Site 1088 (proposed site TSO-2B) is located on the Agulhas Ridge in the southeast Atlantic Ocean at a water depth of 2082 m (Figs. F1, F5; Table T1). This bathymetric setting places the site near the interface between NADW and CDW (Fig. F2). The primary objective of Site 1088 was to recover a long Cenozoic carbonate sequence that could be used to study paleoceanographic change near the Subtropical Front, which today is located north of the Agulhas Ridge.
Three holes were drilled representing a combined 223.4-m section (Table T2 ,also in ASCII format in the TABLES directory), consisting predominantly of carbonate microfossils representing sediment deposition from the Holocene to middle Miocene (~13-14 Ma). The sediments recovered are predominantly nannofossil ooze, foraminifer-bearing nannofossil ooze, foraminifer nannofossil ooze, and nannofossil foraminifer ooze. Carbonate percentages vary from 85 to 95 wt% and the abundance of foraminifers decreases progressively downhole.
Sedimentation rates average 10 m/m.y. in the Pleistocene, 7 m/m.y. in the Pliocene, 17-30 m/m.y. in the late Miocene, and 11 m/m.y. in the middle Miocene section (Fig. F10 ).Only two short depth intervals (0-5.5 and 122-129 m composite depth [mcd]) were cored in more than one hole and, as a result, the continuity of the sedimentary section could not be documented and a continuous spliced record was not construc-ted. Magnetic inclinations were low and less than expected for the site location (60°) and declinations were highly scattered, suggesting drill-string remagnetization of the core.
High-resolution samples (one per section) of interstitial waters were taken in Hole 1088B, between 1.5 and 51 meters below seafloor (mbsf), for major-ion and stable isotopic analysis. Many of the results from these samples must await shore-based analyses, but shipboard analyses show that, as expected, chlorinity increases downhole, with a slight local maximum at ~40 mbsf that probably resulted from diffusion of high-salinity water associated with the last glaciation.
Variations in diffuse spectral reflectance suggest the presence of marine isotopic Stages (MISs) 1 to 13 in Core 177-1088B-1H in the top 5.5 mbsf. Although the upper Pleistocene sequence was deposited with relatively low sedimentation rates (~10 m/m.y.), the record is similar to that from Site 704 to the south (47°S, 7.5°E, 2532 m), and comparison of the two holes will be useful for studying glacial-interglacial changes in NADW flux to the Southern Ocean. Sediments recovered at Site 1088 will also be useful for studying paleoceanographic changes during the Neogene at a temporal resolution of 10 to 100 k.y. The 140-m section of upper Miocene sediments recovered in Holes 1088B and 1088C is particularly promising in that variations in magnetic susceptibility show evidence of cyclicity in the Milankovitch frequency band. Although a complete composite section was not retrieved, the sediments should provide a detailed record of late Miocene changes in surface- and deep-water circulation.
Site 1089 (proposed site SubSAT-1B) is located in the southern Cape Basin in the southeast Atlantic Ocean, close to the northern flank of the Agulhas Ridge (Figs. F1, F5; Table T1). High sedimentation rates (84-180 m/m.y.) resulted in an expanded sedimentary sequence that is ideally suited for studying environmental changes in response to climate variability on orbital and suborbital time scales. Given these high sedimentation rates, Site 1089 is comparable to the North Atlantic drift deposits drilled during Legs 162 and 172, and it will be useful for determining the response of the Southern Ocean to orbital forcing and the phase relationships to climate change in the North Atlantic region. The high sedimentation rates at Site 1089 will also permit detailed correlation between marine sediment records and ice-core records, especially the Vostok ice-core signal.
A 264.9-m-thick sedimentary section, spanning the interval from the Holocene to late Pliocene (~2.4 Ma), was recovered at Site 1089 (Table T3, also in ASCII format in the TABLES directory). The sediments are predominantly composed of diatoms, nannofossils, and terrigenous mud in varying proportions. Calcium carbonate contents in Hole 1089A average 27.0 wt% and range from 0.6 to 69.3 wt%, whereas total organic carbon (TOC) varies between 0 and 0.82 wt% with an average value of 0.43 wt%. Although Site 1089 is deep (4620 m) and near the carbonate compensation depth (CCD), foraminifer abundance is fairly constant downhole to ~220 mbsf, below which it drops to zero. It should be possible, therefore, to produce a stable isotopic stratigraphy in the upper 220 mbsf. No major lithologic boundaries are present within Site 1089, and only one lithologic unit was identified.
