All published and unpublished biostratigraphic results have been compiled into tables showing first or last occurrences of biostratigraphic marker species (Tables T3, T4, T5). This data, together with the interpreted paleomagnetic polarity pattern, is shown in downhole log format for each Site in Figures F3, F4, and F5 and graphically as age-depth plots (Figs. F6, F7, F8). There is a disagreement between the biostratigraphy of biosiliceous fossils and the magnetostratigraphy at Site 1095. Iwai (2000a, 2000b) noted the possibility of a Miocene-Pliocene hiatus at this site. Table T6 summarizes the magnetic anomaly correlatives identified in the Leg 178 Initial Reports volume (Barker, Camerlenghi, Acton, et al., 1999) and the alternative postcruise correlation by Iwai (2000a, 2000b).
Site 1095 (66°59.1´S, 78°29.3´W; 3842 m water depth) is located on the continental rise along the Pacific margin of the Antarctic Peninsula. A sedimentary section, 570 m thick, extending from the Holocene to the upper Miocene (~9.6 Ma), was recovered from a hemipelagic sediment drift (Rebesco et al., 1997) (Fig. F1). Drilling at Site 1095 was targeted to recover an older part of the sedimentary section than at Site 1096 to extend the studies of the history of glaciation back in time. Four holes (1095A-1095D) were drilled (Barker, Camerlenghi, Acton, et al., 1999). Recovery was generally excellent (average recovery = 89% for the entire hole), although there is a short interval from 209.2 to 228.4 mcd (Cores 178-1095B-15X and 16X) where recovery was <50%, a gap resulting from zero recovery in Core 178-1095B-19X at 247.6-257.2 mcd, and poor recovery in the basal cores of Hole 1095B (Cores 178-1095B-43X through 52X).
The recovered sediments are mostly diatomaceous silty clays derived from distal turbidite, contourite, and hemipelagic sedimentation (Barker, Camerlenghi, Acton, et al., 1999). The uppermost 50 m consists of laminated and massive, often extensively bioturbated, diatom-bearing silty clays (Unit I). These deposits show a marked cyclic pattern of alternating gray, terrigenous, and brown biogenic-rich silty clays, which are the stratigraphic expression of successive glacial and interglacial cycles. Lithostratigraphic Unit II extends from 46.0 mcd down to 429.5 mcd and consists mainly of green laminated silts and muds. Sediments below 429.5 mcd (Unit III) consist of nonbioturbated parallel-laminated siltstone-claystones with sporadic occurrences of diatomaceous claystone. This facies of Unit III does not show the cyclic pattern observed in overlying sediments. One possible Pleistocene hiatus was suggested at ~56 mcd (~60 mbsf) (Acton et al., Chap. 37, this volume).
The stratigraphic position of microfossil zonal boundaries at Site 1095 is shown in Tables T3 and T5 and in Figure F3. Diatoms and radiolarians provide the majority of age constraints for Site 1095. Diatoms are common in the upper Miocene to lower Pliocene section and are rare and sporadic in the upper Pliocene through Quaternary section from Cores 178-1095A-1H through 9H and below Core 178-1095B-46X (>498 mcd; >9.4 Ma).
The diatom biostratigraphy of Holes 1095A, 1095B, and 1095C is complex and contains an incomplete record of Pleistocene through upper Miocene datums and events. The Pliocene to Pleistocene is contained in glacial and interglacial hemipelagic sediments, whereas the Miocene consists mainly of turbidites. Many core intervals are barren or have a low abundance and diversity of diatomaceous material. In samples that have diatoms, reworking of older species is often noted. In spite of the obvious reworking, there is a strong biostratigraphic signal for the early Pliocene and late Miocene. Material used in the biostratigraphic analysis of this site was taken from the core catchers and within the split cores (usually one sample per section).
The samples in Cores 178-1095B-29X through 45X contained a typical late Miocene diatom assemblage, with a large proportion of Denticulopsis species dominating. The species of Denticulopsis that are used as zonal boundary markers in older sediments, D. dimorpha and D. praedimorpha, were absent from the assemblage. One sieved sample from Core 178-1095B-37X contained Asteromphalus kennetii, which has a first occurrence age of 10.29 Ma. This datum also helps constrain the age of the lower part of Hole 1095B to the late Miocene.
