Undoubtedly, one of the most exciting results of Leg 177 was the successful recovery of expanded sequences arrayed across the ACC from 41° to 53°S (Fig. 11). Average sedimentation rates during the Pleistocene varied from 132 m/m.y. at Site 1089, to 140 m/m.y. at Site 1094, 145 m/m.y. at Site 1091, and 250 m/m.y. at Site 1093. Detailed sampling and measurements of proxy variables in these cores will permit us to reconstruct changes in paleotracers and lithology on time scales of hundreds to thousands of years. For example, we intend to use isotopic, faunal, and sedimentologic methods to reconstruct changes in the position of the oceanic frontal systems of the ACC, and diatom sea-ice indicators to assess changes in sea-ice distribution during glacial-to-interglacial cycles of the Pliocene-Pleistocene interval. Foraminifer, diatom, and radiolarian transfer functions as well as UK37' temperature estimations will be used to reconstruct variations in past SST. Accumulation rates of carbonate, opal, and organic matter, as well as stable-isotope studies, radiotracer studies, and microfossil distribution patterns will be used to study variation in productivity and export production of the Southern Ocean. Faunal, isotopic, and trace-element studies of benthic-foraminifer and clay-mineral distribution will be used to study changes in deep-water masses, including the variable input of NADW into the Southern Ocean during glacial-to-interglacial cycles. Variations in coarse-grained IRD, magnetic properties, sediment particle size and geochemistry, and clay mineralogy will be used to study variations in the accumulation rate, source, and transport (aeolian, ice rafted, or bottom water) of terrigenous material. These studies will produce multi-proxy data sets for the reconstruction of the interglacial and glacial modes of Southern Ocean surface and deep circulation. They will also provide insight into the impact of Southern Ocean paleoceanographic variability on global ocean biochemical cycles and atmospheric gas concentrations (CO2), as well as on past current-velocity rates, wind fields, and the stability of the Antarctic ice sheets.
The high temporal resolution of Leg 177 sediments will permit detailed correlation of paleotracer signals with those from other rapidly deposited sediment cores from the North Atlantic (Legs 162 and 172) and with ice-core records from Greenland, Antarctica, and tropical glaciers. Leg 177 sediments will be used to study the origin of millennial-scale climate variability that was first recognized in ice cores on Greenland (Daansgard et al., 1993) but now appears to be manifested globally (Broecker, 1997). A particularly interesting time period recovered in Leg 177 sediments is MIS 11 (423 to 363 ka), which is marked in all sites by white, carbonate-rich sediments that display the highest values in color reflectance (Fig. 16). MIS 11 was one of the warmest periods of the late Pleistocene, and the Polar Front may have been further south than during succeeding interglacials (Howard, 1997). The transition from MIS 12 to 11 (Termination V) represents the largest change in oxygen isotopic values during the late Pleistocene, yet insolation forcing at 65°N was very weak during this termination ("Stage 11 problem" of Imbrie et al., 1993). What role did the Southern Ocean play in Termination V? Leg 177 scientists will take a multi-proxy approach to addressing this question by generating detailed stable-isotope, geochemical, faunal, and sedimentological paleotracers in the transect of high sedimentation-rate cores across the ACC. Another time period of great interest is MIS 5, which was recovered in Leg 177 cores with a total thickness of up to 15 m. This includes up to ~3 m of sediment in several cores representing Substage 5.5 (Eemian), which show significant variations in sedimentary physical properties that are tentatively interpreted to represent environmental change. Detailed studies can elucidate the stability of climate conditions during the last climatic optimum and other interglacial stages, including the Holocene, which remains a controversial issue in the light of results from the Greenland Ice-core Project (GRIP, 1993).