The upper 100 m of the section, representing approximately the Brunhes Chron (0.78 Ma to present), is nearly complete. Variations in spectral color reflectance, CaCO3, bulk density, and magnetic susceptibility permit the prediction of glacial and interglacial MISs 1 to 19, constrained by diatom and calcareous nannofossil biostratigraphy (Fig. F11). With four holes drilled to more than 118 mbsf, a continuous spliced record was constructed to a depth of 94 mcd by aligning features in the records of closely spaced physical properties measurements. Core logging data obtained at 2- to 6-cm sampling intervals show cyclic variations at Milankovitch frequencies as well as variations at higher (sub-Milankovitch) frequencies. Postcruise analysis of these signals will be useful for delineating climatic variability on these time scales. Preservation of remanent magnetization is good in the upper 100 m and preliminary results are encouraging for constructing the first detailed Southern Hemisphere record of geomagnetic paleointensity during the Brunhes Chron.
A series of 3- to 15-m-thick deformed sediment units, possibly slump or slide deposits, was cored between 95 and 156 mcd. Soft-sediment deformation is manifested in the sediment by dipping and/or contorted beds, sharp color contacts, and microfaults. The Brunhes/Matuyama boundary is present in the interval from 105 to 114 mbsf in Hole 1089B; however, the transition is not well preserved because of soft-sediment deformation that affects the interval from the lowest Brunhes Chron to the top of the Jaramillo Subchron. Even within the deformed sediment interval, however, laminations and burrow structures are preserved and suggest that the stratigraphic section is relatively intact. Postcruise analysis may allow us to piece together a composite section that eliminates several of the deformed intervals.
Polarity transitions in cores from Hole 1089B define the lower boundary of the Jaramillo Subchron at 151.6-153.6 mbsf, and the upper and lower boundaries of the Olduvai Subchron at 213.8-215.8 and 225.5-227.6 mbsf, respectively. We place the Pliocene/Pleistocene boundary at ~229 mcd at Site 1089.
Biostratigraphic and magnetic datum levels provide an age-depth relationship that indicates continuous sedimentation at Site 1089 since the late Pliocene (~2.4 Ma). Sedimentation rates average ~128 m/m.y. in the upper 94 mcd (~0.7 Ma), ~180 m/m.y. between 94 and 156 mcd (~0.65-1 Ma), ~110 m/m.y. from 156 to 230 mcd (1-1.8 Ma), and ~84 m/m.y. from 230 to 280.6 mcd (1.7-2.4 Ma) (Fig. F12 ). The high--sedimentation-rate interval from 94 to 156 mcd corresponds to the disturbed section, but the superposition of biostratigraphic datums is as expected in this interval.
Interstitial water profiles from Site 1089 indicate reducing conditions such that sulfate reduction is complete by 50 mbsf and methane concentrations are high below this depth in the hole. The Ca2+ profile shows a dramatic decrease in the sulfate reduction zone, reaching a minimum at 50 mbsf. This Ca2+ decrease results in unusually high Mg/Ca values because Mg2+ concentrations remain near seawater values.
Site 1090 (proposed site TSO-3C) is located in the central part of the Subantarctic Zone on the southern flank of the Agulhas Ridge (Figs. F1, F5; Table T1). The water depth (3702 m) places it near the boundary between NADW and underlying lower CDW (Fig. F2), and above the CCD. Together with Sites 1088 (2082 m) and 1089 (4620 m) it forms a depth transect that intersects most of the major water masses of the South Atlantic. Five holes were drilled to 397.5 mbsf, spanning the Holocene to middle Eocene (~46 Ma), and including a ~14-m.y. hiatus at ~70 mcd that includes much of the lower Pliocene to lower Miocene record (Table T4, also in ASCII format in the TABLES directory). We constructed a continuous spliced record to 212 mcd (and perhaps 245 mcd), corresponding to the early Oligocene.
