MST and color reflectance data (650-750 nm) collected from Holes 1091A-1091E were used to determine depth offsets in the composite section. Magnetic susceptibility, gamma-ray attenuation (GRA) bulk density, and color reflectance measurements were the primary parameters used for core-to-core correlation at Site 1091. GRA and magnetic susceptibility data were collected at 2- to 4-cm intervals on cores recovered from Holes 1091A-1091E. Color reflectance data were collected at 4- to 6-cm intervals on cores from Holes 1091A, 1091B, 1091D, and 1091E (see "Physical Properties").
The composite data show that the cores from Site 1091 provide a nearly continuous overlap to 234 mcd (base of Core 177-1091A-23H). The data used to construct the composite section and determine core overlaps are presented on a composite depth scale in Figures F11, F12, and F13. The depth offsets for cores from Holes 1091A-1091E are given in Table T3 (also in ASCII format in the TABLES directory).
Stretching and compression of sedimentary features in aligned cores indicate distortion of the cored sequence. Because much of the distortion occurred within individual cores on depth scales of <9 m it was not possible to align every feature in the MST and color reflectance records accurately by simply adding a constant to the mbsf core depth. Postcruise processing will be required to align sedimentary features within individual cores. Only after allowing variable adjustments of peaks within each core can a more precise estimate of core gaps be made.
Following construction of the composite depth section for Site 1091, a single spliced record was assembled for the aligned cores over the upper 234 mcd, primarily by using cores from Holes 1091A, 1091B, and 1091D. The composite depths were aligned so that splice tie points between adjacent holes occurred at exactly the same depths in mcd. Intervals having significant disturbance or distortion were avoided if possible. The Site 1091 splice (Table T4, also in ASCII format in the TABLES directory) can be used as a sampling guide to recover a single sedimentary sequence between 0 and 234 mcd. Spliced records of magnetic susceptibility, GRA bulk density, and color reflectance (Oregon State University Split Core Analysis Track [OSU-SCAT] data only, see "Lithostratigraphy" in the "Explanatory Notes" chapter) are shown in Figure F14.
One known gap exists between the base of Core 177-1091B-1H and the top of Core 2H. Correlation of MST data sets between Site 1091 and site-survey cores TTN057-10 and 11 suggests only about 0.5 m of section is missing across this gap. A second gap in the composite section may exist between the base of Core 177-1091A-8H and the top of Core 177-1091B-9H. None of the MST or color reflectance data sets could unambiguously verify the existence of an overlap.
At Site 1091, a relatively large number of splice tie points (about one-third) could not be based on prominent features identified in more than one of the MST and/or color reflectance data sets. In some cases, the splice tie points between cores could only be constructed using the known core-section depth of laterally correlative lithologic features such as biogenic turbidites, color banding, and diatom mats. We have identified these problematic splice tie points in Table T4.
Sediments recovered from Site 1091 provide a nearly continuous record of the Pleistocene-Pliocene interval. Calcareous nannofossil assemblages are characterized by common to rare abundances and medium to poor preservation. Several barren intervals mark the Pliocene-Pleistocene record and nannofossil events are, therefore, poorly defined. The biozones of Martini (1971) and Okada and Bukry (1980), as well as some additional events defined by Raffi et al. (1993) and Wei (1993) (see "Biostratigraphy" in the "Explanatory Notes" chapter), were recognized within the Pleistocene interval. Figure F15, and Tables T5 and T6 (both also in ASCII format in the TABLES directory), summarize the main calcareous nannofossil biostratigraphic results.