Studying the response of the Southern Ocean to orbital forcing and determining the phase relationships to climatic changes in other regions is important for assessing the role that the Antarctic region played in glacial-to- interglacial cycles of the late Pleistocene. Only the combination of marine, terrestrial, and atmospheric paleoclimatic records from key areas on our globe will elucidate the mechanisms driving global climate. As such, the expanded sequences recovered during Leg 177 provide much needed deep-sea records from the southern high latitudes for such global comparisons.
Important questions that now can be addressed with the Pleistocene sequences recovered during Leg 177 include the following: Is there evidence for millennial-scale variability in SST and sea ice in the Southern Ocean? If so, how does it relate to short-term climatic events recorded in Antarctic and Greenland ice cores? What role does Antarctic sea ice play in internal feedback mechanisms driving rapid climate change? Sea ice represents a fast-changing environmental parameter with multiple impacts on Earth's heat budget, oceanic and atmospheric circulation, and export productivity. Did pulse-like surges occur in the Antarctic Ice Sheet during the late Pleistocene, and is there a record of these events preserved in the Southern Ocean sediments, similar to the Heinrich events preserved in the North Atlantic? What was the nature and structure of terminations in the Southern Hemisphere during the late Pleistocene? What role did thermohaline circulation (NADW flux to the Southern Ocean) play in coupled ice-sheet and ocean oscillations on millennial and longer time scales? To what extent do processes in the Southern Ocean control atmospheric CO2 variations? What is the phase relationship between millennial-scale climate change in the high-latitude Southern and Northern Hemispheres, and what is the mechanism linking climate in the polar regions? Could the paleoclimatic record of the southern high latitudes represent a potential forecast for future millennial-to-centennial climate change during the Holocene (Howard and Prell, 1992; Labeyrie et al., 1996)?
At about 900 ka, a shift occurred in the dominant power of climatic variability from 41 to 100 k.y., the so-called Mid-Pleistocene Revolution (MPR; Berger and Jansen, 1994). The MPR has not been well studied from the Southern Ocean because sediments are disturbed in the only existing record of this event at ODP Site 704 (Hodell and Venz, 1992). Interestingly, between 0.7 and 1.6 Ma the area of the present PFZ is characterized by deposition of laminated diatom mats deposited at high sedimentation rates, as documented in Sites 1091 and 1093 (Fig. 11). What was the role of the Southern Ocean in the shift from 41-k.y. to 100-k.y. climatic variability? Was the phase relationship between the polar oceans different during the 41-k.y. world of the early Pleistocene compared to the 100-k.y. world of the late Pleistocene? How is the fast deposition of biosiliceous deposits at the transition in the late early Pleistocene linked with the MPR? To address these questions, groups of Leg 177 scientists will focus on the 100-k.y. world, 41-k.y. world, and MPR in Leg 177 sediments using multi-proxy approaches.
Although early and early late Pliocene sequences were recovered partially at only a few sites (Sites 1088, 1090, 1092, and 1093) during Leg 177, combined isotopic and microfossil distribution studies of these sediments may contribute to the debate of the extent and volume of the Antarctic ice sheet during the early-late Pliocene. There are those who assume an essentially stable, combined East and West Antarctic ice sheet since the early Pliocene (Kennett and Barker, 1990; Clapperton and Sugden, 1990), and those who envision a highly dynamic Antarctic ice sheet during the early and early-late Pliocene (Webb and Harwood, 1991; Hambrey and Barrett, 1993). Shipboard diatom studies on Leg 177 sequences indicate changes in surface-water parameters during the early-late Pliocene transition. Although sequences assigned to the upper Gauss Chron contain assemblages reflecting rather glacial-type conditions, the lower Gauss Chron sequences are characterized by warm-water diatoms, such as the Hemidiscus ooze found in Site 1091. This preliminary result may suggest that the mid-Pliocene was punctuated by a time period of significant warming, as suggested by Dowsett et al. (1996). Pliocene sediments will also be scanned for traces of the Eltanin asteroid impact that occurred ~2.15 Ma in the Southern Ocean (Bellingshausen Sea) to constrain the maximum size of the bolide that is now estimated to have been at least 1 km in diameter (Gersonde et al., 1997).