Quaternary sediments, consisting of alternating foraminifer nannofossil ooze, diatom-bearing nannofossil ooze, and mud-bearing nannofossil ooze, extend to 44 mcd with sedimentation rates averaging 33 m/m.y. (Fig. F10). In Hole 1090C, the Brunhes/Matuyama boundary (0.78 Ma) lies between 18.0 and 19.2 mbsf. The top (0.99 Ma) and base (1.07 Ma) of the Jaramillo Subchron lie in the 24.6-25.4 and 27.7-28.4 mbsf intervals, respectively. Two hiatuses may be present between 0.42 and 0.64 Ma and from 1.3 to 1.8 Ma; shore-based analysis is needed for confirmation. Variations in color reflectance permit the identification of glacial and interglacial MISs 1 to 12 in the upper 18 mcd, supported by identification of biostratigraphic events. MIS 11 is particularly prominent because of its exceptionally white color and high nannofossil carbonate content. Cyclic variations in the color reflectance (Fig. F13 ) and gamma-ray attenuation (GRA) bulk density signals may reflect the shift from the 41-k.y. world to the 100-k.y. world at ~30 mcd.
The upper Pliocene sequence was deposited at sedimentation rates of 11 to 13 m/m.y. In Hole 1090C, the top (1.77 Ma) and base (1.99 Ma) of the Olduvai Subchron were recognized in the 35.3-36.0 and 37.6-38.2 mbsf intervals, respectively. Diatom biostratigraphy indicates a hiatus at ~55 mcd that spans the Matuyama/Gauss boundary from 2.5 to 2.6 Ma.
A pronounced hiatus was encountered at ~70 mcd, marked by a lithologic change from white nannofossil ooze to reddish mud-rich nannofossil ooze. Below the hiatus, a tephra sequence composed of vitric ash and admixed with hemipelagic biogenic and terrigenous particles was recovered. Sedimentary structures and textural features suggest deposition of the tephra components by turbidity currents. Sediments above the hiatus are early Pliocene in age and contain manganese nodules. Below the hiatus, approximately 330 m of sediment was recovered consisting of mud-bearing diatom ooze and mud- and diatom-bearing nannofossil ooze and chalk ranging in age from early Miocene to middle Eocene. Sedimentation rates averaged 10 m/m.y. in the early Miocene to middle Eocene, and increased to 30 m/m.y. during deposition of upper Eocene opal-rich sediments that include intervals of well-laminated diatom ooze. Middle Eocene carbonate-rich sediments were deposited at lower sedimentation rates (~10 m/m.y.) (Fig. F10).
The potential for paleomagnetic reversal stratigraphy below the hiatus is excellent, even in the cores that were recovered by XCB. Site 1090 holds much promise for detailed correlations of biostratigraphic datums to the geomagnetic polarity time scale during the early Miocene to middle Eocene.
Interstitial waters of Site 1090 can be characterized as suboxic, with sulfate reduction occurring at very low rates. Interstitial water profiles indicate a sharp break at ~290 mbsf, corresponding to an impermeable layer (presumably a chert layer recovered as fragments in the top of Core 177-1090B-32X) that posed a barrier to diffusion of interstitial waters. Interstitial water regimes above and below the diffusion barrier evolved independently because of the isolation imposed by the impermeable chert.
In summary, the importance of Site 1090 is twofold: (1) the Pleistocene to upper Pliocene section deposited at sedimentation rates of 30 m/m.y. will be useful for the reconstruction of the high-latitude Southern Hemisphere paleoclimate at moderate temporal resolution, and (2) the middle Eocene-lower Miocene section below the hiatus will potentially be an important section for biomagnetostratigraphic correlations, astronomical tuning of time scales, and paleoceanographic studies of the late Paleogene and early Miocene. Cyclic variations in lithology may permit the development of an astronomically tuned time scale for the late Paleogene-early Neogene, similar to that developed during Leg 154 (Weedon et al., 1997). The Paleogene sequence is especially significant because it spans the time period associated with the onset of Antarctic glaciations, early production of cold Antarctic surface and bottom waters, and paleogeographic changes (e.g., the separation of Australia and Antarctica and opening of the Drake Passage) that led to the establishment of the ACC (Kennett, 1977).
