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:
- 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)?
- 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).
- 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.
- 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.
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