The Pleistocene interval is represented from 0 to ~260 mcd (Fig. F15). The first occurrence (FO) of medium Gephyrocapsa (4-5.5 µm) approximates the Pliocene/Pleistocene boundary between 231.6 and 233.9 mcd (Fig. F15). The FO of Emiliania huxleyi is found from 26.34 to 27.58 mcd (base of Zone NN21). The last occurrence (LO) of Pseudoemiliania lacunosa is placed between 57.44 and 60.41 mcd, defining the base of Zone NN20. Reticulofenestra asanoi is recognized in Hole 1091A from 123.48 to 147.60 mcd, although specimens of this species are recorded in scattered samples to a depth of 45.72 mcd and are interpreted as reworked (Table T5). The reentrance of medium Gephyrocapsa is present between 127.62 and 132.37 mcd. Large Gephyrocapsa (>5.5 µm) are observed from 158.06 to 188.83 mcd in Site 1091. The LO of Calcidiscus macintyrei is not identified in Hole 1091A because this species is very rare or absent (Tables T5, T6).
The presence of Pseudoemiliania lacunosa (FO within Zones NN14/NN15 according to Rio et al., 1990) as well as the absence of other typical cosmopolitan and relatively dissolution-resistant lower Pliocene species, such as Reticulofenestra pseudoumbilicus (LO at the top of Zone NN15) (Table T5), allow us to assign the lower part of Hole 1091A (below 260.0 mcd) to the late Pliocene. This is in agreement with other biostratigraphic information (Fig. F15). However, characteristic dissolution-resistant and relatively warm-water upper Pliocene markers (e.g., Discoaster brouweri and Discoaster pentaradiatus) are absent in the analyzed samples and prevent a more accurate zonal assignment.
At Site 1091, the abundance of planktic foraminifers varies considerably between the studied core-catcher (CC) samples (Table T7, also in ASCII format in the TABLES directory). All samples were either dry-sieved at 150 µm (Samples 177-1091A-1H-CC, 9-14 cm [6.84 mbsf], through 12H-CC, 0-10 cm [109.25 mbsf]) or wet-sieved at 63 and 150 µm (starting with Sample 177-1091A-13H-CC, 6-16 cm [121.08 mbsf]). Although, in many cases, the abundance of planktic foraminifers is low in the examined CC samples, the number of specimens present (almost always Neogloboquadrina pachyderma [sinistral]) is high enough for stable isotopic analyses in most cases. It should be noted, however, that the abundance estimates given in Table T7 are based on a sample volume of ~20 cm3. The preservation of planktic foraminifers at Site 1091 is good to moderate and fragmentation is low. The low abundance of planktic foraminifers recorded in many of the studied CC samples seems mainly to be the result of dilution by siliceous microfossils or low productivity rather than dissolution of carbonate. Alternatively, dissolution might have reached the state where fragments start to dissolve. Even in samples with extremely few planktic foraminifers, however, there are no signs of corrosion on the foraminifer tests. Hence, it is likely that the relatively high sedimentation rate at this site has kept dissolution of calcium carbonate to a minimum.
Generally, N. pachyderma (sinistral) dominates the planktic foraminifer assemblages and this species is present in all foraminifer-bearing CC samples. Additional species present in the >150-µm fraction are Globigerina quinqueloba, Globigerina bulloides, Globigerinita glutinata, Globigerinita uvula, Globorotalia inflata, Globorotalia puncticulata, Globorotalia puncticuloides, Globorotalia truncatulinoides, and N. pachyderma (dextral).
At Site 1091, benthic foraminifers are generally not very abundant and vary considerably in their state of preservation. Problems were encountered with picking benthic foraminifers from the highly abundant, needle-shaped remains of the diatom genus Thalassiothrix in the >63-µm fraction. Under the given time constraints it was necessary to wet-sieve sediment samples at >150 µm after Sample 177-1091A-12H-CC had been processed. The change in sieve size is clearly apparent in Table T8 (also in ASCII format in the TABLES directory). The general absence of small tests of Alabaminella weddellensis and marked increase in large, robust Melonis pompiliodes below 109.25 mbsf in Hole 1091A are clearly an artifact of this change in sample preparation. A cursory examination of the 63- to 150-µm fraction residues confirms the presence of A. weddellensis in most samples.