Site 1091 (proposed site TSO-5C) is located in the PFZ on the western flank of the Meteor Rise, ~2° north of the present-day PF (Fig. F1; Table T1). The water depth of 4363 m places the site within lower CDW (Fig. F2). The primary objective at Site 1091 was to recover a high-resolution Pliocene-Pleistocene sequence within the PFZ that could be used to study (1) the history of migration of the PF and Antarctic sea-ice field, (2) glacial-interglacial changes in biological export production, (3) millennial scale climate oscillations recorded in the Southern Ocean sediments and their relation to climate changes documented in North Atlantic sediments and polar ice cores, (4) the melting history of the Antarctic ice sheet and associated meltwater plumes during glacial-interglacial cycles of the late Pleistocene, and (5) changes in lower CDW properties in response to variable NADW flux to the Southern Ocean.
Five holes were drilled at Site 1091 to obtain a complete section that was deposited with high sedimentation rates. Hole 1091A was the deepest hole cored with the APC to a depth of 310.9 mbsf with a recovery rate of 90% (Table T5, also in ASCII format in the TABLES directory). Basal sediments are early Pliocene (~3.4 Ma) in age. The remaining four holes provided overlap to fill coring gaps in Hole 1091A and resulted in a continuous spliced section to 234 mcd (~1.7 Ma). One lithologic unit was defined consisting of diatom-rich ooze, with minor and varying amounts of nannofossils, foraminifers, and mud. Calcium carbonate contents in Hole 1091A are relatively low, ranging from 0.2 to 58.9 wt% with an average value of 5.7 wt%. Despite the low carbonate content, planktic and benthic foraminifers are sufficiently abundant in most samples for stable isotopic analysis. TOC contents vary between 0.17 and 0.91 wt% with an average value of 0.60 wt%.
Because of the proximity of Site 1091 to the PF, the underlying sediments should document past movements of this front. Extensive laminated Thalassiothrix diatom-mat deposits, analogous to those recovered in sediments from the eastern equatorial Pacific Ocean (Kemp and Baldauf, 1993), are present at several horizons. Sedimentation rates were high, averaging 140 m/m.y. during the Pleistocene (Fig. F12). Carbonate-rich interglacial periods are easily recognized by their brightness in the signal of diffuse color reflectance (Fig. F11). For example, peaks in red reflectance (650-750 nm) at 26.5, 38, and 49 mcd correlate to MISs 7, 9, and 11, respectively, which is supported by biostratigraphic information. Downhole variations of physical properties (diffuse spectral reflectance, GRA bulk density, and magnetic susceptibility) show distinct evidence of cyclicity at Milankovitch and suborbital time scales.
A transition in sedimentation occurred at ~2.0 Ma. Sediments older than ~2.0 Ma were marked by relatively low opal content and sedimentation rates, whereas younger sediments were diatom rich and accumulated at high sedimentation rates. This event represents an important change in opal export production in the Southern Ocean during the latest Pliocene and was recognized previously at Site 704 (Froelich et al., 1991a; Hodell and Venz, 1992), which is located only ~60 km to the east of Site 1091 on the crest of Meteor Rise.
Natural remanent magnetization (NRM) at Site 1091 was affected by a drill-string overprint that was largely removed at peak demagnetization fields in excess of 10 mT; however, the resulting inclination values are highly scattered especially during the Matuyama Chron. The Brunhes/Matuyama boundary can be identified in the 95.5- to 102.4-mbsf interval in Hole 1091A. The Matuyama/Gauss boundary is tentatively identified between 285.8 and 288.7 mbsf in Hole 1091A.
The redox conditions in Site 1091 sediments can be classified as reducing on the basis of dissolved H2S (by scent), but sulfate concentrations decrease only modestly from near-bottom-water concentrations of ~28 mM at 4 mbsf to ~22 mM around 300 mbsf (Fig. F14).
In summary, Site 1091 represents the midpoint (at 47°S) of a Pliocene-Pleistocene north-south transect across the ACC (Fig. F15). The high sedimentation rates during the Pleistocene at Site 1091 (~145 m/m.y.) complement the records obtained at Sites 1089 (~128 m/m.y.), 1093 (~250 m/m.y.), and 1094 (~140 m/m.y.) at 41°, 50°, and 53°S, respectively. This north-south transect of sites with high sedimentation rates will be used to reconstruct the past movement of frontal boundaries and the Antarctic sea-ice field during the Pliocene-Pleistocene, and to study the impact of climate variability on processes in the Southern Ocean on orbital and suborbital (millennial) time scales.
