Nine sites on the Pacific margin of the Antarctic Peninsula were drilled during Leg 178 in three depositional environments. Four (1097, 1100, 1102, and 1103) were on the glacial, overdeepened outer continental shelf to investigate sediments deposited by grounded ice over the past 10 m.y. Three (1095, 1096, and 1101) were on sediment drifts on the continental rise to examine the fine-grained component of glacial shelf sediments. These sediments, moved there by turbidity and bottom currents, contain a more complete, higher resolution, more easily recovered and dated glacial record. The two remaining sites (1098 and 1099) were drilled in Palmer Deep, an isolated basin of the inner shelf that contains an ultrahigh-resolution record of Holocene paleoenvironmental change.
The outer continental shelf sites and the rise sites are complementary: the more direct but less complete record on the shelf helps interpretation of the record on the rise. Together they provide insights useful in attempts to recover the much longer but possibly less-accessible record of East Antarctic glaciation. The inner-shelf basin record was assumed to represent regional paleoclimate and will be compared with records from similar environments drilled by ODP (Cariaco Basin, Santa Barbara Basin, and Saanich Inlet) and with ice-core records.
Sites 1095, 1096, and 1101 on the continental rise sediment drifts provided virtually continuous cores, back to about 10 Ma at Site 1095, 4.5 Ma at Site 1096, and 3.1 Ma at Site 1101 (Figs. 2, 3, 7, 10, 12). The depositional environments at the sites were different, ranging from a dominantly hemipelagic mode on the rise crest and center to a dominantly turbiditic mode at the distal site (Fig. 8). However, none of the sites was an end-member environment: the distal site received mainly fine-grained distal turbidites but with a substantial hemipelagic/pelagic component, and the rise-crest sites were not isolated from deposition of fine-grained graded beds within a dominantly hemipelagic environment. All sites revealed a cyclicity in sedimentation that, whatever the dominant depositional mode at the site, was considered to reflect the cyclic provision of glacial sediments to the uppermost continental slope. Concerning the value of drilling at this margin as a guide to drilling at other Antarctic margins, this mixed depositional environment was an advantage: few of the other drifts around Antarctica are as isolated from distal turbidite deposition as Drift 7 (Site 1096). It is therefore reassuring to know that a signal of cyclic glacial loading of the upper slope is provided also, within even a dominantly turbiditic environment such as Site 1095.
Sedimentation rates were highest on the drift crest (Site 1096 [Fig. 11]) and lowest on the distal flank (Site 1095 [Fig. 9]) as expected. At all three sites, the rates decreased through the Pliocene and into the Pleistocene (Fig. 21). The high rates make possible a detailed study of cyclicity in deposition. No such investigation has progressed far as yet, but it is reassuring to know that several parameters show a variability with a period similar to the Milankovic cyclicity seen in lower latitudes (notably 40 k.y. within the Pliocene). This includes lithologic alternations (turbidite abundance, bioturbation, possibly IRD, and color), magnetic susceptibility, and density (cores and logs). Clay mineralogy appears to have glacials and interglacials variability (Fig. 22) and could become a useful additional proxy. We even recovered nannofossils and foraminifers through part of the Pliocene-Pleistocene, an unexpected bonus. It would seem clear that the continental rise is sensitive to variations in the glacial state of the continent and that these reflect the orbital variation in insolation through much of the period examined. A downward change at Site 1095 that sees no cyclicity before about 9 Ma marks a change in the level or nature of glaciation on the shelf, if not its initiation.
The objectives of drilling on the continental shelf were to test the pre-existing depositional model, to date major changes in depositional geometry (so these could be compared with changes in continental rise deposition), and to improve understanding of shelf sedimentation ahead of similar proposed drilling elsewhere around Antarctica. We drilled three sites on a dip transect of a depositional lobe (1102, 1100, and 1103, in landward order [Fig. 16]), and 1097 was drilled in an interlobe area farther south (Fig. 14). Drilling was severely hindered by swell, and recovery was made difficult (as expected) by the unsorted nature of unconsolidated subglacial tills and related diamicts. In drilling we were largely limited to the basic RCB technique, and recovery was very poor until the fine-grained matrix of a diamict became hard enough to support the large clasts that were inevitably encountered. Recovery then improved (e.g., to an average 34% below 250 m at Site 1103).