All previously defined standard radiolarian zones for the late Miocene to earliest Pliocene can be identified in Hole 1095B between 180 and 460 mcd, including the basal Tau, A. challengerae, A. labrata, S. vesuvius, and upper A. australis Zones of Lazarus (1992). As noted, however, by Lazarus (Chap. 13, this volume), radiolarian occurrences are sporadic and the placement of zonal boundaries are thus somewhat imprecise.
The biostratigraphy of calcareous nannofossils in material from Site 1095 is also discontinuous and incomplete but provides biostratigraphic age control in the upper intervals of the site. Emiliania huxleyi, whose appearance defines the base of the calcareous nannofossil Zone CN15 (Okada and Bukry, 1980), is found only in Sample 178-1095A-1H-2, 70 cm (2.16 mcd). Samples 178-1095A-3H-5, 18.5 cm, through 5H-1, 90 cm (15.44-27.79 mcd), mostly contain Pseudoemiliania lacunosa without E. huxleyi. It suggests that this interval is assigned to Zone CN13.
Paleomagnetic results from Site 1095 provide a magnetostratigraphy that correlates well with the GPTS from the termination of Chron C4Ar.2n (9.580 Ma) at ~510 mcd up to the Brunhes Chron (Table T7; Fig. F3). Exceptions occur in a few intervals where core deformation, dropstones, magnetic overprints, or low recovery bias or obscure the paleomagnetic signal, as discussed below.
The initial interpretation of the shipboard paleomagnetic data was difficult for the upper 100 m of the section owing to conflicting results from Holes 1095A and 1095D. This was due mainly to depth offsets that occur in the mbsf depth scales, which were overcome by using the mcd depth scale (Barker, Chap. 6, this volume), and to paleomagnetic overprints. Even with more detailed stepwise demagnetization of U-channel samples, Acton et al. (Chap. 37, this volume) observed that the interval from 17 to 55 mcd is more complexly magnetized than intervals above or below. As a result, none of the short polarity intervals occurring in the past 2.5 m.y., including the Jaramillo Subchron (Subchron C1r.1n), the Cobb Mountain Event (Cryptochron C1r.2r-1n), the Reunion Event (Subchron C2r.1n), and Cryptochron C2r.2r-1n can be confidently identified. Within the complexly magnetized interval, Acton et al. (Chap. 37, this volume) interpret the normal polarity zone from 34.68 to 37.33 mcd as Chron C2n, which places the termination of Chron C2n about 20 m above the depth given by the Shipboard Scientific Party (1999b). (In the interpretation of the Shipboard Scientific Party [1999b], all reversals from the termination of Chron C2n to the onset of C2An.1n were lost in a hiatus).
Downhole magnetic logging with the GHMT was able to provide a complete polarity stratigraphy to the base of Hole 1095B, allowing the paleomagnetic polarities from core measurements to be extended into the interval of low core recovery below ~474.5 mcd. The interval from 485.5 to 517.5 mcd is dominantly of reversed polarity, whereas below 517.5 mcd to the base of the logs (551 mcd) it is of normal polarity, with the exception of two or three shorter reversed polarity events.
Variations in the remanent anomaly signal calculated from data collected on the second pass of the GHMT logging tool in Hole 1095B (Williams et al., Chap. 31, this volume) mimic the variations in the paleomagnetic inclination signal from the cores from Hole 1095B (Fig. F9). In this comparison, we have transformed the remanent anomaly signal, which was in the logging depth scale, to the mcd scale using Table T16 from Acton et al., (Chap. 37, this volume). The depth conversion uses linear interpolation between the published tie points. The remanent anomaly data in mcd, mbsf, and logging depth scales are included in Table T8 along with the full data set from the GHMT logging tool. The geomagnetic reversals, which are evidenced by a change in the sign of the inclination and remanent anomaly signals, correlate well as do the more subtle variations in the signals within intervals of constant polarity. Thus, the paleomagnetic data and magnetic logging data give a consistent magnetostratigraphy for Site 1095.
