The Eastern Margin

This subregion includes the Weddell Sea margin of the Antarctic Peninsula and Larsen Basin (Fig. AP-1). Persistent sea ice covers the region (Gloersen et al., 1992), hence relatively few research cruises have been conducted here. Macdonald et al. (1988) used regional geologic and aerogeophysical data to infer that a large Mesozoic-Cenozoic sedimentary basin extends ~ 700 km south from James Ross Island. Four main seismic stratigraphic units are identified from SCS reflection data on the shelf and upper slope (Anderson et al., 1992; Sloan et al., 1995; Fig. AP-2): Unit 4: acoustic basement interpreted as Jurassic and younger volcanic rocks; Unit 3: seaward-dipping reflections interpreted as Late Cretaceous to Oligocene marine shelf deposits, the older part of which are coeval with those on nearby Seymour Island; Unit 2: prograding sequences with truncated foresets that downlap onto Unit 3, and that are thought to have been deposited by multiple advances of grounded ice across the shelf in the Miocene and early Pliocene;

Figure AP-1: (a) Track lines of multichannel (thick lines) and single-channel (thin lines) seismic data in the Antarctic Peninsula region, DSDP and ODP drill sites (filled circles, annotated with site numbers) and SHALDRIL sites (open squares). SHALDRIL sites are only marked where either >10m subseafloor penetration was achieved or pre-quaternary sediments were recovered. Bathymetric contours are at 1,000 m intervals down to 4,000 m, with 500 m contours included locally on the shelf. Bathymetric data are based on Smith and Sandwell (1997) east of the Peninsula and north of 62°S. Bathymetry for most of the area west of the Peninsula is from Rebesco et al. (1998), the exception being the 500 m contours in the southern Bellingshausen Sea, which are from O Cofaigh et al. (2005). To the east of the Peninsula, 500 m contours south of James Ross Island (JRI) are based on figures in Evans et al. (2005), and the 500 m contour north of James Ross Island is based on multibeam echo sounding data maps produced from data collected on RV Nathaniel B. Palmer Cruises 0003 and 0107. Bold line is location of seismic profile in Fig. AP-2. SOM: South Orkney Microcontinent; JB: Jane Basin; PB: Powell Basin; BS: Bransfield Strait; RT: Robertson Trough. (b) Expanded map of the northern Antarctic Peninsula with SHALDRIL site numbers labelled (I/II indicates first/second cruise). Same bathymetry contours shown as in (a), except 500 m contours around JRI omitted. SSI: South Shetland Islands.

Unit 1: aggrading reflections interpreted as deposits from fluctuating dynamic ice sheets in the Pliocene and Pleistocene.

MCS reflection data show at least 8 km of sediment at the base of the continental slope, overlying likely Early Cretaceous age basement (Barker and Lonsdale, 1991). The northern continental slope has plastered contourite drift deposits up to 900 m thick, thought to have been deposited by north-flowing glacially influenced bottom currents. Pudsey (2002) suggests that drift deposition began in the early Miocene at the onset of bottom water flow, or in the latest Miocene at the onset of volumnious glacially derived sediment supply to the western Weddell Sea.

Late Quaternary shelf sediments have been sampled by seafloor coring (e.g. Domack et al., 2001a, b, 2005; Pudsey and Evans, 2001; Pudsey et al., 2001; Brachfeld et al., 2003; Evans et al., 2005). These researchers infer that grounded ice converged into major ice streams and advanced to the shelf edge during the LGM, that the Prince Gustav Sound Ice Shelf collapsed and reformed in the mid-Holocene, and that the recent collapse of the Larsen B Ice Shelf is unprecedented during the Holocene. Recent drilling by the SHALDRIL project has obtained the first samples from older sequences (Shipboard Scientific Party, 2005, 2006; Anderson et al., 2006, 2007).

