Topography and Climate

The Antarctic Peninsula is a long, dissected plateau giving the impression of a peneplain. It stands at ~900 m elevation at its northern end (63.5ºS), rising to 1750 m at 64ºS and remaining between that altitude and 2000 m as far as 68.3ºS (Elliot, 1997). The crest is broader in the south but is everywhere dissected to varying degrees by steep-sided fjords and glacial valleys. Ice cover is variable, a few hundred meters thick at most, and does not extend far beyond the central plateau on the Pacific side; it drains rapidly through steep ice-falls and short valley glaciers to sea level at the heads of fjords (Fig. F4). Ice shelves are small and sparse, and few large icebergs are produced. The dissected central plateau is bordered by longitudinal fjords and islands (some with elevations matching those of the mainland) conveniently regarded as forming an inner continental shelf. This region is floored by hard rock, whereas the middle shelf is mainly softer rock (partly sediments and probably preglacial), and the outer shelf is underlain by till. A topographic high separates the middle and outer shelf. In the south, near Alexander Island, the distance from plateau crest to continental shelf edge is 600 km, tapering to 200 km in the north. This distance is the maximum possible extent of the ice catchment, and more than half of it is now submarine (Fig. F5).

The greatest relief is in the inner-shelf region: fjords reach water depths >1000 m and are assumed to have been overdeepened by grounded ice. Relief on the outer and middle shelf is far less, but the shelf is essentially inward sloping. Shelf edge depths range from 300 m to >500 m. Between 70ºS and 63ºS the continental slope is steep, reaching 17º in places.

To the northeast lies Bransfield Strait, a linear trough separating the South Shetland Islands from the Antarctic Peninsula mainland, which reaches depths exceeding 2000 m and contains active volcanoes. Unlike other areas, the continental shelf of the South Shetland Islands is narrow and has shallow depths (~200 m). Opposite the South Shetland Islands lies the South Shetland Trench, at a depth of ~4700 m.

Snow accumulation varies with temperature and is greatest around the continental edge, particularly along the Antarctic Peninsula, which is warmer than East Antarctica. The Antarctic Peninsula acts as a major barrier to tropospheric circulation, and its ice sheet currently receives almost four times the average Antarctic continental snowfall (Reynolds, 1981; Drewry and Morris, 1992). However, modern meltwater-influenced sediments are known only from the South Shetland Islands (Banfield and Anderson, 1995), and the climatic regime elsewhere is fully polar. Snow accumulation governs the rates of ice transport required to maintain mass balance, hence the rate of basal sediment transport; greater accumulation means an expanded sediment record. There is much evidence (below) that grounded ice regularly reached the continental shelf edge during glacial maxima, although the level of precipitation at that time is unknown. In all, it seems reasonable to characterize the Antarctic Peninsula climatic regime, through a late Pleistocene glacial cycle, as a fully glacial, high-relief, high-precipitation, small-reservoir regime, with a rapid response to sea-level and climate change and an expanded sedimentary record.

Regional Oceanography

Present-day regional oceanography is fairly well known. A western boundary current, partly wind driven but containing elements originating in the Weddell Sea, was postulated to flow along the continental margin from northeast to southwest (Gordon, 1966; Heezen and Hollister, 1971) and has since been verified by direct-current meter measurements (Nowlin and Zenk, 1988; Camerlenghi et al., 1997a). Over a 10-month period, flow across the sediment drifts on the continental rise was slow and steady; mean current speeds were ~6 cm/s at depths of 3475 and 3338 m on the flanks of one drift, and the speed never exceeded 20 cm/s (Camerlenghi et al., 1997a). These speeds are enough to maintain fine silt and clay in suspension but are too weak to cause significant erosion on the rise (Pudsey and Camerlenghi, 1998). Flow directions (8 m above the seabed) were parallel to isobaths, and potential temperatures at these depths averaged 0.11º and 0.13ºC, respectively, suggesting (in combination with other data) an origin in Weddell Sea Deep Water (WSDW). There is no direct evidence (beyond the sediments themselves) of glacial-age current strength, but Pudsey (1992) has demonstrated a reduced WSDW flow during the Last Glacial Maximum (LGM).

