A total of 4046 m of sediment ranging in age from middle Eocene to Holocene was recovered during Leg 177. To the extent possible, composite records were constructed at each site from cores in multiple holes by aligning features in the signals of core-logging data (Table T1). We were successful in achieving the following drilling objectives.
Leg 177 sediments are dominated by calcareous and siliceous biogenic components comprising foraminifers, nannofossils, diatoms and subordinate radiolarians, silicoflagellates, and sponge spicules (Fig. F18). Almost pure calcareous sediments were recovered at Sites 1088 and 1092, which were situated well above the regional CCD in intermediate water depths on the Agulhas Ridge (2082 m) and the Meteor Rise (1974 m). South of the Subantarctic Front, diatom-rich sediments predominate at Sites 1091 (4363 m), 1093 (3626 m), and 1094 (2807 m), within the circum-Antarctic opal belt. At all sites, the terrigenous sediment fraction mainly consists of siliciclastic silt and clay. At the southern sites (1091-1094), sand- to gravel-sized ice-rafted debris (IRD) represents a minor but ubiquitous constituent of the sediments.
Pelagic calcareous sediments at Site 1088 (Agulhas Ridge) consist of Quaternary foraminifer nannofossil ooze that grades into nannofossil ooze in Pliocene to middle Miocene sediments. Biogenic opal and siliciclastics represent minor components. Downhole lithologic changes were associated with an increase in clay-sized particles in the terrigenous fraction.
Site 1089 (4620 m) is located on a drift deposit in the southern Cape Basin close to the northern flank of the Agulhas Ridge. Quaternary to Pliocene calcareous sediments contain the highest concentrations (as much as 50 wt%) of terrigenous silt and clay encountered at Leg 177 sites, which will permit the study of relative current strengths of paleobottom water from grain-size parameters of the terrigenous silt fraction (Diekmann and Kuhn, 1997). Lithologic alternations between mud- and carbonate-rich sediments probably reflect sedimentary cycles attributed to glacial-interglacial periods that triggered oscillations in carbonate production and/or terrigenous sediment supply.
At Site 1090 (3702 m) on the southern flank of the Agulhas Ridge, we recovered a 400-m-thick sedimentary succession that yields a long-term record of lithologic and paleoenvironmental change from the Quater-nary to the middle Eocene, interrupted by several hiatuses. A hiatus at 70 mcd, marked by a color change to redder sediments below, separates a Pleistocene to lower Pliocene calcareous ooze from lower Miocene sediments that are more opal and mud rich. The hiatus is underlain by a redeposited tephra layer. The older sediments contain high opal concentrations (as much as 50 wt%) in the late Eocene. The lower part of the section is composed of zeolite-bearing calcareous ooze of middle Eocene age.
In addition to Site 1088, Site 1092 on Meteor Rise provides a low--sedimentation-rate record of pelagic calcareous deposits, spanning the Pleistocene to middle Miocene. The Quaternary part of the section reveals distinct variations in opal and siliciclastics, probably associated with glacial-interglacial cycles.
Sites 1091 and 1093 yielded diatom ooze deposited with high sedimentation rates from the Holocene to the Pliocene. They represent typical pelagic deposits of the southern abyssal portion of the southeast Atlantic (Fig. F9). Distinct carbonate-rich intervals indicate peak interglacial periods, which were frequent in the late Pleistocene to Pliocene. At Site 1093, the Pliocene part of the section accumulated at lower sedimentation rates and is marked by an increase in terrigenous silt and clay. Several millimeter-thick marker beds, consisting of sand-sized foraminifer ooze, are intercalated in the section of Site 1091 and probably represent turbidites.
Rapidly deposited diatom ooze of Pleistocene age was also obtained at Site 1094, which was drilled in 2807-m water depth in a small sedimentary basin north of Bouvet Island. In contrast to Sites 1091 and 1093, only a few carbonate-bearing intervals were found. Downhole lithologic variations are marked by pronounced changes in the abundance of siliciclastics, as illustrated by fluctuations in magnetic susceptibility that show peak values in glacial intervals.
Four porcellanite horizons were penetrated at Site 1094 (Fig. F15) that form discrete layers as documented in Parasound seismograms. Porcellanite mainly is present as brownish amorphous fragments derived from the crushing of concrete porcellanite layers during coring. It also occurs as individual loaf-shaped concretions that are as much as 6 cm in diameter and exhibit internal bedding structures that indicate an early diagenetic growth within the host sediment. Fragments of porcellanite washed downhole during drilling were also found in Site 1093 cores. Shipboard X-ray diffraction measurements indicate an opal-CT composition of the porcellanites. Joint investigations among Leg 177 scientists on the geochemical and mineralogical properties of porcellanite, interstitial water, and host-sediment composition in the context of regional heat flow and spatial distribution patterns of porcellanite layers will provide information about the conditions under which these young porcellanites were formed.
