Upper Pliocene to Holocene planktonic foraminifer assemblages at Site 997 are classified into four groups based on quantitative and principal component analyses: (1) warm water (Gulf Stream) (2) subtropical gyre (mixed-layer species), (3) gyre margin (thermocline/upwelling species), and (4) subpolar faunas. The subtropical gyre and gyre margin assemblages are dominant at Site 997, associated with small amounts of Gulf Stream species such as G. menardii.
Planktonic foraminifer assemblages display major changes in dominance between 0.7 and 1.0 Ma. In sediments older than the Olduvai Subchron and younger than the Matuyama, the planktonic foraminiferal faunas are characterized by alternations in the relative abundance of gyre and gyre margin assemblages, whereas assemblages from ~0.7 to 1.0 Ma are dominated by gyre margin taxa, particularly species characteristic of modern slope-water settings. Low-oxygen conditions expanded over the Blake-Bahama Outer Ridge at Site 997A during this change in planktonic foraminifer assemblages perhaps because of weaker upper North Atlantic Deep Water ventilation.
No distinct changes have been recognized at about 900 ka at Blake Ridge during the switch from 41- to 100-k.y. cyclicity in the well known "Mid-Pleistocene Revolution." Instead, we observed at least two intervals of faunal turnover around 700 and 1000 ka, just above and below the "Mid-Pleistocene Revolution." (Fig. 12). We conclude that the "Mid-Pleistocene Revolution" was not an "event" per se, but involved a gradual refrigeration and speeding-up of gyral circulation that helped expand the frontal zone over the Blake Ridge and establish the slope-water foraminifer assemblage over the Blake Bahama Outer Ridge throughout the late Pleistocene. Our data also indicate that changes in both Gulf Stream intensity and southward movements of northern cooler water masses containing transitional groups are responsible for variations in planktonic foraminifer assemblages over Blake Ridge. We suggest that an increase in trade-wind velocity connected with sea-surface temperature or insolation changes can explain the time-progressive changes of thermocline depths and foraminifer assemblages.