Site 1092 (proposed site SubSAT-3B) is located on the northern Meteor Rise in the PFZ, ~3° north of the present-day position of the PF (Fig. F1; Table T1). The water depth (1974 m) places the site above the regional carbonate lysocline and CCD and within a mixing zone between upper NADW and CDW (Fig. F2). The impetus for drilling Site 1092 came from the record obtained at Site 704, located only 34 nmi southeast of Site 1092. The goal of Site 1092 was to improve upon Site 704 by recovering a continuous upper Miocene to Pleistocene section.
Three holes were drilled by the APC to depths of 188.5 mbsf (Hole 1092A), 168.9 mbsf (Hole 1092B), and 165.5 mbsf (Hole 1092C) with high recovery in all holes (97%, 86%, and 91%, respectively; Table T6. also in ASCII format in the TABLES directory). A continuous spliced section was constructed to 188.5 mcd. The sediments consist of pale brown green to pure white nannofossil ooze with varying mixtures of diatom and foraminifer ooze and mud. One lithologic unit was defined and divided into two subunits at ~54 mcd on the basis of a change from alternating calcareous and diatom ooze above to dominantly nannofossil ooze below. Calcium carbonate is present throughout the sediment column, varying from 16.7 to 94.6 wt% with an average content of 80.2 wt%. TOC contents vary between 0 and 0.70 wt% with an average value of 0.16 wt%.
The section at Site 1092 ranges in age from the Pleistocene to the early Miocene. Sedimentation rates varied from ~10 to ~29 m/m.y. during the Pliocene-Pleistocene and between ~4 and ~38 m/m.y. during the Miocene (Fig. F10). The pattern of sedimentation-rate change at Site 1092 is similar to Site 704, except that Site 1092 rates are considerably lower. For example, the Pleistocene section at Site 1092 appears to be condensed by ~50% relative to Site 704. The pattern of sedimentation rate is also similar to that at Site 1088, including the hiatus in the middle Miocene, with the exception that the lower Pleistocene section is more expanded at Site 1092. Winnowing of sediment is documented in the upper and mid-Pleistocene sections, as indicated by well-sorted foraminifer sands and reduced sedimentation rates averaging 10 m/m.y. Several hiatuses punctuate the sediment record at Site 1092: (1) a lower Pliocene hiatus at ~65 mcd spans the interval from ~3.8 to 4.6 Ma, (2) one or more hiatuses are present across the Miocene/Pliocene boundary that was tentatively placed between 70 and 75 mcd, and (3) a hiatus at ~178 mcd spans the earliest late Miocene to the middle middle Miocene from ~11 to 13 Ma (Fig. F10).
The paleomagnetic inclination records are highly discontinuous in the upper 60 mbsf because of drilling disturbance in poorly consolidated nannofossil ooze. Below 60 mbsf, the polarity reversal stratigraphy is well resolved for the early Pliocene sequence and a particularly good upper Miocene sequence. Correlation of the polarity sequence at Site 1092 awaits detailed shore-based biostratigraphy.
The redox characteristics of Site 1092 can be characterized as generally oxic or suboxic throughout the section. The major cations (Ca, Mg, and Sr) in interstitial waters vary similarly to those from Site 704 (Froelich et al., 1991b).
Measurements of physical properties show evidence of distinct cyclicity throughout the record. In the upper 35 mcd, large-amplitude variations in color reflectance mirror alternations between Pleistocene siliceous and carbonate sediments, whereas the signal is dampened below in sediments dominated by nannofossil ooze. Lithologic cyclicity in the upper Miocene section of Site 1092 may offer the opportunity to test the new late Miocene time scale derived by Shackleton and Crowhurst (1997) from the Site 926 (Leg 154) sedimentary record.
Site 1093 (proposed site TSO-6A) is located north of Shona Ridge, near the present PF and north of the average winter sea-ice edge (Fig. F1; Table T1). The site is characterized by moderately laminated pelagic sediments deposited at high sedimentation rates (~250 m/m.y.) within the circum-Antarctic opal belt (Figs. F12, F15). The high temporal resolution of biosiliceous sediment offers an excellent opportunity for paleoceanographic studies on millennial-centennial time scales over the last 1 m.y. At Site 1093, sediment cores were recovered for the first time from the circum--Antarctic opal belt in multiple APC holes, permitting the construction of a complete composite section. At 3626-m water depth, the site is located within lower CDW (Fig. F2).
