Technical Note 20/5

LEG 177

SOUTHERN OCEAN PALEOCEANOGRAPHY

ABSTRACT

The general goals of Leg 177 are two-fold: (1) to augment the biostratigraphic, biogeographic, and paleoceanographic history of the earlier Cenozoic, a period marked by the establishment of the Antarctic cryosphere and the ACC; and (2) to target expanded sections of late Neogene sediments that will resolve the timing of Southern Hemisphere climatic events on orbital and suborbital time scales, which can be compared with similar records from other ocean basins and with ice cores from Greenland and Antarctica. Drilling the proposed sites will provide the sedimentary sequences needed to address a number of first-order problems in southern high-latitude paleoclimatology and stratigraphy including (1) the evolution of the ACC and past changes in the position of the Polar Front Zone and the Antarctic sea-ice field; (2) the evolutionary history and stability of the Antarctic cryosphere; (3) changes in Southern Ocean productivity, nutrient cycling, and pCO2 and their role in global biogeochemical cycles; (4) changes in the mixing ratio of various deep and bottom water masses in the Antarctic (e.g., North Atlantic Deep Water); (5) the response of the Southern Ocean to orbital forcing and the phase relationships to climatic changes in the high-latitude Northern Hemisphere; and (6) correlation and comparison of marine sediment cores from the Southern Hemisphere with polar ice-core records and documentation of abrupt climate change on millennial time scales.

Paleoceanographers, climatologists, and geochemists have recognized over the last decade that processes occurring in the Southern Ocean have played a major role in defining the Earth's climate system. The Southern Ocean is an extraordinarily important region because:

1. The Antarctic cryosphere represents the largest accumulation of ice on the Earth's surface. The development and evolution of the Antarctic ice sheets and sea-ice field has had a profound influence on global sea-level history, the Earth's heat budget, atmospheric circulation, surface and deep-water circulation, and the evolution of Antarctic biota.

2. The Southern Ocean is one of the primary sites of intermediate, deep, and bottom water formation. For example, almost two-thirds of the ocean floor is bathed by AABW that mainly originates in the Weddell Sea region. AABW depresses the temperature of at least 55%-60% of the world´s ocean volume to below 2°C (Gordon, 1988). In addition, the Southern Ocean represents the "junction box" of deep-water circulation where mixing occurs among water masses from other ocean basins (Fig. 3). As such, the Southern Ocean is perhaps the only region where the relative mixing ratios of deep-water masses can be monitored (e.g., fluxes of NADW production). As one of the primary sites of deep and intermediate-water mass formation, the geochemical and climatic fingerprint of Southern Ocean processes is transmitted throughout the world's deep oceans.

3. The Antarctic continent is thermally and biogeographically isolated from the subtropics by the Antarctic Circumpolar Current, a circum-global ring of cold water that contains complex frontal features and upwelling/downwelling cells. The zonal temperature, sea-ice distribution, and nutrient structure within the ACC control the biogenic sedimentary provinces that are characteristic of the Southern Ocean. Upwelling of deep, nutrient-rich water in the Southern Ocean results in significant primary productivity and constitutes nearly one-third of total marine productivity (Berger, 1989). As a result, about two-thirds of the silica supplied annually to the ocean is removed as hard parts of planktonic siliceous microorganisms in the Southern Ocean. This leads to high accumulation rates of biogenic opal and the formation of a circum Antarctic biogenic silica belt, located between the Polar Frontal Zone (PFZ) and the northern seasonally sea-ice covered Antarctic Zone of the ACC (e.g., DeMaster, 1981; Lisitzin, 1985). Surface waters in the circum-Antarctic are also important globally because upwelling of deep water and sea-ice formation link the thermal and gas composition of the ocean's interior with the atmosphere through air-sea exchange. As a result, most paleogeochemical models of atmospheric CO2 are highly sensitive to changes in nutrient utilization and/or alkalinity of Antarctic surface waters.

Another deficiency in the distribution of ocean-drilled cores is the lack of Quaternary, Neogene, and older sequences from the southern high latitudes that would permit the generation of high resolution stratigraphic and paleoenvironmental signals. Compared to the superb records now available from the high-latitude North Atlantic Ocean (ODP Legs 94, 154, 162, and 172), the Southern Ocean has relatively few sites that are suitable for high-resolution paleoclimatic studies. One of the fundamental tasks in paleoclimatology today is documenting and explaining phase relationships between climatic proxies from different oceanographic regions. Cores with high sedimentation rates are needed to study the timing and response of the southern high latitudes to orbital forcing and the phase relationships to climatic changes in the high-latitude Northern Hemisphere (Imbrie et al., 1989, 1992). Targeting ultra-high-resolution sequences is also important to study rapid climate change on suborbital (millennial) time scales, and to understand the nature of abrupt triggering and feedback processes in the ocean/sea-ice/atmosphere system.

