MST and color reflectance data (650-750 nm) collected from Holes 1093A-1093F were used to determine depth offsets in the composite section. Gamma-ray attenuation (GRA) bulk density and magnetic susceptibility data were collected at 2- to 4-cm intervals on cores recovered from Holes 1093A-1093F. Color reflectance data were collected at 4- to 6-cm intervals on cores from Holes 1093A-1093F (see "Physical Properties" for details about these MST and color reflectance data sets).
The composite data show that the cores from Site 1093 provide a continuous overlap to 252 mcd (base of Core 177-1093D-10H). The data used to construct the composite section and determine core overlaps are presented on a composite depth scale in Figures F10, F11, and F12. The depth offsets that comprise the composite section for Holes 1093A-1093F 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. Within-core scale changes will require postcruise processing to align smaller sedimentary features. Only after allowing variable adjustments of peaks within each core can an accurate estimate of core gaps be made.
Following construction of the composite depth section for Site 1093, a single spliced record was assembled for the aligned cores within the upper 252 mcd by using cores from Holes 1093A-1093D and 1093F. The composite depths were aligned so that 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 1093 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 252 mcd. Spliced records of magnetic susceptibility, GRA bulk density, and color reflectance are shown in Figure F13.
In the Site 1093 spliced record, there are eight ambiguous tie points. Ambiguities result from either insufficient overlap between adjacent cores (e.g., <1 m), lack of strong features that can be correlated across the overlapping segments of adjacent cores (e.g., diatom-mat intervals), or disagreement in magnetic susceptibility, GRA bulk density, and/or color reflectance data across the spliced interval. We have identified these ambiguous tie points with an asterisk in Table T4.
Sediments recovered from Site 1093 provide a Pliocene-Pleistocene record. Calcareous nannofossils in this interval are abundant to rare and are characterized by low diversities and by medium to poor preservation. Some nannofossil events are poorly defined at this site because several barren intervals occur in the Pliocene-Pleistocene record (Table T5, also in ASCII format in the TABLES directory). The biostratigraphic zones of Martini (1971) and Okada and Bukry (1980), as well as some additional events defined by Raffi et al. (1993) and Wei (1993) (see "Explanatory Notes" chapter), are recognized within the Pleistocene interval. Tables T5 and T6 (both also in ASCII format in the TABLES directory) and Figure F14, summarize the main calcareous nannofossil biostratigraphic results.
The Pleistocene interval is represented from 0 to ~460 mcd at Site 1093 (Fig. F14). The base of the Emiliania huxleyi acme is well identified in Hole 1093A between 11.98 and 14.98 mcd, defining the base of Subzone NN21b. The first occurrence (FO) of Emiliania huxleyi is present from 74.43 to 75.18 mcd (base of Zone NN21). The last occurrence (LO) of Pseudoemiliania lacunosa is present between 152.43 and 159.16 mcd, defining the base of Zone NN20. The LO of Reticulofenestra asanoi is recognized from 220.79 to 227.33 mcd, although rare specimens of this species are recorded in scattered samples above 220.79 mcd and are interpreted as reworked (Table T5). The reentrance of medium Gephyrocapsa (4-5.5 µm) is present between 245.54 and 246.79 mcd, although the abundance of this morphotype is few to rare in this interval. The FO of R. asanoi is identified from 262.77 to 263.96 mcd. The LO of large Gephyrocapsa (>5.5 µm) is not well defined because of its scarcity above 314.53 mcd, whereas its FO is present from 323.6 to 345.1 mcd. Calcidiscus macintyrei is very rare at Site 1093 (Table T5) and its LO is not clearly defined.
The upper Pliocene markers are absent or few; this fact prevents an accurate identification of zonal boundaries or events in this time interval. Some fragments of calcareous sediments recovered in Sample 177-1093D-50X-CC (614.70 mcd) contain a calcareous nannofossil assemblage composed mainly of the same Reticulofenestrid species observed in the upper Miocene interval of Sites 1088 and 1092.