Six holes were drilled with a maximum penetration of 597.7 mbsf (Table T7, also in ASCII format in the TABLES directory). A continuous spliced section was constructed to a depth of 252 mcd, representing the early Pleistocene (~1.0 Ma) to Holocene. For Holes 1093A and 1093B, the Brunhes/Matuyama boundary (0.78 Ma) was found in the interval between 205 and 210 mcd, yielding an average sedimentation rate of ~250 m/m.y. (Figs. F12, F15). Shipboard multisensor track (MST) data (natural gamma radiation [NGR], GRA bulk density, and magnetic susceptibility) and diffuse color reflectance document lithologic variations on orbital and suborbital time scales, which can be used to interpret climatic changes during the past 1.0 m.y. (Fig. F11). Preliminary age models derived from shipboard MST results indicate that in some interglacial intervals sedimentation rates were as high as 300 to 700 m/m.y., permitting a temporal sampling resolution of 100 yr or less!
The lithology at Site 1093 consists almost exclusively of diatom ooze, including distinctive intervals of laminated diatom mats up to several meters thick (Fig. F16). Calcium carbonate contents are generally low (<15 wt%) with occasional peaks of as high as 56.9 wt%. Despite the low carbonate content, core-catcher samples indicate that a nearly continuous planktic foraminifer isotopic stratigraphy (Neogloboquadrina pachyderma sinistral) and a more-or-less continuous benthic stable isotopic record should be possible over the last 1 m.y.
Relatively carbonate-rich interglacial intervals are recognized by their brightness in the diffuse color reflectance signal (Fig. F11). On this basis, we were able to predict MISs 1 through 11 (~400 k.y.). MIS 11, at ~124 mcd, stands out as the brightest, most carbonate-rich interglacial interval of the Pleistocene section. The transition from glacial MIS 12 to interglacial MIS 11 (Termination V) occurs over an 8-m interval (from ~133 to 125 mcd) and is marked by a thick laminated interval of Thalassiothrix diatom mats. This expanded interval provides an unprecedented opportunity to study abrupt climate changes associated with Termination V.
The recovery of the deeper section below ~255 mcd, averaging only 26%, was disappointingly low in Hole 1093D. This is thought to be the result of thick intervals of laminated diatom mats that were difficult to recover using the APC and particularly the XCB coring systems. Apparently, a brief hiatus (spanning 0.2 m.y.) marks the Pliocene/Pleistocene boundary, and sedimentation rates were lower (57 m/m.y.) below this level. A hiatus spanning ~2.5 m.y. also marks the latest early Pliocene to latest late Miocene. The oldest sediment recovered at 595 mcd was latest Miocene (6.3-6.9 Ma) in age and contained a Neobrunia mirabilis diatom ooze with similar composition to that recovered at ODP Site 701 (Fig. F12).
Closely spaced (one per section) interstitial water samples were taken from cores from Hole 1093A between 0 and 63 mbsf for major-ion and stable isotopic analysis. One to three samples per core were taken at Holes 1093A and 1093D to a maximum depth of 498 mbsf. Shipboard analyses show a chlorinity increase downhole, with a well-defined Cl- maximum in the interval from 50 to 60 mbsf, probably resulting from diffusion of higher salinity water associated with the last glaciation. The chloride profile is identical (within analytical uncertainty) to its lower resolution counterpart from Site 1091 (Fig. F14). Both sites are characterized by high sedimentation rates, and the presence of diatom mats may be responsible for creating such a distinct chloride maximum. Addi-tional shore-based isotopic analyses and modeling of interstitial water profiles may permit the estimation of the oxygen isotopic composition and salinity of bottom waters at Site 1093 during the last Ice Age.
Hole 1093D was wireline-logged between 70 and 560 mbsf using the triple combination tool and the geological high-sensitivity magnetic tool. Good quality resistivity, NGR, and magnetic susceptibility data were obtained that should permit core-log integration using the MST core-logging data. The magnetic susceptibility record obtained from the Pliocene section of Site 1093 shows close similarities to the Pliocene record obtained at Site 1092, deposited at lower sedimentation rates.
