Here we synthesize Leg 177 results by placing our findings in the historical context of Southern Ocean paleoceanographic evolution. For readability, we omitted references and refer the reader to the detailed discussion and citations in the body of the paper.
The development of the ACC during the Paleogene was a key event in the evolution of the Southern Ocean because it permitted interocean transport among ocean basins and made possible a global thermohaline circulation cell. Its development was closely tied to the opening of tectonic gateways south of Australia (Tasmanian Seaway) and South America (Drake Passage). The timing is only loosely constrained because of complex tectonics in both regions and may be easier to determine by studying Southern Ocean sediments. During Leg 177, we cored a 330-m-thick section at Site 1090 that ranges in age from the early Miocene to middle Eocene and possesses an excellent geomagnetic polarity reversal stratigraphy. This unique section represents a new Southern Ocean archive of Cenozoic climate change and has led to several important findings regarding the early history of the Southern Ocean:
The opening of the Drake Passage to at least shallow-water flow is inferred at ~33 Ma from increased supply of detrital matter with affinities to oceanic crust derived from rifting west of Site 1090. This timing is near synchronous with the opening of the Tasmanian Seaway between Australia and Antarctica, suggesting circum-Antarctic flow of surface waters by ~33 Ma in the early Oligocene.
A continuous Paleogene isotope record could not be obtained at Site 1090 because of the low abundance and poor preservation of foraminifers in parts of the section; however, a high-resolution record was produced for the upper Oligocene to lower Miocene section (25-16 Ma) and tied to the geomagnetic polarity timescale. Comparison of carbon isotope gradients between the North Atlantic, Southern Ocean (Site 1090), and Pacific Ocean reveals a general lack of a 
13C gradient among ocean basins during the majority of the latest Oligocene and early Miocene. In contrast, a strong oxygen isotopic gradient existed between the Southern Ocean (Site 1090) and other ocean basins prior to 17 Ma, indicating that the deep Southern Ocean was colder and/or more saline than the deep North Atlantic or the Pacific.
Prior to Leg 177, Site 704 was one of the few sites available for high-resolution Neogene paleoceanographic studies in the Southern Ocean. Site 1092 was drilled on Meteor Rise to improve and complement the existing record at Site 704, located only 34 nmi to the southeast. An enigmatic observation from Site 704 was that benthic 18O values were greater than those at deep Pacific Site 849 between 3.5 and 2.7 Ma. This observation is suspicious because it requires that deep Pacific waters (at a depth of 3850 m) were warmer or less saline than those at a depth of ~2000-2500 m in the subantarctic South Atlantic. Benthic 18O results from Site 1092 confirm the Site 704 measurements and demonstrate that Southern Ocean 18O values were indeed higher than those in the Pacific throughout much of the Pliocene. Changes in the flux and/or salinity of North Atlantic Deep Water do not offer a complete explanation of this observation, and other mechanisms must be sought to reconcile the Pliocene isotope records from the Southern Ocean and eastern Pacific.
The first identifiable IRD above background levels at Site 1092 is present in the late Pliocene at ~3.18 Ma, and IRD peaks become progressively larger thereafter, reaching a maximum at 2.8 Ma. This pattern is similar to the one noted at nearby Site 704 and sites from the high-latitude Northern Hemisphere, suggesting a possible link to the expansion of Northern Hemisphere ice sheets. The latest Gauss Chron (~2.8-2.5 Ma) was a time of pronounced change in the subantarctic region that included surface waters cooling, a northward shift of the Polar Front, and establishment of the modern circum-Antarctic opal belt.
Pliocene-Pleistocene changes in deepwater circulation have been inferred using benthic 13C at Site 1090 (3702 m) and Site 1092 (1974 m). Southern Ocean benthic 13C values decreased in two pronounced steps relative to records in the North Atlantic and Pacific. Prior to 2.75 Ma, Southern Ocean benthic 13C oscillated between those of the North Atlantic and Pacific. At ~2.75 Ma, Southern Ocean 13C values decreased abruptly, indicating a reduction of deepwater ventilation at the same time as expansion of Northern Hemisphere ice sheets. At ~1.55 Ma, benthic 13C values dropped below those in the Pacific during glacial periods, thereby establishing a pattern that persisted throughout the late Pleistocene. The onset of "lower-than-Pacific" 13C values at Site 1090 may have been related to expansion of the Antarctic sea ice field during glacial periods after 1.55 Ma, which led to a reduction in the ventilation of Southern Component Water.
The carbon isotope signal of middepth Site 1088 (2082 m) evolved differently from deeper sites in the South Atlantic sector of the Southern Ocean during the Pleistocene. At no time during the Pleistocene were benthic 13C values at Site 1088 lower than those in the deep Pacific. Reconstruction of vertical 13C gradients supports the existence of a sharp chemocline between 2100 and 2700 m during glacial periods of the last 1.1 Ma, which separated nutrient-depleted middepth waters above 2100 m from poorly ventilated deepwater masses below. The vertical carbon isotope gradient between middepth and deep water parallels variations in atmospheric pCO2 for the last 400 k.y., lending support to changes in Southern Ocean deepwater ventilation as a mechanism of CO2 change.