The abundance of planktic foraminifers is highly variable at Site 1093. In general, abundance increases below ~200 mcd. Preservation is good to moderate in most samples. Neogloboquadrina pachyderma (sinistral) is by far the most abundant species (Table T7, also in ASCII format in the TABLES directory). Additional species include Globigerina bulloides, Globigerina quinqueloba, Globigerinita uvula, Globorotalia inflata, Globorotalia puncticulata, and Globorotalia puncticuloides. These species are generally present only in low abundance. Based on the examination of CC samples, it seems likely that a planktic isotopic stratigraphy may be established on N. pachyderma (sinistral), but it might not be continuous throughout the entire composite section of Site 1093. Low recovery between 255 and 450 mcd and below 520 mcd will also prevent the construction of a continuous record.
Benthic foraminifers at Site 1093 are generally not very abundant and vary considerably in their state of preservation from poor to good. Highly abundant, needle-shaped remains of the diatom genus Thalassiothrix in the >63-µm fraction made it necessary to wet sieve sediment samples at 150 µm.
Benthic foraminifers are highly variable, typically constituting between 5% and 100% of the total foraminifer fauna from the >150-µm fraction studied. Absolute foraminifer abundances are variable and low, reaching a maximum of 9 specimens/cm3 in Sample 177-1093A-12H-CC, 0-10 cm. Low benthic foraminifer abundances may be explained by relatively high sedimentation rates (see below). A number of barren intervals occur, notably below 400 mcd, suggesting that a continuous benthic foraminifer isotopic record from this site may be difficult to obtain. However, Cibicidoides wuellerstorfi appears to be present during the Pleistocene interglacials, and a benthic stable isotopic record, albeit discontinuous, should be achievable through the late and mid-Pleistocene.
Quantitative estimates of relative species abundance were made from Holes 1093A and 1093D, with counts of up to 145 specimens/20-cm3 sample. Species richness is variable, with a maximum of 32 taxa recorded in Sample 177-1093A-12H-CC, 0-10 cm (124.69 mcd), which corresponds to a peak in color reflectance during marine isotope stage (MIS) 11 (Fig. F13). Low values correspond to glacial intervals (e.g., the adjacent intervals to MIS 11). Samples 177-1093A-11H-CC (114.11 mcd) and 13H-CC (134.10 mcd) yield very sparse assemblages containing one and two taxa, respectively (Table T8, also in ASCII format in the TABLES directory).
The most common benthic taxa recorded at Site 1093 include C. wuellerstorfi, Eggerella bradyi, Globocassidulina subglobosa, Melonis pompiliodes, Pullenia bulloides, Pullenia quinqueloba, and Pullenia subcarinata. The assemblages are very similar to those recorded at Site 1091 and do not provide any basis for biostratigraphic subdivision.
For biostratigraphic age assignments we used the zonation proposed by Gersonde et al. (1998). The late Pliocene-Pleistocene zonation proposed for the northern area of the Southern Ocean by Gersonde and Bárcena (1998) could be applied only partially because of the diachronous or scattered presence of certain marker species, which preferentially dwell in warmer waters. All diatom stratigraphic information from the six holes was combined and converted to the mcd scale (Tables T6, T9, both also in ASCII format in the TABLES directory).
Diatoms are generally abundant and display good preservation in all investigated samples. During examination of the diatom assemblages, we also encountered silicoflagellates in few to trace numbers (Table T10, also in ASCII format in the TABLES directory).