In summary, the purpose of Site 1093 was to obtain an expanded record of biosiliceous sediments near the present-day position of the PF to study past changes in Antarctic surface-water properties and sea-ice extent. We succeeded in obtaining Pleistocene sediments that were deposited at the highest sedimentation rates yet recovered in any pelagic deep-sea section, affording the opportunity to study paleoceanographic processes in response to climate variability on millennial and even centennial time scales. In particular, the high temporal resolution of the sedimentary record will permit detailed correlation of the paleoceanographic history at Site 1093 with climate signals from the Greenland and Antarctic ice cores.
Site 1094 (proposed site TSO-7C) is located in a small sedimentary basin north of Bouvet Island (Fig. F1; Table T1). It was the highest latitude site drilled during Leg 177 and represents the southernmost anchor of sites drilled along a north-south transect across the ACC. The site is located in the southern part of the ice-free Antarctic Zone, but it was covered by sea ice during the last glaciation and preceding glacial intervals. The water depth of 2807 m places the site within the core of CDW (Fig. F2).
Four APC holes were drilled to depths of 159.6 mbsf (Hole 1094A), 38.0 mbsf (Hole 1094B), 73.1 mbsf (Hole 1094C), and 171.1 mbsf (Hole 1094D; Table T8, also in ASCII format in the TABLES directory). The oldest sediments recovered are Pleistocene in age (~1.4 to 1.5 Ma) and the section consists predominantly of olive gray to gray diatom ooze, with minor and varying amounts of foraminifers, nannofossils, and siliciclastic mud. A continuous spliced section was constructed to ~121 mcd, representing the last 1 m.y., with one gap at the bottom of Core 177-1094A-7H.
Lithologic variations (as expressed by lithologic compositional signals of magnetic susceptibility, GRA bulk density, color reflectance, NGR, and geochemistry) reflect in great detail the glacial and interglacial cycles of the late Pleistocene (Fig. F17). MISs 1-12 are readily identifiable in the upper 80 mcd. Glacial intervals are marked by relatively high magnetic susceptibility, high NGR, low GRA bulk density, and low color reflectance (450-550 nm). Glacial terminations are marked by abrupt decreases of magnetic susceptibility and NGR and by increases in GRA bulk density and color reflectance. Sedimentation rates averaged ~140 m/m.y. during deposition of the diatom-dominated middle to upper Pleistocene sequence above ~80 mcd (Fig. F12). Below MIS 12, sedimentation rates decreased to ~91 m/m.y. during the early mid- and early Pleistocene. The transition between the Brunhes and Matuyama Chrons is identified between 98.20 and 101.58 mcd in Hole 1094A (Fig. F15).
Fragments of porcellanite layers (opal-CT) were recovered at 68 mbsf (Holes 1094A and 1094D), 104 mbsf (Hole 1094A), and 164 mbsf (Hole 1094D), and an individual loaf-shaped porcellanite concretion was found at 141 mbsf (Hole 1094A). These porcellanite layers were also detected in Parasound sediment echosounding lines by high-amplitude reflectors. The upper porcellanite horizon occurs in the lower portion of MIS 11 and is the same as that previously described in piston cores from this area (Bohrmann et al., 1994). An anomalously low temperature gradient (~8°/km) was measured at Site 1094, indicating that the porcellanites formed under low (near bottom-water) temperatures. The upper porcellanite (at 68 mbsf) coincides with a sharp discontinuity in interstitial chloride concentrations, suggesting that the porcellanite layer may have acted as a diffusion barrier (Fig. F14). Shore-based geochemical analyses of interstitial water and solid-phase samples taken from near these porcellanite beds will be important for studying early silica diagenesis in Site 1094.
In summary, the purpose of Site 1094 was to obtain a record of biosiliceous sediments at high temporal resolution south of the present-day position of the PF. Together with Sites 1089 (41°S), 1091 (47°S), and 1093 (50°S), Site 1094 (53°S) represents the southernmost end member of the north-south transect of sites across the ACC needed to reconstruct past changes in frontal boundaries and sea-ice distribution during glacial-interglacial cycles of the Pleistocene (Fig. F15). The expanded upper and mid-Pleistocene sedimentary succession at Site 1094 will permit study of rapid climate change on suborbital time scales, including comparison with paleoclimatic signals from Antarctic and Greenland ice cores.