4. Site 10891

Shipboard Scientific Party2

BACKGROUND AND OBJECTIVES

Site 1089 (proposed site SubSAT-1B) is located in the southern Cape Basin close to the northern flank of the Agulhas Ridge at a water depth of 4620 m (see Figs. F1, F5, both in the "Leg 177 Summary" chapter). This area is presently located south of the Subtropical Front (STF), which bounds the Antarctic Circumpolar Current (ACC) to the north. A geophysical survey of the area near Site 1089 was conducted by Thompson Cruise TTN057 (Fig. F1). The site was selected in an area of thick sediment cover that overlies topographically irregular basement consisting of normal oceanic crust (Fig. F2). Site 1089 is located on a drift deposit that was formed by bottom-water currents in the southern Cape Basin. Much of the seafloor of the Cape Basin is marked by erosion above 4 km water depth (Tucholke and Embley, 1984), and some of the eroded sediment suspended by bottom-water currents may be deposited as current velocities wane in the southern Cape Basin north of the Agulhas Fracture Zone Ridge. The high sedimentation rates recorded at Site 1089 were also traced 175 km to the west (Lamont piston core V14-65), suggesting that the drift deposit is laterally continuous. Although Site 1089 is deep (4620 m) and close to the carbonate compensation depth, the high supply of terrigenous mud by currents increases bulk sedimentation rates and promotes quick burial of sediment and, thus, the preservation of calcium carbonate, which averages 35 wt% in nearby piston cores.

Previous Ocean Drilling Program (ODP) drilling of sediment drifts during Legs 162 and 172 has now shown that these high-sedimentation-rate environments provide excellent archives of past climate variability at high temporal resolution. We expected Site 1089 to represent the Southern Hemisphere analog to the drift-deposit sequences of the North Atlantic that were drilled during Legs 162 and 172. We targeted Site 1089 because of the high-resolution isotopic record of the last climatic cycle that had been obtained from nearby piston core RC11-83 (Fig. F3; Charles et al., 1996). Our goal was to extend this record into the early Pleistocene or late Pliocene. With few exceptions, deep-sea sedimentary sequences generally lack the resolution needed to delineate climatic variability on millennial time scales. Site 1089 is an exception because sedimentation rates average 150 m/m.y. for the late Pleistocene section.

The primary objective of Site 1089 was to recover a sediment sequence with ultra-high temporal resolution from north of the present-day Polar Front Zone that could be used to address the following problems:

  1. The response of the Southern Ocean environment to orbital forcing and the phase relationships to climate events occurring in low and high northern latitudes. An important question in modern paleoceanography is whether climate changes recorded in sediments of the Southern Ocean lead, lag, or are in phase with climate changes in the high-latitude North Atlantic region. For example, Sowers and Bender (1995) and Bender et al. (1994) found that warming in Antarctica during the last deglaciation preceded that of Greenland by almost 2000 yr. By analysis of core RC11-83 (located near Site 1089), Charles et al. (1996) found that Northern Hemisphere climate fluctuations during the past 80 k.y. lagged those of the Southern Hemisphere by 1500 yr. Was the early response of Southern Ocean surface and deep waters relative to paleoceanographic proxies from other regions a persistent pattern during the entire late Pleistocene as suggested by Imbrie et al. (1992)?
  2. Rapid climate change on suborbital time scales in the Southern Ocean region and its relation to climate signals from polar ice cores. The high accumulation rates at Site 1089 permit correlation of millennial-scale climate oscillations from marine-sediment records in the Southern Ocean to ice-core signals from Greenland and Antarctica. For example, the similarity between the two planktic foraminifer 18O time series in core RC11-83 and the Vostok Dice signal is striking over the past 70 k.y. (Fig. F3; Charles et al., 1996). With the extension of the Vostok ice-core signal over the past four climatic cycles (Petit et al., 1997), Site 1089 will be useful for detailed correlations between ice and marine-sediment cores for the past 450 k.y. and for studying the linkages between atmospheric (temperature and pCO2) and oceanic dynamics (sea-surface temperature, productivity, and deep-water circulation).
  3. Glacial-interglacial variations in the physical and chemical properties of bottom-water masses in the South Atlantic Ocean and their relation to high-latitude climate change. Site 1089 (4620 m) is located in lower Circumpolar Deep Water (CDW) (Fig. F2 in the "Leg 177 Summary" chapter), and benthic 13C variations at this site should be sensitive to the varying flux of North Atlantic Deep Water (NADW) to the Southern Ocean. By comparing the benthic 13C record with the planktic 18O record from Site 1089, we hope to determine the phase relationship between variations in NADW and the surface-ocean response in the Southern Ocean (Charles et al., 1996). We plan to address the question whether changes in NADW flux to the Southern Ocean were an important interhemispheric mechanism for linking climate change between the polar oceans.
  4. Glacial-interglacial variations in Southern Ocean nutrient cycling, productivity, and pCO2 and their role in global biogeochemical cycles. The Southern Ocean has become a region of paleoceanographic focus because of the key role it plays in global deep-water circulation and its potential significance for the global carbon cycle. For example, it has been proposed that new production (sinking flux of organic carbon) was higher and utilization of preformed nutrients in surface waters was more efficient in the glacial Southern Ocean than today, effectively lowering the glacial atmospheric CO2 concentration. Martin (1990) proposed enhanced eolian iron fertilization from a dustier atmosphere as one potential mechanism that enhances Antarctic phytoplankton production during glaciations. The tests for such productivity changes using various tracers in Antarctic deep-sea sediments have yielded equivocal results (Boyle, 1989; Mortlock et al., 1991; Kumar et al., 1995; Frank et al., 1996, in press; Francois et al., 1998). Site 1089 should provide the northern end-member along the north-south transect of sites needed to test for glacial-interglacial changes in export production of organic carbon and biogenic silica in the South Atlantic.

1Examples of how to reference the whole or part of this volume.
2Shipboard Scientific Party addresses.

Ms 177IR-104