The glacial nature of the youngest sequence group (S1) was amply confirmed, and much of it could be given a Pleistocene or latest Pliocene age, but recovery was poor everywhere (the outermost shelf was particularly unfriendly). The most information on S1 is likely to come from the broad suite of logs obtained at Site 1103 (Fig. 23). Sequence Group S2 was sampled only at Site 1097, where it is thin; its older part is early Pliocene in age (Fig. 24). Sequence S3, whose age and origin were in doubt before drilling, was sampled at Sites 1097 and 1103. This sequence is continuous and similar in seismic expression (Fig. 16) along the West Antarctic margin to at least 105°W, and it lacks the focus into depositional lobes of the overlying S2 and S1. It was clearly established as a mostly glacial sequence, although probably reflecting a greater range of environments than S2 and S1 (from subglacial to glacial marine) and likely a lesser level of glaciation. The conformity of its top with the base of S2, deduced from seismic profiles in the region of Site 1097, was essentially confirmed by the age range for the boundary (between 4.5 and 4.6 Ma) established by drilling. Seismic data suggest that the part of S3 sampled at Site 1103 is older than at Site 1097. The inferred depositional environment at Site 1097, near the paleoshelf edge, was perhaps more open marine than that at Site 1103, although the depositional environment was of lower energy. It is uncertain whether this reflects a time change in climate or in depositional environment. Dating of the lower part of S3 is uncertain as yet but should be improved by a range of shore-lab studies. It seems unlikely that the recovered sediments range back to 9 Ma, the time of a change in shelf environment inferred at Site 1095 on the rise.
Preliminary results of drilling in Palmer Deep accorded with expectations based on piston cores. Basin I, the narrowest basin with the thinner sediment fill, is less affected by turbiditic sedimentation than Basin III. The 45-m-thick sediment fill of Basin I (Site 1098) contains mainly laminated and massive muddy diatom ooze, with laminae probably reflecting changes in surface productivity (Fig. 20). In Basin III (Site 1099), the alternation of laminated and massive muddy diatom oozes is interrupted more frequently by turbidites with a terrigenous graded base. The highly reflective lower seismic unit (Fig. 19) is an alternation of thin turbidites and laminated diatom ooze, with a strong impedance contrast with the upper semitransparent unit. At this stage, we cannot say how the sedimentary records of the two basins correlate. The downhole change of diatom and foraminifer assemblages indicating more restricted oceanographic conditions at the base of Basin I and in the lower part of Hole 1099B suggests that the two recovered sections underwent a similar evolution and may therefore be approximately coeval.
Shipboard analysis of Palmer Deep cores was limited, with sampling mainly postponed to the Bremen Core Repository. It is worth noting, however, that the only high-resolution shipboard study (of pore-water composition) revealed anomalous and still-puzzling chemical gradients. Despite the inferred rapid but steady accumulation of sediment, pore waters in the upper 20 m at Site 1098 reveal a homogeneous composition that suggests almost complete mixing. The explanation of these gradients will come from postcruise studies of the relationships between pelagic settling of organic-rich material, turbiditic sedimentation, and bioturbation.
Several additional opportunities offered by Leg 178 drilling are being exploited by the shipboard party. The continuous, high-resolution, partly terrigenous record of the continental rise drift sites‹and the high southern latitudes of this leg‹provide excellent paleomagnetic data (e.g., Fig. 25). They also present opportunities for a wide range of studies, including field paleointensity, rock magnetism, detailed examination of particular reversals, and the nature of certain cryptochrons. The Palmer Deep sediments should also provide high-resolution paleointensity and secular variation data. The existence of a detailed high-latitude magnetostratigraphy and the location of the rise sites in a low-energy environment securely within the Antarctic water masses provide further opportunities to check and refine high-latitude biostratigraphy. These will be further enhanced if an orbital chronology can be established.
Pore-water geochemistry is of interest at several sites, notably Palmer Deep, where it seems capable of informing the wide range of high-resolution investigations currently planned. The logging effort, though curtailed by hole conditions in places, also shows promise. In general terms, we have a wide range of physical properties and log data from a cluster of generically related environments that will repay closer study. This leg has seen the first-ever FMS examination of subglacial tills (at Site 1103), and the comparison between high-quality core magnetostratigraphy and the GHMT record at Site 1095 is likely also to be productive. VSP and sonic logs will aid correlation between the hole and the seismic record and allow the results of drilling to feed back into the exceptionally large regional seismic reflection data set.
An additional opportunity, not considered within either proposal or discussed in the leg prospectus, was to detect at the continental rise sites the late Pliocene Eltanin asteroid impact. The renewed interest was generated by Gersonde et al. (1997). The Leg 178 sites lay 1300 km distant from the likely impact area. The rise drifts, being at their crest perhaps only three-quarters of the water depth of the direct path, seemed areas where a sedimentological event might be detectable, helping to guide us to geochemical and paleontological anomalies that would otherwise be difficult to find. At present, until stratigraphic control has improved, the impact is not clearly identified within the sediments. Each rise site, however, shows a single anomalous depositional or erosional event that may be associated with it.