Although highly variable, benthic foraminifers typically constitute between 5% and 10% of the total foraminifer fauna from the >150-µm fraction studied. Absolute foraminifer abundances are variable and low, reaching a maximum of 21 specimens/cm3 in Sample 177-1091B-1H-CC, 9-14 cm. Low benthic foraminifer abundances may be explained by the relatively high sedimentation rates (see "Stratigraphic Summary"). Several barren intervals (5 out of 62 samples) suggest that a continuous benthic foraminifer isotopic record from this site will be difficult to obtain. However, Cibicidoides spp. and Oridorsalis umbonatus are recorded in 33 of the 62 samples examined and, given sediment volumes >20 cm3, it may be possible to generate a combined benthic stable isotopic record from these taxa at Site 1091.
Quantitative estimates of relative species abundance were made from Holes 1091A and 1091B, with counts of up to 315 specimens per sample. Species richness is variable, with a maximum of 29 taxa recorded in Sample 177-1091A-27H-CC, 11-16 cm, and a minimum of 1 taxon recorded in a number of samples (Table T8). Not all of this variability can be accounted for by sample size (see "Biostratigraphy" in the "Explanatory Notes" chapter), but may be a function of generally poor preservation and selective loss in certain intervals. However, low sample abundance and poor foraminifer preservation are not well correlated. Closer sampling intervals and well-constrained age-depth models on millennial time scales will be required to determine whether or not benthic foraminifer production, dilution, or preservation best explains this variability.
The most common benthic taxa recorded at Site 1091 include A. weddellensis, Cibicidoides aff. wuellerstorfi, Eggerella bradyi, M. pompiliodes, O. umbonatus, Pullenia bulloides, Pullenia quinqueloba, and Pullenia subcarinata. The assemblages present are mostly dominated by infaunal taxa and presumably responded to the high primary productivity in the surface waters above this site.
In addition to the CC samples obtained from all holes, we have examined smear slides from sections in Hole 1091A (Table T9, also in ASCII format in the TABLES directory). All diatom stratigraphic information from the four holes was combined and converted to the mcd scale (Tables T6, T10, both also in ASCII format in the TABLES directory). Diatoms are abundant in almost all the samples studied above ~270 mcd at Site 1091. In the lowermost part of Site 1091, characterized by low sedimentation rates below 270 mcd, diatom abundance varies between few and abundant. The preservation of diatom assemblages is generally moderate or good (Fig. F16; Table T9). During examination of the diatom assemblages, we also encountered silicoflagellates in trace numbers, as well as sporadic Actiniscus specimens and sponge spicules (Table T9).
The Thalassiosira lentiginosa Subzone b, which ranges from the top of marine isotope Stage (MIS) 7 to the base of MIS 11, is placed between 21.0 and 49.50 mcd (Fig. F15). This indicates that peak values in color reflectance (650-700 nm) recorded at 26.5, 38, and 49 mcd (Fig. F14) are associated with the climatic optima of MISs 7, 9, and 11, respectively. This interpretation is supported by calcareous nannofossil stratigraphic events, such as the FO of Emiliania huxleyi at 27 mcd and the LO of Pseudoemiliania lacunosa at 58.9 mcd that occur in MISs 8 and 12, respectively (Table T10). The top of the Actinocyclus ingens Zone, marked by the LO of A. ingens, can be placed at ~80.7 mcd. Below this, the A. ingens Subzone b ranges between 147.9 and 205.8 mcd. Assemblages assigned to the upper Pliocene P. barboi Zone, which