Previous ODP drilling in the Southern Ocean has resulted in only a few sites that recovered Pleistocene and Neogene sections with high-sedimentation rates (Site 704, Meteor Rise; Site 594, Chatham Rise; Site 695, east of South Orkney; Site 514, Subantarctic southwest Atlantic). Sediment drifts and the region of the circum-Antarctic biogenic silica belt in the South Atlantic are ideal targets for the recovery of sediments deposited at high and ultra-high-sedimentation rates. On Leg 177, we expect to recover expanded late Neogene sections with good biocalcareous and biosiliceous preservation at several sites (SubSAT-1, TSO-6A, and TSO-7C) between 41° and 53°S. Piston cores at these sites indicate sedimentation rates in excess of 20 cm/k.y. In addition, the high flux of organic materials to the seafloor in the circum-Antarctic biogenic silica belt may lead to the formation of laminated biosiliceous sediments, which develop when oxygen is depleted periodically in sediment pore waters. Biosiliceous sediments deposited above the carbonate compensation depth (CCD) in the biogenic silica belt also generally contain sufficient calcareous microfossils (foraminifers) for stable isotopic analysis. Together with quantitative reconstructions of paleoenvironmental conditions using statistical methods developed recently for diatoms (Pichon et al., 1987; Zielinski, 1993; Zielinski and Gersonde, 1997) and radiolarians (Brathauer, 1996; A. Abelmann, pers. comm.), these records can be used to reconstruct surface water hydrography, nutrients, and productivity. In addition, sea-ice distribution can be deciphered using diatom taxa that are indicative of sea-ice (Gersonde et al., 1996; Zielinski and Gersonde, 1997).

1. To augment the biostratigraphic, biogeographic, paleoceanographic, and paleoclimatic history of the Southern Ocean during the Cenozoic, including the evolution and stability of the Antarctic cryosphere; and

2. To construct high and ultra-high-resolution records during the Quaternary and late Neogene to better understand the role of the Southern Ocean in climate change on orbital and suborbital time scales.

• Evolutionary history and stability of the Antarctic cryosphere. The Leg 177 transect will permit reconstruction of latitudinal isotopic gradients and analysis of biogeographic distribution and abundance patterns of microfossil assemblages that should lead to improved understanding of the growth and stability of the Antarctic ice sheets and help address existing discrepancies between land-based and marine records.

• Thermal isolation of Southern Ocean surface waters by development of the ACC and its associated frontal systems. Thermal isolation of the Antarctic continent was intimately linked to tectonic and paleoceanographic changes that led to the establishment of a zonal circulation system, the ACC. Knowledge of the timing and strength of thermal isolation is important for understanding polar heat transport and its effect on the development and stability of the Antarctic ice sheets. The establishment and expansion of the ACC has also influenced intermediate-, deep-, and bottom-water formation in the Southern Ocean. Accurate reconstruction of frontal boundaries requires a latitudinal transect of sites that encompass the dynamic range of the frontal movements. Leg 177 sites will permit us to reconstruct the changes in the paleolatitudinal position of frontal boundaries, similar to studies carried out on piston cores during the late Quaternary (Prell et al., 1979; Morley, 1989; Howard and Prell, 1992).

• History and distribution of sea ice and its seasonal variation to better understand its role in the global climate system. Sea ice is a fast changing environmental parameter, which is presently characterized by strong seasonal variations. Changes in sea-ice distribution have been among the most important controls on the Southern Hemisphere climate during the late Pleistocene and affect gas and heat exchange between ocean and atmosphere, ocean circulation and the formation of water masses by the rejection of salt, atmospheric circulation and wind speeds, surface albedo, and the biological production and distribution of organisms.

• History of primary productivity in the Southern Ocean and evolution of the Antarctic biogenic silica belt. Since about 36 Ma, the Southern Ocean has acted as a major sink for biogenic opal reflecting increased surface-water productivity as a result of polar cooling and upwelling in the circum-Antarctic (Baldauf et al., 1992). Changes in Southern Ocean productivity and the expansion of the biogenic silica belt have significantly influenced the distribution of nutrients in the World Ocean and has probably played a role in atmospheric pCO2 variation (Keir, 1988).