Foraminiferal transfer functions at Site 1090 indicate that relatively cool SSSTs prevailed at 43°S during the early Pleistocene (1.8-0.9 Ma) as isotherms in the South Atlantic shifted north by ~7°. To the south, the early Pleistocene is marked by high rates of opal accumulation on Meteor Rise (Site 1092) and thick deposits of diatom mats often containing a near-monospecific diatom assemblage of Thalassiothrix sp. At both Sites 1091 (47°S) and 1093 (50°S), the most significant diatom mat sediment was deposited in the late early and mid-Pleistocene. This early Pleistocene diatom maximum between 46° and 50°S in the South Atlantic may bear an inverse relationship to the latest Pliocene (early Matuyama Chron) diatom maximum recorded in Leg 175 sediments off Namibia. The two regions may be linked by production of thermocline water via subduction in the subantarctic region and subsequent upwelling off southwest Africa.
Obtaining a Southern Ocean record of the mid-Pleistocene climate transition was an important objective of Leg 177, which was met at Site 1090. An increase in the 100-k.y. cycle is first observed in power spectra of planktonic foraminiferal SSST, benthic 18O, and sediment composition at ~1.2 Ma and then intensifies greatly after 0.9 Ma. Clay mineral assemblages at Site 1090 indicate a shift toward more arid conditions in southern Africa at 0.9 Ma. Across the MPT, no change is observed in the pacing or amount of IRD delivered to Site 1094 (53°S). The power of the 100-k.y. cycle increases progressively in the 13C gradient between middepth and deep waters in the South Atlantic during the mid-Pleistocene and is consistent with possible CO2 forcing of this climate transition.
Faunal and isotopic studies provide strong evidence for a mid-Brunhes climate transition in the subantarctic South Atlantic. Beginning with MIS 11, interglacial periods were marked by warmer temperatures and possibly less ice volume than those before 420 k.a. MIS 11 stands out in Leg 177 sediment as the brightest, most carbonate-rich period of the Pleistocene. Diatom-based estimates of SSST indicate that values during the MIS 11 thermal maximum did not exceed those obtained during other interglacials of the past 450 k.y., including the climatic optima of MIS 5 or MIS 1 from the Antarctic Zone. However, the duration of warm conditions was distinctly longer for MIS 11 than any other interglacial, and MIS 11 was also the longest period devoid of IRD in the late Pleistocene.
One unexpected postcruise result was the late Pleistocene carbonate record at Site 1089, which is unlike any other Leg 177 site because it displays a Pacific-type carbonate stratigraphy (i.e., high carbonate glacials and low carbonate interglacials). The Site 1089 carbonate record is controlled by dissolution, and the signal is nearly identical to cores from the Indo-Pacific Ocean. The fidelity of this record is unmatched by other cores, because it is free from many of the complications that limit other records (low sedimentation rates, blurring by chemical erosion, bioturbation, etc.). As such, the Site 1089 carbonate record serves a qualitative, high-resolution proxy of the temporal evolution of the carbonate saturation state of the deep sea.
Expanded late Pleistocene sequences at Sites 1089, 1091, 1093, and 1094 have been used to study the role of the Southern Ocean in glacial-to-interglacial climate change. Several studies have focused on the timing and structure of Southern Ocean changes during the last several deglaciations. Minimum planktonic 18O values and increases in SSST estimated by transfer functions lead the minimum in benthic 18O by several thousand years at glacial terminations, confirming the "lead" of Southern Ocean temperature with respect to global ice volume. At the highest latitude Site 1094, sea ice is the first parameter to change at Termination I followed by nutrient proxies and SSST. Several terminations are punctuated by significant cold reversals, similar to the Antarctic cold reversal at the last termination, that may be triggered by short-term meltwater discharges from Antarctica.
Sites 1089 and 1094 have been correlated to Greenland and Antarctic ice cores, which permits analysis of the phase relationships between oceanographic and atmospheric variables at glacial terminations. For the last four major deglaciations, changes in the temperature and chemistry of the deep Southern Ocean were synchronous with changes in polar air temperatures and atmospheric CO2 recorded in the Vostok ice core. This relationship supports a physical mechanism for glacial-to-interglacial pCO2 variations and is consistent with recent models that emphasize the role of sea ice and deep ocean ventilation in controlling atmospheric pCO2.
Much effort has been devoted to studying millennial-scale variability in the high-latitude North Atlantic, and Leg 177 sediments now allow us to extend these studies to the South Atlantic sector of the Southern Ocean. At Site 1089, radiolarian-based estimates of SSST indicate very large millennial-scale changes in SSST during MIS 3 and MIS 4 that are almost as great as those observed at Terminations I and II. Millennial-scale variability in the delivery of IRD, consisting mostly of quartz and ash, were found to coincide across the Polar Front from Sites 1089 (41°S) to Site 1094 (53°S) during MIS 3. South Atlantic IRD peaks are associated with times of warming (interstadials) and increased NADW production in the North Atlantic, suggesting an antiphase relationship between these regions. The linking mechanism may have been sea level rise associated with melting of the Laurentide Ice Sheet during strong interstadial events that unpinned grounded Antarctic ice shelves, releasing armadas of icebergs to the South Atlantic.
Millennial-scale variability in SSST and IRD was not limited to the last glacial period. At Site 1094, sub-Milankovitch variability in IRD occurred during glacial periods for the past 1 m.y., with a pacing similar to that of the last glacial period. Over the last four climate cycles, the early part of each interglacial period was marked by low IRD abundance during hypsithermal conditions, followed by the cooling and the resumption of IRD delivery during the onset of neoglaciations.