Figure AP-2: Single-channel seismic line across the eastern margin of the Antarctic Peninsula at 65°15'S, collected on RV Nathaniel B. Palmer in 1993. Line drawing interpretation shows the internal geometries and boundaries (dashed lines) between the main seismic units. Vertical exaggeration at the seafloor is 82:1. Adapted from Sloan et al. (1995). Line location is shown in

Sequences drilled by SHALDRIL are principally shallow marine sands, sandy and silty muds, and pebbly muddy sands from neritic environments, with mollusc shells distributed throughout the cores. The cores are dated as: late Eocene to early Oligocene, Oligocene, middle Miocene, early Pliocene and Holocene.

5.6.2. The South Orkney Islands Region

This subregion includes the South Orkney Microcontinent (SOM) and the adjacent deep-water Jane and Powell basins (Fig. AP-1). The SOM extends about 350 km from east to west and 250 km from north to south, and is underlain by Mesozoic metamorphic and sedimentary rocks (Thomson, 1981; Dalziel, 1984). Offshore, the SOM includes four Cenozoic sedimentary basins (King and Barker, 1988) with up to 5 km of sediment (Harrington et al., 1972; Busetti et al., 2001, 2002). Powell Basin (up to 3,600m deep) formed as the SOM rifted and drifted away from the tip of the Antarctic Peninsula in late Eocene to late Oligocene time (King and Barker, 1988; Lawver et al., 1994; Coren et al., 1997; Eagles and Livermore, 2002). Opening of Jane Basin (up to 3,300 m deep) probably began slightly later (Lawver et al., 1991, 1994), and may have continued until the middle Miocene according to Maldonado et al. (1998).

From SCS data on the SOM, King and Barker (1988) defined pre-rift, syn-rift, and post-rift units. The post-rift sediments are less than 1 km thick (e.g. Busetti et al., 2001, 2002) and were drilled at ODP Site 695 (1,300m water depth) and ODP Site 696 (600 m water depth) (Fig. AP-3). They comprise Oligocene or early Miocene to Quaternary terrigenous sediments, with rare coarse-grained IRD until the late Miocene (~8.7 Ma) and common IRD thereafter. Middle Miocene to Quaternary sediments are hemipelagic and diatomaceous muds and oozes (Barker et al., 1988a, b). ODP Site 696 also sampled syn-rift Eocene sandy mudstones (Sequence 2) that have nannofossil assemblages and clay minerals suggesting a relatively warm climate, and palynoflora indicating temperate beech forests and ferns on West Antarctica. Drilling results suggest intermittent glaciation with little sea ice during most of the Miocene and a persistent ice cap to sea-level on West Antarctica since the late Miocene. Herron and Anderson (1990) place the maximum late Quaternary grounding line advance at the 300 m isobath, and consider open-marine conditions to have existed over the SOM since 6,000 y. B.P. based on SCS and seafloor core data.

In Powell Basin, post-early-rift sediments are up to 3 km thick. King et al. (1997) identified two seismic units with low reflectivity below and high

Figure AP-3: Part of seismic line AMG845-18, showing the setting of ODP Site 696 in relation to the seismic units (S1-S3) described by King and Barker (1988). Vertical exaggeration at the seafloor is 3.3:1. Adapted from Barker et al., 1988a. ODP Site 696 location shown in Fig. AP-1.

reflectivity above. They interpret the change as recording the onset of glacial-interglacial cyclicity in the supply of coarse detritus to the basin in the late Miocene. A similar upward change in reflectivity is observed in Jane Basin (Maldonado et al., 1998). The reflectivity change may also be due to silica diagenesis (e.g. Lonsdale, 1990; Volpi et al., 2003). Maldonado et al. (2006) identify five seismic units in Jane and neighbouring ocean basins, and relate changes in seismic characteristics to variations in bottom water flow since the middle Miocene. ODP Site 697 was drilled in Jane Basin (Fig. AP-1) to ~323mbsf, and recovered mainly early Pliocene and younger hemipelagic sediments with IRD throughout; however, IRD is abundant only near the base of the sequence (Barker et al., 1988a, b). Other seismic studies of Powell Basin (e.g. Kavoun and Vinnikovskaya, 1994; Coren et al., 1997; Viseras and Maldonado, 1999) focus on the post-early Oligocene rift history of the basin, but are limited in paleoclimate interpretations by the lack of drilling data.

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