Sediments from Sites 1091, 1093, and 1094 contain scattered IRD throughout and should provide a high-temporal resolution record of past ice-rafting activity in response to Antarctic ice-sheet dynamics. IRD mainly consists of volcaniclastic particles along with minor quartz and crystalline rock fragments.
A significant proportion of the sediment at southern Sites 1091, 1093, and 1094, consists of mats of the needle-shaped diatom Thalassiothrix, which proved difficult to recover with the APC or XCB coring systems (Fig. F15). The mats occur as intervals of laminated sediment as much as 20 m thick (Fig. F16), as intermittently laminated sediment, or as bioturbated mat fragments or burrow-fills of mat material. This mat sediment is common in the transitions to and from interglacial, carbonate-rich sediment resulting in expanded sections of glacial terminations (e.g., the 8-m-thick MIS 12/11 boundary at Site 1093; see Frontispiece 2, p. ii). At the two southernmost sites (1093 and 1094), diatom mats were recovered in upper and mid-Pleistocene sediment. At Site 1091, located in the PFZ, the youngest diatom mats were noted at the lower/mid-Pleistocene boundary (Fig. F15). At both Sites 1091 and 1093, the most significant diatom-mat sediment was deposited in the late early and mid-Pleistocene. Diatom mats also occur in the mid-Pliocene section. The Leg 177 Thalassiothrix diatom-mat deposits are remarkably similar to the vast Neogene laminated diatom-mat deposits of the eastern equatorial Pacific Ocean (Kemp and Baldauf, 1993). Such deposits are thought to form beneath intense frontal zones (Kemp et al., 1995) and, in the Leg 177 sites, the Thalassiothrix mat intervals may track the paleoposition of the Southern Ocean frontal systems. These laminated sequences also represent a paleosediment trap that preserves individual flux events and provides the potential to generate pelagic records of climate/ocean change at key time intervals at a resolution that rivals that of ice cores.
Primary age control points were provided by calcareous nannofossil, diatom, and radiolarian biostratigraphy, integrated in some sites with magnetostratigraphy. The calcareous nannofossil assemblages show a clear difference between the northern and southern sites, with an important decrease in diversity to the south. Datums previously calibrated in middle- and low-latitude areas were used for the Pleistocene time interval. A more accurate age model will provide the possibility to recalibrate these events and estimate their synchronism or diachronism. The Pliocene-Eocene time interval offers the opportunity to generate a new biostratigraphic scheme for the Southern Ocean, as well as to correlate these events with low-latitude zonations. Furthermore, calcareous nannofossil assemblages are marked by cyclic abundance variations at all sites and ages, offering a potential tool for paleoceanographic investigations.
Paleoceanographic reconstructions using foraminifer-based stable isotopic results will be possible for most sites. Although the absolute abundance of both planktic and benthic foraminifers is low in many cases, particularly at the southernmost deep-water sites, the high sedimentation rates in these areas have clearly increased the preservation of foraminifers. Radiolarian assemblages sharply change along the north-south transect, which makes it possible to elucidate temporal and spatial distributions of radiolarian assemblages from mid- to high-latitude regions. In addition, abundant radiolarians from high-resolution sites may allow us to obtain detailed paleoceanographic information such as biogenic opal productivity and sea-surface temperature (SST) changes. A detailed late Pliocene to Pleistocene biostratigraphic diatom zonation developed for subantarctic waters was successfully applied throughout most of the north-south transect (Gersonde and Bárcena, 1998). Diatom analyses of Leg 177 sediment provide a great potential to improve the diatom biostratigraphic zonation for the Southern Ocean. In particular, the diatom record of the Miocene-Eocene time interval, when correlated to a nearly continuous paleomagnetic record, will provide a detailed biostrati-graphic zonation for the Paleogene. Recovered material from the two southernmost sites located within the circum-Antarctic opal belt will allow reconstructions of paleoenvironmental parameters such as SST (by means of diatom transfer functions) and sea-ice occurrence (by means of diagnostic diatom taxa).