The LO of Hemidicus karstenii, which falls in the lower portion of MIS 6 and defines the boundary between Thalassiosira lentiginosa Subzones c and b, was noted at 49.05 mcd (Fig. F14). Downhole variations in physical properties (compare Fig. F22) allow tentative identification of MISs in the upper 220 mcd of Site 1093. On this basis, the LO of H. karstenii falls in the lower part of an interval interpreted to represent MIS 6. The climatic optima of MISs 7, 9, and 11 can be tentatively placed at ~76, 106, and 125 mcd, respectively. However, H. karstenii, which is present in the Subantarctic and northern Polar Front Zone during these intervals (compare "Biostratigraphy" sections, "Site 1089" and "Site 1091" chapters), was found only in sediments tentatively identified as MISs 7, 8, and 9. H. karstenii is apparently absent in sediments assigned to MIS 11. This age assignment is supported by the LO of the calcareous nannofossil Pseudoemiliania lacunosa at 156 mcd, which is known to disappear in MIS 12. A similar occurrence pattern of H. karstenii was also observed in piston cores recovered in the area south of the Polar Front (R. Gersonde, unpubl. data). For this reason, T. lentiginosa Subzones a and b (Gersonde and Bárcena, 1998) cannot be discerned and must be combined at Site 1093. Below 124.77 mcd, in the lower part of the interval assigned to MIS 11, we observe rare but consistently well-preserved specimens of Actinocyclus ingens. This taxon has its LO in the lower portion of MIS 16 (Gersonde and Bárcena, 1998). Tentative MIS identification, and recognition of the Brunhes/Matuyama boundary at ~210 mcd (see "Paleomagnetism") indicate that the section below 124.77 mcd is not disturbed by a hiatus. We suggest that the presence of A. ingens is caused by reworking of diatoms from older sediments. The reworked sediments possibly originate in late Pleistocene deposits belonging to the upper portion of the A. ingens Zone, because no stratigraphically older taxa (e.g., Fragilariopsis barronii, which marks the A. ingens Subzone a) were identified besides A. ingens. Because A. ingens is consistently few to abundant in number only below 174.6 mcd (Table T10), we tentatively place the top of the A. ingens Zone at this depth. A possible source area for the reworked sediments can be identified on a multichannel seismic line crossing the location of Site 1093 (Fig. F2). This line shows erosional features in sediments possibly related to the upper A. ingens Zone ~4 nmi north of Site 1093. A nearby source area is also suggested by the good preservation of the reworked A. ingens valves, which are unlikely to have been transported very far.
Similar to the sediments of the T. lentiginosa Zone, the underlying sediments assigned to the A. ingens Zone were also deposited at average sedimentation rates as high as 250 m/m.y. The base of A. ingens Subzones b and a has been placed at 272.52 and 339.75 mcd, respectively. However, the lower portion of the A. ingens Zone is in a section with poor recovery (Fig. F14), possibly because of the presence of diatom mats (see "Lithostratigraphy"). High recovery occurred below 450 mcd, and this sedimentary interval belongs to the lowermost portion of the A. ingens Subzone a of earliest Pleistocene age. Close sample spacing allowed us to establish that assemblages representing the underlying Proboscia barboi Zone are not present at Site 1093. This indicates a hiatus at ~460.5 mcd that separates sequences assigned to the A. ingens Zone and the Thalassiosira kolbei-Fragilariopsis matuyamae Zone, and which spans at least 0.2 m.y. Below this hiatus we note a distinct drop in sedimentation rates to average values of ~80 m/m.y. (Fig. F15). The top of the Thalassiosira vulnifica Zone, marked by the LO of the nominate species, is noted at 494.25 mcd. This zone straddles the Matuyama/Gauss boundary. The preliminary depth assignment, however, places the base of this zone, marked by the LO of the nominate species of the underlying Thalassiosira insigna Zone, 1 m above the Matuyama/Gauss boundary, which has been identified at 506.85 mcd (Table T9; Fig. F14). Assemblages representing the underlying Fragilariopsis interfrigidaria Zone, which straddles the Gauss/Gilbert boundary and thus the late/early Pliocene boundary, were observed between 543.1 and 585.5 mcd. In the lower portion of this interval, magnetic inclination data suggest the possible placement of the Gauss/Gilbert boundary at 574.2 mcd (see "Paleomagnetism"). Sediments assigned to the F. interfrigiaria Zone occur within an interval of incomplete recovery, which continues downhole to the base of Site 1093. Sample 177-1093D-48X-CC, 8-9 cm (594.98 mcd), contains dominant Neobrunia mirabilis. The co-occurrence of rare Hemidiscus triangularus indicates a late Miocene age for the Fragilariopsis reinholdii Zone. A similar ooze was recovered at Ocean Drilling Program (ODP) Site 701 with a thickness of ~30 m. This ooze has been dated as latest Miocene (between 6.3 and 6.9 Ma) (Shipboard Scientific Party, 1988). For this reason, we suggest that a hiatus separates the uppermost lower Pliocene from the uppermost upper Miocene interval, spanning a time interval of ~3 m.y. The hiatus is located between Samples 177-1093D-47X-CC, 0-10 cm, and 48X-CC, 8-9 cm, and thus occurs at ~590 mcd (Fig. F15).
