Surface and Deep Ocean Circulation in the Subpolar North Atlantic During the Mid-Pleistocene Revolution

Two high sedimentation rate (about 10 cm/1000 years (k.y.)) subpolar North Atlantic Ocean Drilling Program sites, Feni Drift Site 980 (55° 29'N, 14° 42'W, 2179 m) and Bjorn Drift Site 984 (61° 25'N, 24° 04'W, 1649 m) were used to investigate surface and deep ocean variability from 500 to 1000 thousand years ago (ka). This interval encompassed the Mid Pleistocene Revolution (MPR), the period from about 900 to 600 ka when glacial/interglacial cycles changed from 41 k.y. obliquity to 100 k.y. eccentricity dominated periodicity. We test the hypothesis that oceanographic changes in the North Atlantic region were associated with the initiation of the 100-k.y. world using stable isotopes, planktic foraminiferal faunas, IRD data, and faunally based sea surface temperature estimates. At Feni Drift Site 980, glaciations and interglaciations were slightly cooler relative to Marine Isotope Stages (MIS) 5 and 6 prior to about 620 ka in the %Neogloboquadrina pachyderma (s) and Transfer Function Technique Sea-Surface Temperature (SST) records. At Bjorn Drift Site 984, interglaciations were significantly cooler than MIS 5 throughout the 500-900 ka interval studied, based on the %N. pachyderma (s) record. At 5 of the 6 glacial terminations from 500-1000 ka the initial benthic δ ¹⁸O decrease was followed by a benthic δ ¹³C decrease and %IRD increase accompanied by a cooling of the surface water indicators. This was followed by a gradual δ ¹³C increase that often continued into the interglacial. The end of each deglacial was marked by a rapid %N. pachyderma (s) decrease, SST increase and % IRD decrease. Benthic δ ¹⁸O change that preceded surface water warming and North Atlantic Deep Water (NADW) recovery suggests that ice volume decrease may have been driven by processes other than North Atlantic surface circulation, although subducted meltwater is a possible complication. North Atlantic polar front position was traced through comparison of polar water indicator %N. pachyderma (s) and sea ice edge indicator %Globigerina quinqueloba. Starting within MIS 16 at ca. 660 ka, the %G. quinqueloba at Site 980 significantly increased during warm intervals, indicating that the polar front may have fluctuated over the Feni Drift area. A similar increase occurred at Site 984 during MIS 15 at ca. 610 ka, indicating that the polar front may have migrated north to more strongly influence the Bjorn Drift area. The more northerly position of the polar front would have allowed greater moisture transport to the subpolar North Atlantic while SSTs were generally cool, and may have helped contribute to the onset of the 100- k.y. world.