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

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Geography and Geology


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 (32degrees29'N, 76degrees20'W) is located on the continental slope (water depth: 2167 in) 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 delta(13)C, which are both associated with a weak Western Boundary Undercurrent and low North Atlantic Deep Water production, persist for 6 kyr after the benthic delta(18)O record indicates that ice volume has begun to decrease. Recovery of carbonate and benthic delta(13)C values is abrupt and rapid. Millennial-scale variation (similar to 3-4 kyr) is apparent in the glacial intervals of the planktonic delta(18)O records and is more pronounced in a Deltadelta(18)O record, which represents the temperature range in the photic zone. Semi-precessional (10-12-kyr) cycles are apparent in the planktonic Deltadelta(13)C record. The millennial-scale cycles are largely caused by an increase in G. sacculifer delta(18)O 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 delta(13)C 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 delta(18)O 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 Deltadelta(18)O record are accompanied by decreases in diversity, which is consistent with the interpretation of surface warming during these events. (C) 2002 Elsevier Science B.V. All rights reserved.

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