All Leg 177 sites, with the exception of Site 1088, yielded magnetic polarity reversal stratigraphies to augment other chronostratigraphic information. Of the sites with high sedimentation rates, the primary magnetization was most clearly recorded at Sites 1089 and 1094. At the two other sites (1091 and 1093) with expanded sections, the magnetization is affected by secondary components that were not entirely removed by shipboard demagnetization treatments. Nonetheless, all four high-resolution sites along the north-south transect have high potential for detailed (U-channel) studies of directional and bulk magnetic properties (Fig. F15). The objectives of these studies will be to: (1) generate the first geomagnetic paleointensity records from the Southern Ocean for long-distance stratigraphic correlation; (2) generate proxies for magnetic grain size and mineralogy that can be used for paleoenvironmental interpretation, and monitor detrital fluxes; and (3) obtain detailed polarity transition records from the high-latitude Southern Hemisphere.
Sites 1090 and 1092, which are marked by lower sedimentation rates, both yielded well-defined magnetic stratigraphies, although the upper part of the section at both sites was severely compromised by drilling-related core deformation. Below ~60 mcd at both sites, the shipboard magnetic polarity stratigraphies are well defined; however, correlation of polarity zones to the geomagnetic polarity time scale is ambiguous in the absence of detailed biostratigraphic analyses. Even in the XCB section of Site 1090, magnetic stratigraphies were well defined mainly because of the exceptional quality (lack of drilling deformation) of these cores. The middle Miocene to early Pliocene magnetostratigraphic record at Site 1092 and the exceptional Eocene to early Miocene record at Site 1090 will provide important new biomagnetostratigraphic correlations, and may allow orbital tuning of this part of the time scale.
Closely spaced measurements of sedimentary physical properties were obtained from all cores recovered during Leg 177, using the standard ODP whole-round MST. The Oregon State University Split Core Analysis Track (OSU-SCAT) was deployed for diffuse color reflectance and resistivity measurements. Downhole logging data were obtained from Hole 1093D.
Measuring the cored sediments every 2 to 4 cm with the MST provi-ded us with the highest temporal resolution data set collected during Leg 177. Physical properties are a function of sediment composition, structure, and porosity. Moreover, they are a tool for hole-to-hole correlations and comparisons among sites. Glacial-interglacial fluctuations in sediment composition were observed in GRA bulk density. High bulk-density values are consistent in sediments at the northern sites with overall high carbonate contents, particularly in interglacial intervals. High percentages of biogenic opal (high porosity) result in a decrease of sediment bulk density in interglacial sediments at southern Sites 1093 and 1094. The opposite is observed in sediments deposited during glacial stages that are marked by higher percentages of terrigenous material. At Site 1094, magnetic susceptibility and NGR show high signal amplitudes, but with different character. The shape of the magnetic susceptibility signal is rectangular, whereas NGR displays an asymmetric, sawtooth pattern with highest intensities toward the end of glacial periods (Fig. F17). This indicates that both signals contain different information regarding terrigenous sediment components. The signals are strongly cyclic in the Pleistocene sequences at Sites 1089, 1091, 1093, and 1094, and also in the continuous early Miocene to late Eocene sequence at Site 1090. These cyclic variations in lithologic parameters may permit the development of orbitally tuned age models in conjunction with biomagneto- and stable-isotopic stratigraphies.
Diffuse spectral reflectance measurements obtained with the OSU-SCAT and the Minolta CM-2002 spectrophotometers contributed greatly to the overall success of the leg, providing a high-resolution proxy for lithostratigraphic records in real time. These data provided important stratigraphic constraints for hole-to-hole correlation during the generation of shipboard spliced composite sections. At Sites 1088, 1089, and 1092, interglacial carbonate-bearing sediments were easily discernible from darker, diatom-rich glacial sediments. The spectral reflectance signals were especially important for correlation of the biosiliceous oozes at Sites 1091, 1093, and 1094, where magnetic susceptibility signals dropped below measurable values in interglacial sediments. Records of reflectance also proved extremely useful as geochronologic tools during Leg 177. In conjunction with biostratigraphic and magnetostratigraphic datums, preliminary estimates of MISs were inferred on the basis of sediment brightness (Fig. F11).
Some of the oldest sediments thus far measured for diffuse spectral reflectance were recovered in the Miocene to Eocene sequences from Sites 1088, 1090, and 1092. The continuous lower Miocene to upper Eocene sequence at Site 1090 is noteworthy for the high-amplitude OSU-SCAT signal in the APC cores that span from early Miocene to Oligocene time, as well as in the deeper XCB sequence from which the cores were measured with the CM-2002 spectrophotometer (Fig. F13). Leg 177 spectral reflectance records hold great potential for development of high--resolution age models and proxy estimation of sediment mineralogy.