The magnetostratigraphy proposed by Acton et al. (Chap. 37, this volume) results in sedimentation rates that vary from 10 to 114 m/m.y., with the exception of the normal polarity zone from 455.37 to 480.03 mcd, interpreted to be Chron 4Ar.1n (9.230-9.308 Ma), over which the sedimentation rate is 316 m/m.y. Overall, the sedimentation rate increases progressively downhole, so a higher rate near the base of the section where Chron 4Ar.1n is located may not be exceptional. Below 480 mcd, the magnetostratigraphy becomes uncertain owing to poor core recovery. Both the core measurements and the magnetic logging data indicate that a reversed polarity zone is present from 480.3 to ~515 mcd, which we interpret to be Chron C4Ar.2r. A normal polarity zone appears to extend from ~515 mcd to the base of the hole, which could represent Chron C4Ar.2n or some combination of Chrons C4Ar.2n through C5n.2n.
Site 1096 (67°34.0´S, 76°57.8´W; 3152 m water depth) was drilled near the crest of a hemipelagic sediment drift on the continental rise off the northwest Pacific margin of the Antarctic Peninsula; a more proximal, shallower location than Site 1095 (Fig. F1). Site 1096 was drilled in order to obtain the shallower part of the stratigraphic section, where it is most expanded. At this site we recovered a complete double advanced hydraulic piston corer (APC) section to ~140 mcd and an extended core barrel (XCB) cored section to 613.2 mcd, and the site was logged using multiple downhole tools.
The sediments recovered are predominantly fine grained and terrigenous, consistent with drift deposition, and are divided into three depositional units. A detailed lithologic description is given in the "Site 1096" chapter of the Leg 178 Initial Reports volume (Shipboard Scientific Party, 1999c). The age of the sediments extends from the Holocene to the early Pliocene.
The stratigraphic position of microfossil zonal boundaries at Site 1096 is presented in Tables T3 and T5 and in Figure F4.
Site 1096 has better-preserved microfossils than Site 1095. Above 175 mcd (Units I and II) the sediments contain calcareous nannofossils and foraminifers. Below this depth, the biostratigraphy is based on the biogenic silica record. In the upper 110 m of Hole 1096A and the upper parts of Holes 1096B and 1096C, diatoms are rare or absent and the diatom zonal boundary markers are not seen. The exception to this is noted in brown-colored, possibly interglacial sediments present in the lower part of Cores 178-1096A-2H and 178-1096B-2H and 3H in which diatoms are common and the assemblage is dominated by Fragilariopsis kerguelensis, Hemidiscus karstenii, Thalassionema spp., and Thalassiothrix spp. fragments. Samples studied from 115 to 170 mcd are barren of diatoms. From 175 to 613 mcd (Unit III), siliceous microfossils dominate. The diatom record for this interval is virtually complete, with zonal datums being available for nearly all the zones. The record of radiolarians is relatively complete for Unit III, and most of the marker species are present through the entire Upsilon Zone. Radiolarians are present in several intervals, but the marker species for the two zones below Psi (Chi and Phi) were not found. Foraminifers are rare to abundant only in the upper interval of Site 1096. Neogloboquadrina pachyderma sinistral dominates the planktonic foraminiferal assemblage and is found to the base of the hole. Benthic foraminifers are rare and consist mostly of shallow-water species presumed to be reworked from the continental shelf. The calcareous nannofossil biostratigraphy of Site 1096 is discontinuous and incomplete but provides biostratigraphic age control in the upper 175 m the section.
The paleomagnetic results from Site 1096 provide a magnetostratigraphy that correlates well with the GPTS from the onset of Chron C3n.2n (4.6 Ma) at ~489.68 mcd up to the Brunhes Chron (Table T7; Fig. F4). As at Site 1095, there are a few intervals where core deformation, dropstones, magnetic overprints, or low recovery obscure the paleomagnetic signal, as discussed below. Coring gaps in the double-APC cored section above ~140 mcd are negligible (Holes 1096A and 1096B). Below this, gaps of several meters may be present between cores, with potential loss of some shorter polarity zones. Also, as at Site 1095, the interval from below the Brunhes/Matuyama reversal to just above Chron C2An.1n, spanning roughly 0.8 to 2.4 Ma, has the most complex magnetization and is the most difficult to interpret.