• Southern high-latitude calcareous and siliceous biozonations. ODP Legs 113, 114, 119, and 120 provided an enormous improvement in southern high-latitude stratigraphy, but further refinement of these stratigraphies is desirable. Leg 177 will provide the opportunity to improve dating of Neogene biostratigraphic markers by correlation with orbital-tuned paleoenvironmental signals. In addition, Leg 177 sequences will permit study of evolutionary processes (patterns, modes, and timing of speciation and diversification), the development of Southern Hemisphere bioprovinces (e.g., endemism), and the response of the biota to long- and short-term environmental changes.

• Changes in the production and the mixing ratios of various deep and bottom water masses, and their role in affecting the global climate system . The Southern Ocean is unique in that its deep water (mainly Circumpolar Deep Water) is a mixture of deep-water masses from all ocean basins. As such, monitoring changes in the chemistry of Southern Ocean deep water provides an opportunity to reconstruct changes in the mean composition of the deep ocean. The Southern Ocean is perhaps the only region where fluctuations in the production rate of NADW can be monitored unambiguously (Oppo and Fairbanks, 1987; Charles and Fairbanks, 1992). The Leg 177 depth transect will be ideal for reconstructing the long-term evolution of the dominant subsurface water masses in the Southern Ocean (Fig. 2).

• Timing and response of Southern Ocean surface and deep waters to orbital forcing, including the phase relationships to climatic changes in the Northern Hemisphere. Relatively little is known about the interhemispheric phase response (lead, lag, or in-phase) between the high latitude northern and Southern Hemispheres. Based upon limited data from the Southern Ocean, Imbrie et al. (1989, 1992) suggested an early response of surface and deep waters in the Southern Ocean relative to other regional proxy data. This early response has also been observed by other studies (Charles et al., 1996; Bender et al., 1994; Sowers and Bender, 1995), implying that the Antarctic region plays a key role in the driving mechanism of glacial-to-interglacial climate change. Leg 177 sediments will provide the material needed to study the response of the Southern Ocean to orbital forcing and the phase relationships to climatic changes in other regions.

• Rapid (suborbital) climate change in the Southern Ocean by correlation and comparison of millennial signals from the Southern Hemisphere with polar ice cores and marine records. Leg 177 will recover cores with highly expanded sections at three sites (SubSAT-1, TSO-6A, TSO 7C), which will permit the study of climatic variations in the Southern Ocean at suborbital (millennial) time scales. These cores will test if abrupt climate changes occurred in the southern high latitudes similar to those documented in Greenland ice cores (Dansgaard et al., 1993) and marine records from the high-latitude North Atlantic (Bond et al., 1993; Bond and Lotti, 1995). Expanded sections will also permit study of the structure of glacial and interglacial cycles in the Southern Ocean, including the trajectories of deglacial meltwater from the Antarctic continent (Labeyrie et al., 1986). Lastly, correlation between Antarctic sediment cores and ice cores from Greenland and Antarctica will reveal the phase relationships between various variables in the atmosphere and ocean systems, and contribute to identifying the mechanisms responsible for rapid climate change.

Successful recovery at the primary sites will provide the following:

• Two deep holes (700 to 800 m) at Sites TSO-2B and TSO-6A that will complete a Cenozoic latitudinal transect between 35° and 72°S when combined with ODP Site 704 (Meteor Rise, and other Leg 114 sites), Sites 689 and 690 (Leg 113, Maud Rise), and Site 360 (South African Continental Rise).

• A depth transect on the Agulhas Ridge around 43°S that includes Sites TSO-2B (2104 m), TSO 3C (3718 m), SubSAT-1C (4620 m) and one site on the Meteor Rise around 47°S when Site TSO-5B (4418 M) is combined with Site 704 (2532 m).

• Complete composite sections of the late Neogene with moderate sedimentation rates by triple APC penetration to 200 m at Sites TSO-3 (3 cm/k.y.) and TSO-5B (8 cm/k.y.).

• One ultra-high resolution transect between 41° and 53°S (SubSAT-1C, TSO-6A, and TSO-7C) recovered by triple APC/XCB.

Downhole logging will augment and complement the results of coring. Only holes deeper than 400 m will be logged using standard tools: Quad combo, geochemical (GLT), GHMT, and FMS.

The primary objective at Site TSO-2B is to recover a Cenozoic carbonate record that can be used to reconstruct long-term changes in:

• surface water parameters, and the evolution of the Subtropical Front (STF) and its response to southern-high latitude climate variability;
  
• paleoproductivity north of the PFZ;
  
• the mixing ratio between lower upper CPDW and upper NADW properties and the evolution of these water masses during the Cenozoic; and
  
• the paleodepth history of the Agulhas Ridge.

• the response of the Southern Ocean to orbital forcing and the phase relationships to climatic events occurring in the high-latitude Northern Hemisphere;
  
• rapid climate change on suborbital time scales in the Southern Ocean and its relation to climate signals from ice cores from Greenland and Antarctica;
  
• glacial-to-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; and
  
• glacial-to-interglacial variations in Southern Ocean productivity, nutrient cycling, and pCO2 and their role in global biogeochemical cycles.