Radiolarian biostratigraphy at Site 1093 is based on the examination of 58 samples (Table T11, also in ASCII format in the TABLES directory). Radiolarians at Site 1093 are generally well preserved except for the lowermost sample (177-1093D-50X-CC, 0-2 cm [614.7 mcd]), but their abundance is variable because of dilution by abundant diatoms. All the Antarctic Pleistocene to Pliocene radiolarian zones (Omega to Upsilon Zones) are recognized at Site 1093.
Stylatractus universus is persistently present below 131.47 mcd, which suggests that the boundary of the Omega and Psi Zones lies at ~130.72 mcd (Table T9). The boundary of the Psi and Chi Zones, marked by the LO of Pterocanium trilobum, is somewhat difficult to recognize because rare to few occurrences of P. trilobum are observed even in the Omega Zone. Judging from the continuous presence of Pterocanium trilobum below ~219 mcd, however, the last appearance datum of P. trilobum at 0.83 Ma can be placed at 213.68 mcd (Table T9). The boundary of the Chi and Phi Zones, defined by the LO of Eucyrtidium carvertense at 1.92 Ma, is tentatively placed at 484.08 mcd (Table T9). As for Pterocanium trilobum, reworked specimens of Eucyrtidium carvertense are observed in some horizons of the overlying Chi and Psi Zones, which makes the recognition of the boundary difficult. Moreover, the Chi/Phi boundary is apparently in the lower reversed polarity portion of the Matuyama Chron (Fig. F14), which suggests that the zonal age assignment by Lazarus (1992) cannot be applied at Site 1093. Therefore, further investigations will be necessary to clarify the details of the Chi/Phi boundary at Site 1093. The Upsilon Zone, whose top is defined by the LO of Helotholus vema at 2.42 Ma, is encountered within the lowermost portion of Hole 1093D, below 521.77 mcd. The lowermost sample (177-1093D-50X-CC, 0-2 cm [614.7 mcd]), yields poorly preserved Prunopyle haysi that has its LO in the late Miocene (Lazarus, 1992). This assemblage is quite different from that of the Upsilon Zone, which suggests the presence of a hiatus around 590 mcd as shown in Figure F14.
Archive halves of APC and XCB cores recovered at Site 1093 were measured using the shipboard pass-through magnetometer. Measurements were made at 5-cm intervals. Sections obviously affected by drilling disturbance were not measured. The majority of core sections from Site 1093 were measured after alternating-field (AF) demagnetization at peak fields of 0 (natural remanent magnetization [NRM]), 5, 10, 15, 20, and 25 mT. The rate of cores processed through the laboratory ("core flow") at this site generally allowed this six-step measurement sequence. Shorter measurement sequences comprising four steps (0, 10, 20, and 25 mT) were used for Cores 177-1093B-1H through 9H, and for all XCB cores from Hole 1093D.
NRM intensities were about 1 × 10-2 A/m at the top of each hole, decreasing through the upper 20 mbsf to ~1 × 10-3 A/m, below which they remained fairly uniform throughout most of the APC section. After AF demagnetization at peak fields of 25 to 30 mT, intensities generally decreased to ~5 × 10-4 A/m in the APC sections (above 250 mbsf) of all holes. Magnetization intensities were slightly higher in the XCB section of Hole 1093D, particularly below 440 mbsf. 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 (Fig. F16). Magnetization directions attributed to the Matuyama Chron are particularly inconsistent, probably because 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 is identified in the 205- to 210-mcd interval at both Holes 1093A and 1093B (Fig. F16; Table T9). The boundaries of the Gauss Chron are tentatively identified in the XCB section of Hole 1093D (Fig. F17; Table T9).