The interstitial water chemistry of Leg 177 sites can be divided into two broad categories: (1) sites with a high biogenic carbonate content, low biosiliceous content, and low sedimentation rates (10-30 m/m.y.) that are located to the north of the PFZ (Sites 1088, 1090, and 1092); and (2) sites with low carbonate content, high opal content, and high sedimentation rates (140-250 m/m.y.) that are located within or to the south of the PFZ (Sites 1091, 1093, and 1094). The interstitial water geochemistry of the carbonate-rich sites is, in general, quite similar to that of many other carbonate-rich sites drilled on previous ODP and DSDP legs; that is, oxic to suboxic sediments with ample evidence for carbonate diagenesis occurring at depth. The closely spaced interstitial water sampling employed during Leg 177 will permit more detailed analyses of some interesting features observed from these carbonate-rich sites. However, the highlight of the interstitial water geochemistry obtained during Leg 177 derives from the unique (and still somewhat enigmatic) results observed at sites with sequences dominated by diatom ooze deposited at high sedimentation rates within or south of the PFZ, and also from the early diagenetic porcellanites (opal-CT) observed at Site 1094. The collection of a series of closely spaced interstitial water samples across several of the porcellanite intervals should provide important insights into the formation of these early porcellanites.
A synthesis of several interstitial water profiles from Sites 1091, 1093, and 1094 is shown in Figure F14. The chloride profiles show evidence for the downward diffusion of higher salinity glacial-age seawater (McDuff, 1985). The uppermost porcellanite layer at ~68 mbsf at Site 1094 has apparently interrupted this downward diffusion and presents the intriguing suggestion that the porcellanite may have formed in the past 10 to 20 k.y. These diatom oozes were suboxic to mildly reducing. H2S was detected by scent at Sites 1091 and 1093 throughout most of these profiles, but H2S was not detected at Site 1094 except very faintly in one whole-round near the top of the section. In addition, sulfate depletion is much less than would be expected based on sedimentation rate, and it appears to be inversely correlated with our preliminary estimates of TOC (see site chapters for data not shown here). Phosphate profiles show little correlation with alkalinity and ammonium except at Site 1094, and dissolved manganese is observed to varying degrees throughout the profiles. We offer the following preliminary interpretation of these observations. The highest sedimentation-rate site is Site 1093 (~250 m/m.y.), which is located very near the contemporaneous PF at ~50°S. Sedimentation rates at Site 1093 were least likely to be affected by glacial-interglacial migrations of the PF compared to Sites 1091 and 1094 located about 3° to the north and south, respectively. Thus, the mildly reducing conditions at Site 1093 have likely persisted through glacial-interglacial cycles as evidenced by the low downhole dissolved-Mn profile. Sites 1091 and 1094 have undergone much more drastic perturbations in average sedimentation rates (both ~140 m/m.y.) and are out of phase with each other over the glacial-interglacial climate cycles. This resulted in a periodicity in the redox state of the sediments that has permitted reactive Mn to persist at depth. The low sulfate reduction rates observed (despite the high sedimentation rates) may result from the fact that a significant, if not major, fraction of the organic carbon in these diatom-rich oozes is highly refractory opal-intrinsic organic carbon that is unavailable for degradation until the opal has dissolved. Opal-intrinsic organic carbon may have relatively low phosphate content, thus offering some explanation for the nature of the phosphate profiles observed in these diatom-rich oozes.
In summary, this preliminary and general interpretation of these first deep interstitial water profiles from the circum-Antarctic opal belt will need to be verified and enhanced with additional shore-based analyses. Additionally, shore-based analyses of closely spaced interstitial water samples across some of the porcellanite intervals observed in the sediments at Site 1094 may offer important insight into the mechanisms involved in the transformation of diatom opal to opal-CT.
Sediments recovered during Leg 177 will be used to study the paleoceanographic history of the southeast Atlantic sector of the Southern Ocean on a variety of time scales, including suborbital (102 to 103 yr, centennial to millennial), orbital (104 to 105 yr, Milankovitch), and supraorbital (105 to 106 yr, Cenozoic). Future research will focus on generating signals of faunal, isotopic, and sedimentologic paleotracers that will be used to study the role played by the Southern Ocean in the global climate system.