• past migrations in the position of the PFZ;
  
• changes in the mixing ratios of lower NADW and CPDW in the Southern Ocean and its relation to high-latitude climate change;
  
• response of the Southern Ocean to orbital forcing and the phase relationships (leads and lags) to climatic changes in the high-latitude Northern Hemisphere; and
  
• stability of the Antarctic cryosphere during "warmer-than-present" climate during the Pliocene prior to the initiation of Northern Hemisphere glaciation.

• surface water parameters and the evolution of the PFZ;
  
• paleoproductivity changes in the present PFZ region; and
  
• lower CPDW and AABW properties and their response to high-latitude climate and the changes in flux of NADW during glacial and interglacial cycles.

• surface water parameters and the evolution of the Polar Front;
  
• sea-ice distribution in the Southern Ocean;
  
• paleoproductivity changes (e.g., silica, carbon export rates) and the history of the circum Antarctic biogenic silica belt in relation to surface water mass changes, deep water circulation, and sea-ice distribution;
  
• deep water circulation including changes in the physical and chemical properties of CPDW; and
  
• silica diagenesis (see also Site TSO-7C).

• surface water parameters south of the present PFZ;
  
• sea-ice distribution in the Southern Ocean-
  
• paleoproductivity changes (e.g., silica, carbon export rates) in relation to surface water mass changes and sea-ice distribution;
  
• deep-water circulation including changes in the physical and chemical properties of CPDW; and
  
• early low-temperature silica diagenesis.

Site SubSAT-3B
Proposed Site SubSAT-3B is located on the northern Meteor Rise in 2008 m water depth, approximately 34 nmi to the northwest of Site 704. During ODP Leg 114, a 500-m-thick Neogene sequence was recovered from Site 704, but significant core disturbance (e.g., missing and double cored sections) is evident in Holes 704A and B during some intervals, and the core breaks occur at almost the same depth in the two holes making it impossible to retrieve missing or disturbed sections in the companion hole. Site SubSAT-3B is expected to provide an identical record to Site 704 in terms of age, sedimentation rate, and lithology, but will be far superior in terms of coring disturbance, gaps, and turbidites. Sediments at Site SubSAT-3B should be composed of alternating calcareous and siliceous ooze during the Pleistocene, grading into dominantly calcareous ooze with a significant biosiliceous component during the Pliocene. Average sedimentation rates are expected to be 3 cm per tens of years during the Brunhes Chron, increasing to an average of 7 cm per tens of years during the Matuyama Chron, and decreasing again to 2 cm per tens of years during the Gauss and Gilbert Chrons. Site SubSAT-3B is approved to be cored by triple APC in the upper 200 m. Because Hole 704B drilled 672 m of sediment to the early Oligocene, recovery beyond the depth of APC penetration (>200 m) at Site SubSAT-3B is not warranted. Site SubSAT-3B should penetrate to the early Pliocene (~4.0 Ma) according to the section recovered at Site 704. Specific objectives at Site SubSAT-3B include the recovery of a continuous composite section of the Plio-Pleistocene approximately 2.5°N of the present-day position of the PFZ in 2008 m of water.

Site SubSAT-4B
Site SubSAT-4B is located to the south of the Davis Seamounts in a region where high sedimentation rates occur, and is south of the present-day position of the Polar Front of the ACC in a water depth of 3661 m (Fig. 1). This site is located at 52°S in between Sites TSO-6A and TSO-7C and serves as a good backup to either of these sites in the event that weather or ice bergs prevent us from occupying either of them. Site SubSAT-4B is approved to be drilled using triple APC to 200 m, XCB to 400 m, and RCB to 800 m to recover late Cenozoic biosiliceous and calcareous sediments at high resolution south of the present day position of the PFZ. The objectives are essentially the same as those outlined for Sites TSO-6A and TSO-7C.

SubSAT-4C
Site SubSAT-4C has been selected as an alternate to Site SubSAT-4B in the unlikely event that an iceberg prevents occupation of the primary site.

Site TSO-6B
Site TSO-6B is an alternate to Site TSO-6A in the unlikely event that an iceberg prevents occupation of the primary site. The objectives and drilling strategy at TSO-6B are identical to those described for TSO-6A.

Site TSO-7B
Site TSO-7B is an alternate to Site TSO-7C in the unlikely event that an iceberg prevents occupation of the primary site. The objectives and drilling strategy at TSO-7B are identical to those described for TSO-7C.

To Leg 177 Proposed Site Information

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