At Site 1093, the sedimentary section recovered is continuous only to a depth of 252 mcd (base of Core 177-1093D-10H, early Pleistocene), and has a total thickness of ~600 mcd (base of Core 1771093D-50X, late Miocene) (Figs. F10, F11, F12, F13). Holes 1093A-1093F were cored by APC to 241.5, 221.8, 169.5, 307.0, 42.5, and 43.5 mbsf, respectively. Holes 1093A and 1093B were drilled by XCB to a TD of 309.4 and 597.7 mbsf, respectively. The offset of cores below the spliced record was arbitrarily chosen to be the same as the cumulative offset of the overlying cores. Holes 1093E and 1093F are short holes that were drilled to ensure continuous overlap in the uppermost part of the sedimentary section.
At Site 1093, an extremely expanded Holocene to mid-Pliocene section was recovered. However, this record is incomplete because of intervals of low recovery at 260-460 mcd and below 520 mcd. The lowermost biostratigraphic datum at Site 1093 was dated at 6.3-6.9 Ma as latest Miocene. The late Miocene is apparently separated by a hiatus from the overlying early Pliocene sediments. Another hiatus may be present in the late Pliocene at ~460.5 mcd (Figs. F14, F15).
All biostratigraphic datums, including calcareous nannofossil, diatom, and radiolarian events and available magnetostratigraphic interpretations, yield consistent age assignments throughout the record at Site 1093. The sedimentation rates in the diatom-dominated Pleistocene sequences average ~250 m/m.y. The transition between the Brunhes and Matuyama Chrons could be placed between 205 and 210 mcd. Preliminary correlation of the physical properties records with MISs during the Brunhes Chron indicates slightly higher sedimentation rates during the last 0.45 m.y., ranging between ~200 and ~720 m/m.y. In the lower portion of the Brunhes Chron, sedimentation rates are lower, ranging between ~100 and ~450 m/m.y. Because of low recovery rates (25%) in the interval between ~260 and ~460 mcd, the lower Pleistocene sedimentary record older than ~1-1.1 Ma is only sparsely documented. The top of the underlying high-recovery interval (450-520 mcd) is earliest Pleistocene in age. The absence of the Proboscia barboi diatom zone, which correlates with the Olduvai Subchron, may indicate that a hiatus separates the Pleistocene from the Pliocene, probably spanning at least 0.2 m.y.
Late Pliocene sedimentation rates are ~80 m/m.y. Such a distinct drop from high Pleistocene to lower late Pliocene sedimentation rates was also observed at Sites 1090, 1091, and 1092, and is probably related to the general change in biosiliceous export rates in the Southern Ocean at this time. The interval of low recovery from below 520 mcd to the bottom of Site 1093 is correlated with the Gauss Chron. The boundary between the Gauss and Gilbert Chrons is tentatively placed at 574.2 mcd. Only the uppermost early Pliocene section is present at Site 1093 because of a hiatus at ~590 mcd. Below this hiatus, which spans ~3 m.y., uppermost Miocene sediments were penetrated with low recovery.
The extremely expanded upper and mid-Pleistocene sediments provide a unique opportunity for paleoceanographic reconstructions at very high time resolution in a pelagic environment. Sedimentation rates between 100 and 700 cm/k.y. allow sample spacing with a temporal resolution of less than 100 yr. This record can be correlated in detail with paleoclimatic records from Greenland and Antarctic ice cores. On the basis of CC samples, it seems likely that a nearly continuous planktic isotopic stratigraphy (N. pachyderma [sinistral]) and a more or less continuous benthic isotopic record can be established for the late and mid-Pleistocene sections at Site 1093.