Farther north lies the axis of the east-flowing Antarctic Circumpolar Current (ACC) and Polar Front (e.g., Orsi et al., 1995). The ACC is of higher energy, and eddies are common. However, there is no indication that the ACC or Polar Front has extended so far south in the recent past. The drill sites on the rise lie beneath the Circumpolar Deep Water (CDW), and temperature-depth sections such as that of Figure F6 are typical. Surface-water temperatures range up to +1ºC during the summer, and seasonal sea ice cover above the rise sites lasts for 6 to 9 months on average (Gloersen and Campbell, 1992).

Sea ice cover is more extensive on the continental shelf than on the rise, although coastal polynyas may occur (Zwally et al., 1985; Gloersen and Campbell, 1992). Measurements on the continental shelf in this part of the Antarctic Peninsula (Hoffmann et al., 1996) are largely confined to static hydrographic measurements of temperature and salinity (some dissolved oxygen and sparse drifting buoy data). Shelf water belongs to the CDW. Surface currents on the shelf are mainly toward the southwest. On the basis of sparse data, Hoffmann et al. (1996) hypothesize a slow southwest shelf current with a northeast countercurrent offshore, forming a weak local gyre. Icebergs are sparse in this region, and pack ice was not expected at the time of drilling.

Geology and Tectonics

The tectonic setting of the Antarctic Peninsula is unusual but straightforward. Its onshore geology is a mixture of calc-alkaline plutonic and volcanic rocks, metasediments of an old accretionary prism, and gneissic basement, reflecting a long history of subduction at the Pacific margin of Gondwana (e.g., Adie, 1954, 1955; Saunders et al., 1980; Garrett and Storey, 1987; Barker et al., 1991; Moyes et al., 1994; Leat et al., 1995). A young accretionary prism is not exposed onshore, and the youngest exposed sediments are terrigenous and volcaniclastic sediments of mainly late Mesozoic age (Paleogene on the backarc-Weddell Sea margin).

Subduction of the Pacific Ocean floor at this margin for more than 150 m.y. ended with the progressive subduction (collision) of segments of the Phoenix-Antarctic ridge crest at the trench, earliest (~50 Ma) in the southwest and latest (3-6 Ma) in the northeast (Barker, 1982; Larter and Barker, 1991a; Fig. F7). The geometry of ridge-crest subduction is very well known because the trailing flank of the ridge crest belonged to the Antarctic plate. This meant that motion relative to the margin was zero after collision and that the trailing flank survived at the sea-floor. The observed half-spreading rate was half the subduction rate, and (until a very late stage) the spreading fabric was parallel and perpendicular to the margin. The ridge-crest subduction event is helpful to the preservation and study of margin sediments; the series of young ocean floor ages along the margin places constraints on the ages of overlying sediments that may be traced onto the shelf and rise (Larter and Barker, 1991b; Rebesco et al., 1997). In the collision segment, there was uplift of the outer margin for a few million years after subduction as the forearc above the subsiding ridge crest was heated. The resulting hiatus in terrigenous sediment provision to a corresponding section of the continental rise was recorded at Deep Sea Drilling Project (DSDP) Site 325 (Hollister, Craddock, et al., 1976). It is marked in seismic reflection profiles by the restricted range of a particular depositional unit on the shelf (S3 of Larter and Barker, 1991b) and by likely gaps in terrigenous deposition on the rise (Rebesco et al., 1997). Importantly, however, it is wholly after the time of collision; the continuity of terrigenous supply for 2-3 m.y. before collision (filling and crossing the approaching mid-ocean ridge crest) is shown by a broad zone of very low oceanic magnetic anomaly amplitudes (a "magnetic quiet zone"; Larter and Barker, 1991a) along the margin. The age range of the interval of reduced terrigenous sediment supply is diachronous along the margin, following the progress of the collision itself.

In the far northeast the remaining Phoenix-Antarctic ridge segments ceased spreading 3-4 m.y. ago before reaching the subduction zone, at about the time of the youngest collision. Thus, the South Shetland Trench survives at the seabed. It is considered that subduction continues, accompanied by rollback of the subduction hinge and active backarc extension in Bransfield Strait (Barker and Dalziel, 1983; Barker and Austin, 1994, 1998; Fig. F7).