underlies the A. ingens Zone and corresponds with the Olduvai Subchron of the Matuyama Chron, were found between 262 and 277 mcd. Below this interval, the Thalassiosira kolbei-Fragilariopsis matuyamae Zone, which ranges between 2 and 2.5 Ma, is recognized. In this interval, the age-depth model indicates a drop in sedimentation rates to ~30 m/m.y. (Table T10). The relatively short T. kolbei-F. matuyamae Zone at Site 1091 may indicate the presence of one or more short hiatuses. The Thalassiosira vulnifica Zone, which ranges between 2.5 and 2.6 Ma, has been identified between 294.8 and 312.1 mcd. The range of the T. vulnifica Zone straddles the boundary between the Matuyama and Gauss Chrons at 2.58 Ma. This is consistent with the interpretation of the magnetostratigraphic data obtained at this site (see "Paleomagnetism," Fig. F15). This interval is underlain by only a few meters (312.1-316.4 mcd) containing assemblages assigned to the upper Pliocene Thalassiosira insigna Zone, which ranges between 2.6 and 3.3 Ma. This assignment is based on the co-occurrence of Fragilariopsis weaveri, T. vulnifica, and T. insigna. According to Harwood and Mayurama (1992), the LO of F. weaveri can be placed at ~2.7 Ma and thus falls in the upper portion of the T. insigna Zone. On the basis of this stratigraphic sequence of the marker taxa in the T. insigna Zone, we suggest that the upper portion of this zone is not represented at Site 1091 because of a hiatus (Fig. F15; Table T10). The base of Site 1091, only recovered in Hole 1091A, is assigned to the Fragilariopsis interfrigidaria Zone, which ranges between 3.26 and 3.8 Ma. We note the co-occurrence of the nominate taxon and F. weaveri, which has its FO in the uppermost part of the F. interfrigidaria Zone (Harwood and Maruyama, 1992).
The abundance distribution of diatoms and specific diatom taxa can be used to interpret the general paleoenvironmental evolution at Site 1091 since the early late Pliocene. The transition from diatom-rich sediments, deposited at high sedimentation rates, to sediments with lower diatom contents, deposited at lower sedimentation rates, may indicate a drastic change in opal export productivity rates in the latest Pliocene, at ~2 Ma. This change is possibly related to the establishment of water-mass distributions and oceanic frontal systems that allowed enhanced biosiliceous export rates. Mass deposition of taxa such as Fragilariopsis kerguelensis led to high opal sedimentation in the late and mid-Pleistocene, whereas Actinocyclus ingens and Thalassiothrix longissima were prominent contributors of high opal accumulation rates during the early Pleistocene (Fig. F16). This led to the formation of so-called diatom mats (see "Lithostratigraphy"). A similar distinct acme of A. ingens was also found between ~165 and 225 mcd in the A. ingens Subzones b and a. An A. ingens acme at about the same time as that observed at Site 1091 has been described by Gersonde and Bárcena (1998) from piston cores recovered in the Subantarctic area of the Atlantic sector of the Southern Ocean. The great abundance of the genus Hemidiscus within the F. interfrigidaria Zone (Core 177-1091A-33H) suggests warm surface waters during the mid-Pliocene.
Radiolarian biostratigraphy at Site 1091 is based on the examination of 36 CC samples (Table T11, also in ASCII format in the TABLES directory). All samples yielded well preserved, abundant Pleistocene to late Pliocene radiolarians. The radiolarian assemblages recovered are mostly of an Antarctic origin, and all existing Antarctic Pliocene to Pleistocene radiolarian zones from the Upsilon to the Omega Zone (Lazarus, 1992) are found at this site.
