.7 Ma (Fig. 12).
The gyre margin assemblage in Hole 997A remained an important part of the planktonic foraminiferal faunas through the Brunhes Chron, although the warm subtropical gyre assemblage made a partial reappearance with higher abundance of G. ruber and G. sacculifer in the early Brunhes. Based on our data, the modern ecotone between the Gulf Stream and the subtropical gyre had developed close to 0.7 Ma and just above the Brunhes/Matuyama boundary. The warm foraminiferal component, presumably from the Gulf Stream or southern Sargasso Sea, became less important later in the Brunhes because G. sacculifer decreases in abundance above 32 mbsf. A gyre assemblage with G. hirsuta appeared between 5 and 10 mbsf, suggesting that the climate system favored a more northern subtropical gyre assemblage in the middle Brunhes. Finally, the gyre margin group re-emerged as a dominant component in the last glacial cycle above 5 mbsf (Fig. 12).