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. F15). 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 (Fig. F12). Detailed sampling and measurements of proxy variables in these cores will permit us to reconstruct paleoenvironmental changes on time scales of hundreds to thousands of years. We intend to use isotopic and micropaleontological 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-interglacial cycles of the Pliocene-Pleistocene interval. Foraminifer, diatom, and radiolarian transfer functions, as well as Uk'37 temperature estimations, will be used to reconstruct variations in past SST. Accumulation rates of carbonate, opal, and organic matter, as well as stable isotopic studies, radiotracer studies, and microfossil distribution patterns will be used to study variation in biological export productivity of the Southern Ocean. Micropaleontological, 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-interglacial cycles. Variations in coarse-grained IRD, magnetic properties, sediment particle size and geochemistry, and clay mineral-ogy will be used to study variations in the accumulation rate, source, and transport modes (eolian, ice rafted, or bottom water) of terrigenous material. These studies will produce multiproxy 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 biogeochemical cycles and atmospheric gas concentrations (CO2), as well as on past current velocities, 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 sediments from the North Atlantic (Legs 162 and 172) and with ice-core records from Greenland, Antarctica, and low-latitude 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 in marine and terrestrial sediments (Broecker, 1997). MIS 11 (423-363 ka) is a particularly interesting time period, and sediment of this age was recovered during Leg 177 in several sites. Sediments of MIS 11 are characterized at all sites by white, nannofossil-rich oozes that display the highest values of color reflectance (Fig. F11). MIS 11 may have been one of the warmest periods of the late Pleistocene, and the PF possibly was located farther south than during succeeding interglacials (Howard, 1997). The transition from MIS 12 to 11 (Termination V) is linked to the largest shift in oxygen isotopic values during the late Pleistocene, although 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 during Termination V? We will take a multiproxy approach to addressing this question by generating detailed stable isotopic, geochemical, micropaleontological, and sedimentological paleotracers along the north-south transect of high-sedimentation-rate sites across the ACC.
Another time period of considerable interest is MIS 5, which is represented in several Leg 177 cores with a maximum thickness of as much as ~15 m. In several cores, there is as much as 3 m of sediment representing Substage 5.5 (Eemian). Sediments of this substage show significant variations in sedimentary physical properties that are tentatively interpreted to reflect short-term environmental changes during peak interglacial periods. Detailed studies can elucidate the stability of climate conditions during the penultimate climatic optimum, which remains a controversial issue considering the results of the Greenland Ice-core Project (GRIP, 1993). Natural climate variability in the Holocene is also of great interest in light of anticipated future global warming.
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-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 biological 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 millennial-centennial climate change in the future (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 Site 704 (Hodell and Venz, 1992). Interestingly, between 0.7 and 1.6 Ma the area of the present PFZ was characterized by accumulation of laminated diatom mats deposited at high sedimentation rates, as documented in Sites 1091 and 1093 (Fig. F15). What was the role of the Southern Ocean during 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 rapid deposition of biosiliceous sediments in the Southern Ocean during 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 multiproxy approaches.
Although early and early late Pliocene sequences were recovered partially at only a few sites (1088, 1090, 1092, and 1093) during Leg 177, combined isotopic and microfossil distribution studies of these sediments may contribute to the debate on 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 (e.g., temperature) 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 at 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 examined for traces of the Eltanin asteroid impact that occurred at ~2.15 Ma in the Southern Ocean (Bellingshausen Sea) to constrain the maximum size of the bolide, which is now estimated to have been at least 1 km in diameter, by mapping the flux of impact-related ejecta (Gersonde et al., 1997).
Two upper Miocene sequences were recovered at Sites 1088 and 1090, forming a north-south 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 recovered during Legs 113, 114, and 119 (Gersonde et al., 1990; Ciesielski, Kristoffersen, et al., 1988; Barron et al., 1991b). Combined isotopic and microfossil analyses will focus on late Miocene climate evolution along this north-south 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 ranging 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 excellent chronological control 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 uncompromised stable isotopic stratigraphies.
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 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. Combined with the results from Paleogene sections recovered on Maud Rise during Leg 113 (Kennett and Barker, 1990), Site 1090 provides an opportunity to study major paleoceanographic changes in the Southern Ocean from the middle Eocene to early Miocene (Fig. F19). This time period includes the development and intensification of the ACC and the growth of the East Antarctic Ice Sheet (Barron et al., 1991a, 1991b; Zachos et al., 1992) in conjuction with the changing paleogeography of the high-latitude Southern Hemisphere (Lawver et al., 1992).