The most significant changes to the magnetostratigraphy of Site 1096 proposed by Acton et al. (Chap. 37, this volume) are the identification of Chron C2n, Subchron C2r.1n (the Reunion Subchron), the onset of Chron C2An.1n, and Chron C2An.2n. The relative stratigraphic locations of these newly identified polarity chrons and subchrons (Fig. F7) are compatible with the broader magnetostratigraphic framework completed during Leg 178 (table T21 from Shipboard Scientific Party, 1999c).
Site 1101 (64°22.3´S, 70°15.7°W) was drilled in 3278 m water depth, centrally located within a sediment drift ~500 km northeast of Sites 1095 and 1096.
A single hole (1101A) was APC cored to 142.7 mbsf and extended by XCB drilling to 217.7 mbsf (recovery = 99.1%). Recovered sediments consist mainly of hemipelagic clayey silt that contains a nearly continuous distal glacial record of the past 3.1 to 3.4 m.y. (late Pliocene to present). Unit I (0-53.3 mbsf) and Unit II (53.3-142.7 mbsf) are composed of alternating biogenic-bearing massive clayey silts and laminated clayey silts (Shipboard Scientific Party, 1999d).
All four microfossil groups (diatoms, radiolarians, calcareous nannofossils, and foraminifers) are present in Hole 1101A. The stratigraphic positions of microfossil zonal boundaries in Hole 1101A are given in Tables T3 and T5 and in Figure F5.
Diatom preservation and abundance varied throughout Hole 1101A. Fragmentation of valves is common throughout the upper 160 m, and diatoms in samples below Core 178-1101A-19X were more abundant than in the upper part of the hole. The lowest three cores (178-1101A-22X, 23X, and 24X) contain a diverse assemblage of open-marine diatoms. All diatom zones from the Pleistocene to the late Pliocene were identified at Site 1101.
Radiolarians were generally abundant and moderately preserved, with only two barren intervals. The Psi through Upsilon Zones (Pleistocene-Pliocene) were recovered (Fig. F5).
Calcareous microfossils were found in the uppermost core as well as in carbonate-rich intervals between 50 and 134 mbsf (Unit II). Several planktonic foraminiferal oozes were identified, mostly in Unit II. Over 90% of the individual specimens in these assemblages are Neogloboquadrina pachyderma sinistral, with rare N. pachyderma dextral, Globigerina bulloides, and benthic foraminifers. The carbonate-rich interval appears to be coeval with a similar interval at Site 1096.
Calcareous nannofossils were recovered in the upper 1 m. The first and last occurrence of the large form of Gephyrocapsa spp. was observed, making the nannofossil record at Site 1101 more complete than that at Site 1096.
The paleomagnetic polarity pattern from Site 1101 correlates well with the GPTS from the onset of Chron C2An.1n (3.040 Ma) at ~9.38 mbsf up to the Brunhes Chron (Table T7; Fig. F5). As at the other Leg 178 sites, exceptions occur in a few intervals where core deformation, dropstones, overprints, or low recovery obscure the paleomagnetic signal. Unlike these other sites, Site 1101, however, was cored with a single hole. Thus, several large coring gaps that occur in the upper 60 m are not filled by core recovered at other holes.
The revised magnetostratigraphy of Acton et al. (Chap. 37, this volume) is virtually unchanged from that completed during Leg 178 (see table T21 and associated discussion from Shipboard Scientific Party, 1999d). Slight adjustments made were based mainly on the higher resolution of the U-channel measurements. Acton et al. (Chap. 37, this volume) also defined the boundaries for Subchron C1r.2r-1n (Cobb Mountain Subchron) and Subchron C2r.1n (Reunion Event), which were discussed by Shipboard Scientific Party (1999d) but not included in their table T21. The identification of both of these subchrons is speculative given the short intervals over which they occur and the level of drilling disturbance that affects some intervals cored in Hole 1101A.