In Hole 1091A, the boundary of the Omega and Psi Zones (0.46 Ma) is recognized between Samples 177-1091A-3H-CC, 12-17 cm (25.4 mbsf, 27.63 mcd), and 5H-CC, 9-15 cm (44.09 mbsf, 46.46 mcd). The boundary of the Psi and Chi Zones (0.83 Ma) can be placed between Samples 177-1091A-14H-CC, 16-21 cm (130.54 mbsf, 139.2 mcd), and 16H-CC, 11-16 cm (148.34 mbsf, 162.25 mcd). However, the LO of Pterocanium trilobum, which defines the top of the Chi Zone, is found in Sample 177-1091B-9H-CC, 11-16 cm (82.88 mbsf, 83.6 mcd) (Table T11). This depth corresponds to the middle part of the Psi Zone in Hole 1091A, suggesting reworking of P. trilobum in Hole 1091B. Consequently, the boundary of the Psi and Chi Zones is placed at ~120 mcd between Samples 177-1091A-12H-CC, 0-10 cm (109.25 mbsf, 116.51 mcd), and 177-1091B-13H-CC, 10-15 cm (122.09 mbsf, 124.20 mcd) (Table T10). Likewise, the boundary of the Chi and Phi Zones (1.92 Ma) is recognized between Samples 177-1091B-28H-CC, 20-25 cm (255.60 mbsf, 275.43 mcd), and 29H-CC, 11-16 cm (268.87 mbsf, 285.04 mcd), although the latter sample contains reworked specimens of Helotholus vema, which defines the underlying Upsilon Zone. Finally, the boundary of the Phi and Upsilon Zones (2.42 Ma) is placed between Samples 177-1091A-28H-CC, 22-27 cm (263.22 mbsf, 285.69 mcd), and 30H-CC, 10-15 cm (281.43 mbsf, 303.90 mcd). The first appearance datum (FAD) of Cycladophora davisiana at 2.58 Ma is recognized in the Upsilon Zone between Samples 177-1091A-30H-CC, 10-15 cm (281.43 mbsf, 303.90 mcd), and 32H-CC, 30-35 cm (299.17 mbsf, 321.64 mcd) (Table T10; Fig. F15).
Boundaries of all the aforementioned zones are defined by the last appearance datum of marker species rather than the more reliable FAD. In addition, abundances of marker species, especially Stylatrctus universus and Pterocanium trilobum, are rare to few in the assemblages examined. Additional work will, therefore, be required to determine the precise zonal boundaries at this site.
Archive halves of APC cores recovered at Site 1091 were measured using the shipboard pass-through magnetometer. Measurements were made at 5-cm intervals. Sections obviously affected by drilling disturbance were not measured. Core 177-1091A-1H was measured after alternating-field (AF) demagnetization at peak fields of 0 (natural remanent magnetization [NRM]), 5, 10, 15, and 20 mT. Cores 177-1091A-2H through 10H were measured after peak fields of 0, 10, 20, and 25 mT. Core 177-1091A-11H was measured after peak fields of 0, 10, 20, 25, 30, and 35 mT. Cores 177-1091A-12H through 33H and 177-1091B-1H through 2H were measured after peak fields of 0, 10, 20, 25, and 30 mT. Cores 177-1091B-3H through 29H were measured after peak fields of 0, 10, 20, and 30 mT, and Cores 177-1090E-1H through 14H were measured after peak fields of 0, 20, and 30 mT.
NRM intensities are about 1 × 10-2 A/m at the top of each hole, decrease through the upper 20 mbsf to ~1 × 10-3 A/m, and remain fairly uniform thereafter for most of the cored interval. In Hole 1091A, NRM intensities >1 × 10-2 A/m were found below 250 mbsf. After AF demagnetization at peak fields of 25 to 30 mT, intensities generally decreased to ~3.5 × 10-4 A/m above 250 mbsf and to ~3 × 10-3 A/m below. NRM inclinations are typically steep down as a result of a magnetic overprint, probably largely attributable to the drill string. The drill-string remagnetization was largely removed at peak demagnetization fields in excess of 10 mT; the resulting inclination values, however, are highly scattered (Figs. F15, F17; Table T10). Magnetization directions attributed to the Matuyama Chron are particularly inconsistent, probably as a result of normal polarity magnetic overprints associated with (1) drilling-related core deformation and (2) magnetite dissolution and growth of iron sulfides in a reducing diagenetic environment.
The Brunhes/Matuyama boundary can be identified in the 95.50- to 102.40-mbsf interval of Hole 1091A. The Matuyama/Gauss boundary is tentatively identified in the 285.80- to 288.70-mbsf interval of Hole 1091A (Fig. F17). No other polarity transitions can be identified with any confidence.
