Gulf Stream and Western Boundary Undercurrent variations during MIS 10–12 at Site 1056, Blake-Bahama Outer Ridge

Abstract

The stable isotopic composition of two planktonic foraminifer species ( Globigerinoides sacculifer and Neogloboquadrina dutertrei) and two benthic foraminifer species ( Cibicidoides wuellerstorfi and Uvigerina peregrina) was measured at sub-orbital resolution through the marine isotope stages (MISs) 10, 11, and 12 (345–460 ka) at Site 1056 on the Blake-Bahama Outer Ridge. Planktonic foraminifers were counted for the interval 405–450 ka at 2–4-kyr resolution. Site 1056 (32°29′N, 76°20′W) is located on the continental slope (water depth: 2167 m) beneath the Gulf Stream. The average rate of sediment accumulation through the interval is 11.4 cm/kyr, but sediment accumulation is much more rapid during glacial intervals (15–17 cm/kyr). The decline in percent carbonate during glacial intervals, and its rise during interglacials, indicates that the increased sediment supply is of terrigenous origin. Low carbonate values and low benthic δ 13C, which are both associated with a weak Western Boundary Undercurrent and low North Atlantic Deep Water production, persist for 6 kyr after the benthic δ 18O record indicates that ice volume has begun to decrease. Recovery of carbonate and benthic δ 13C values is abrupt and rapid. Millennial-scale variation (∼3–4 kyr) is apparent in the glacial intervals of the planktonic δ 18O records and is more pronounced in a Δδ 18O record, which represents the temperature range in the photic zone. Semi-precessional (10–12-kyr) cycles are apparent in the planktonic Δδ 13C record. The millennial-scale cycles are largely caused by an increase in G. sacculifer δ 18O and represent surface warming. They are interpreted as representing periodic increases in westward intensification of the gyre. The semi-precessional cycles are driven by changes in the N. dutertrei δ 13C and represent fluctuations in the Gulf Stream itself and therefore likely have a tropical origin. Planktonic foraminifer census counts did not show an expected response to one of the largest glacial/interglacial transitions of the late Pleistocene. The most obvious change was an increase in faunal diversity during MIS 12.2, the interval of maximum δ 18O values. This suggests that cool slope water and warm subtropical gyre water penetrated a more sluggish Gulf Stream with greater frequency at this time. The millennial-scale maxima in the Δδ 18O record are accompanied by decreases in diversity, which is consistent with the interpretation of surface warming during these events.