Valve concentration of Ethmodiscus sp. and carbon in sediment core GeoB3801-6

Within generally calcareous sediment sequences, layers of variable thickness of the giant diatom Ethmodiscus were found in five cores recovered in the Subtropical South Atlantic between 23° and 33°S from both sides of the Mid-Atlantic Ridge. Two types of oozes occur: (almost) monospecific layers of Ethmodiscus and layers dominated by Ethmodiscus, with several accompanying tropical/subtropical, oligotrophic-water diatoms. The two thickest Ethmodiscus layers occur in GeoB3801-6 around 29°S, and accumulated during late MIS 14 and MIS 12, respectively. Downcore concentrations of Ethmodiscus valves range between 3.4 10 4 and 2.3 10 7 valves g -1. We discuss the ooze formation in the context of migration of frontal systems and changes in the thermohaline circulation. The occurrence of Ethmodiscus oozes in sediments underlying the present-day pelagic, low-nutrient waters is associated with a terminal event of the Mid-Pleistocene Transition at around 530 ka, when the ocean circulation rearranged after a period of reduced NADW production.

Stable isotope record and composition of planktonic foraminifera from ODP Site 172-1056

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 delta13C, which are both associated with a weak Western Boundary Undercurrent and low North Atlantic Deep Water production, persist for 6 kyr after the benthic delta18O record indicates that ice volume has begun to decrease. Recovery of carbonate and benthic delta13C values is abrupt and rapid. Millennial-scale variation (~3-4 kyr) is apparent in the glacial intervals of the planktonic delta18O records and is more pronounced in a Delta delta18O record, which represents the temperature range in the photic zone. Semi-precessional (10-12-kyr) cycles are apparent in the planktonic deltadelta13C record. The millennial-scale cycles are largely caused by an increase in G. sacculifer delta18O 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 delta13C 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 delta18O 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 Delta delta18O record are accompanied by decreases in diversity, which is consistent with the interpretation of surface warming during these events.