The middle and outer continental shelves are separated by a mid-shelf high (MSH), best seen by its gravity signature (Fig. F8). This feature is considered to have been a mid-slope high during subduction (Larter and Barker, 1991b; Bart and Anderson, 1995; Larter et al., 1997), and probably marked the edge of the rigid overriding plate: such positive features are common at subducting margins (e.g., Fryer, Pearce, et al., 1990; Taylor, Fujioka, et al., 1990; Barker, 1995b). The MSH may be of continental origin (it is generally nonmagnetic) or may have a serpentinite composition as in the Marianas arc. Thermal uplift after ridge-crest subduction seems to have been focused on the MSH. Inshore from the MSH lies a series of shelf basins, interpreted as precollision upper forearc basins with a gently dipping, possibly depositional inner flank, but an outer flank probably deformed during uplift of the MSH (Fig. F9). They could be Cretaceous or Cenozoic (precollision) in age, with a mixed terrigenous/biogenic composition. These basins and the MSH are prime sources of reworked material within the glacial sediments of the prograded wedge.

Tectonics and Sedimentation

Two instances of the influence of tectonics on sedimentation along the margin have already been mentioned. First, the ages of young ocean floor at the margin in the northeast constrain the age of overlying sediments, aiding the location of appropriate sites for drilling. Second, thermally driven uplift of the middle shelf immediately after ridge-crest collision prevented the supply of terrigenous sediment to a section of the continental rise. Slow subsidence thereafter is marked by basal onlap of the shelf Sequence S3 (below), diachronously along the margin. In planning the drill sites it was necessary to try to avoid examining the onset of a glacial regime, on the continental shelf or on the rise, where this diachronous, tectonics-driven break in terrigenous sedimentation might confuse a climate-driven change. This meant sampling only the younger sediments in the northeast and the older sediments in the southwest.

Tectonics has influenced sedimentation in other ways. Following ridge-crest subduction, the margin has resembled thermally a young extensional margin. Preservation of topsets of the shelf-slope prograded wedge has been assisted by thermal subsidence; on much older margins, this mechanism is far less effective. Also, the isostatic response to sediment load is more local in a younger margin, making it easier to detect and use. Further, it has been suggested that there are links between ridge-crest subduction, uplift of the Antarctic Peninsula, and the onset of glaciation (see Elliot, 1997). These are unproven but may be tested by drilling. The central plateau's resemblance to a peneplain is marked, but constraints on the time of uplift are sparse. Uplift may have been diachronous along the margin, resulting from ridge-crest subduction, but an age as old as Late Cretaceous is not ruled out. It is suggested also that the creation of a 2-km-high ridge of mountains would have affected precipitation and local climate sufficiently to have caused the development of an ice sheet. In a region of high precipitation, uplift should be followed quickly by erosion, so there is the opportunity to examine the provenance of the terrigenous component of the drilled sediments for signs of progressive unroofing of a newly elevated Antarctic Peninsula.

Antarctic Peninsula Glacial History

It is useful to summarize what is known of Antarctic Peninsula glacial history, from work onshore and conjectured from (mainly seismic) work offshore. Onshore evidence has been published (see Birkenmajer, 1992; Elliot, 1997) of early or middle Eocene as well as early and late Oligocene glaciations on the South Shetland Islands (northern Antarctic Peninsula). These conflict with other evidence of regional climate; for example, from ODP Leg 113 (Mohr, 1990; Kennett and Barker, 1990). Late Miocene glacial deposits are found on Seymour Island (Smellie et al., 1988). Sloan et al. (1995) suggested a late Miocene age for sediments on the Weddell Sea margin, identified as glacial and glaciomarine in seismic profiles; Bart and Anderson (1995) suggested a middle Miocene onset on the Pacific margin on similar grounds. It is generally considered that the Antarctic Peninsula can provide a high-resolution record of glaciation back to perhaps 10 Ma. To go back farther could involve entanglement with the tectonics of ridge-crest collision (see above), making a problem out of an asset. However, because of the Antarctic Peninsula's more northerly position, its glacial history is shorter than East Antarctica's; the record before 10 Ma may be largely nonglacial, or may reveal a stage of valley glaciation lacking regular ice-sheet extension to the continental shelf edge.