Two upper Miocene sequences were recovered at Sites 1088 and 1090, forming a latitudinal transect across the Southern Ocean in conjunction with Leg 113 Sites 689 and 690 (Maud Rise). At both sites, the late and middle late Miocene (~5.3-9 Ma) is marked by low sedimentation rates (~15-17 m/m.y.) and the early late Miocene by higher rates (almost double). Similar upper Miocene sequences were recorded during Legs 113, 114, and 119 (Gersonde et al., 1990; Ciesielski, Kristoffersen, et al., 1988; Barron et al., 1991). The high early late Miocene sedimentation rates can be related to a distinct cooling period accompanied with a significant drop in sea level (Fig. 19), succeeded by several less intense warming and cooling periods in the middle and late late Miocene (Barron et al., 1991). Combined isotope and microfossil analysis will focus on late Miocene climate evolution on this latitudinal transect, and may elucidate the waxing and waning of the Antarctic ice sheets during this interval. Evidence of cyclicity in the Milankovitch frequency band at Site 1092 may permit the development of an astronomically tuned time scale (Shackleton and Crowhurst, 1997) that will provide a detailed chronology of upper Miocene changes in surface- and deep-water circulation.
The Cenozoic objectives of Leg 177 will be addressed mainly at Site 1090. This site contains a lower Miocene to middle Eocene sequence that is remarkable for several reasons: (1) a verifiably complete spliced section was constructed using three holes spanning in age from the early Oligocene to early Miocene; (2) the co-occurrence of well-preserved calcareous and siliceous microfossils throughout most of the section will allow intercalibration of foraminifer, calcareous nannofossil, diatom, silicoflagellate, and radiolarian biostratigraphies; (3) the paleomagnetic inclination records indicate clearly defined polarity zones throughout the sequence, offering the potential of a magnetic time scale after correlation of the reversal pattern to the geomagnetic polarity time scale with the aid of detailed shore-based biostratigraphy; (4) the development of geomagnetic paleointensity and/or reversal records may provide long distance stratigraphic correlation; (5) cyclic variations in lithologic parameters may permit the development of an astronomically tuned time scale for the Oligocene to early Miocene; and (6) the shallow burial depth (<370 mbsf) of the section offers an opportunity to produce reliable stable-isotope stratigraphies that have not been compromised by diagenetic alteration.
Approximately 330 m of sediment was recovered below the hiatus at 70 mcd at Site 1090, ranging in age from the early Miocene to middle Eocene. Sedimentation rates averaged 10 m/m.y. in the early Miocene and the middle Eocene, and increased to 30 m/m.y. during the deposition of opal-rich sediments in the late Eocene that include intervals of well-laminated diatom ooze. The spliced Oligocene-early Miocene section at Site 1090 complements the records obtained during Leg 154 (Sites 925, 926, 928, and 929), and comparisons among these records can be used to test orbitally tuned time scales (Weedon et al., 1997), study Milankovitch scale cyclicity of paleotracers during the late Paleogene-early Neogene (Zachos et al., 1997), and calibrate biostratigraphic datums to the geomagnetic polarity time scale. Furthermore, Site 1090 will be used to study major paleoceanographic changes in the Southern Ocean during the middle Eocene to early Miocene. Combined with the results from Paleogene sections recovered on Maud Rise during Leg 113 (Kennett and Barker, 1990), Site 1090 provides a unique opportunity to study major paleoceanographic changes in the Southern Ocean from the middle Eocene to early Miocene (Fig. 19). This will include the development and intensification of Southern Ocean thermal isolation and the ACC, the related growth of the East Antarctic Ice Sheet (Zachos et al., 1992), and their relation to the changing paleogeography of the high-latitude Southern Hemisphere (Lawver et al., 1992).
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