A 310.9-m-thick (332.87 mcd) sedimentary section was recovered at Site 1091. Holes 1091A-1091E were cored with the APC to 310.9, 273.8, 4, 203.1, and 51.7 mbsf, respectively. The combined MST and color reflectance data provide a nearly continuous section to 234 mcd (base of Core 177-1091A-23H) (Figs. F10, F11, F12, F13). One known gap exists (~0.5 m) between the base of Core 177-1091B-1H and the top of Core 2H. A second gap in the composite section may exist between the base of Core 177-1091A-8H and the top of Core 177-1091B-9H. At Site 1091, a relatively large number of splice tie points could not be defined on the basis of prominent features identified in more than one of the MST and/or color reflectance data signals.
The age of the recovered section at Site 1091 is Holocene to late Pliocene, displaying high sedimentation rates in the Pleistocene interval (~145 m/m.y.; Fig. F18). Although the lithology at Site 1091 is dominated by siliceous microfossils (diatoms and radiolaria), the sediments contain enough calcareous nannofossils for the establishment of a combined biosiliceous/calcareous biostratigraphy, particularly in the thick Pleistocene interval. Unfortunately, the shipboard pass-through magnetometer measurements indicate strong disturbances and normal polarity overprints of the magnetization record, probably related to core deformation, magnetite dissolution, and post-sedimentary growth of iron sulfides. This results in highly scattered inclination values that only allow the identification of the Brunhes/Matuyama and Matuyama/Gauss boundaries (Table T10; Fig. F17). Marine isotope stages can be identified for the last 700 k.y. by using a combination of physical properties variations that reflect glacial-interglacial variability in carbonate and opal content, the abundance pattern of the diatom Hemidicus karstenii, and radiolarian and calcareous nannofossil biostratigraphic events. This preliminary interpretation must be confirmed by postcruise stable isotopic measurements of planktic and benthic foraminifers that occur in sufficient numbers throughout the Site 1091 record.
All biostratigraphic datums, magnetostratigraphic results, and interpretation of physical properties records yield consistent age assignments throughout the record at Site 1091. The climatic optima of MISs 7, 9, and 11 have been identified at 26.5, 38, and 49 mcd, respectively (Fig. F14). This indicates average sedimentation rates of ~120 m/m.y. for the last 0.42 m.y. The average sedimentation rates during the Pleistocene, which has its base at ~261 mcd, were ~145 m/m.y. This is consistent with the common occurrence, especially in the early Pleistocene, of so-called diatom mats (see "Lithostratigraphy") that mainly consist of Actinocyclus ingens or diatoms belonging to the Thalassiothrix antarctica-longissima group. A distinct drop in sedimentation rates is seen below ~285 mcd in upper Pliocene sediments assigned to the T. kolbei/F. matuyamae diatom Zone and the Phi radiolarian Zone. The decrease in diatom abundance may indicate that at least part of this drastic drop in sedimentation rates is related to a change in opal export rates, probably caused by changes in hydrographic conditions that controlled biosiliceous production. However, it cannot be ruled out that the presence of the relatively thin sedimentary section in the lowermost Matuyama Chron might also be caused by one or more hiatuses in this interval. A distinct change in sedimentation rates at ~2 Ma was also reported for Site 704, drilled on Meteor Rise east of Site 1091. Hodell and Venz (1992) proposed that the increase in sedimentation rates between the late Pliocene and Pleistocene was related to a northward expansion of nutrient-rich Antarctic water masses.
The absence of diatom assemblages representing the upper portion of the Thalassiosira insigna Zone suggests a possible hiatus at ~311 mcd. This zone is correlated to the upper portion of the Gauss Chron (see "Biostratigraphy" in the "Explanatory Notes" chapter) and the hiatus may span a time interval ranging at least from 2.6 to 2.7 Ma. The oldest sediments recovered at Site 1091, belonging to the upper Fragilariopsis interfrigidaria diatom Zone and the Upsilon radiolarian Zone, are somewhat older than 3.3 Ma. Thus, the normal polarity magnetic interval observed at the base of Hole 1091A may represent part of C2An.3n in the Gauss Chron.