Glacial Variability from the Continental Margin Geological Record

Continuous sedimentary records of Late Pliocene-Pleistocene (last ~2.5myr) glacial variability are generally lacking on the Antarctic continental shelf (defined as the glacially, over-deepened sea floor above the 1,000m isobath, Fig. 11.1A). This is because: (1) ice grounding during Quaternary ice sheet expansions onto the shelf has produced significant hiatuses, and (2) the 'riser-less' rotary coring systems employed by the ODP, and its predecessors, usually result in poor core recovery (<50%) of unconsolidated coarse-grained glacimarine sediments. While, the finer-grained pelagic and hemipelagic sedimentary records recovered from deep-water sediment drifts on the continental rise have yielded more continuous records, these do not provide direct physical evidence of past glacial fluctuations. The history of Antarctic margin Cenozoic geological drilling is summarised by Barrett (this volume, Chapter 3). Here we review in more detail the Late Cenozoic intervals of those records.

Since the mid-1970s, ship-based continental margin drilling of Late Pliocene-Pleistocene sediments has been focused in three main regions (Fig. 11.1): (1) Antarctic Peninsula (ODP Leg 178; Barker et al., 1999) and Weddell Sea (ODP Leg 113; Kennett and Barker, 1990); (2) Prydz Bay (ODP Leg 119; Barron et al., 1989; ODP Leg 188; O'Brien et al., 2001); (3) Ross Sea (DSDP Leg 28; Hayes and Frakes et al., 1975; Hays et al., 1976). Average core recovery on the continental shelf has ranged from 8%

(Leg 178 sites 1097, 1100, 1102, 1103) to 30% (Leg 119, sites 739-743) (see Fig. 11.1 and this volume, Chapter 3). The continental margin off Wilkes Land is scheduled for drilling by the IODP in January 2009 (IODP expedition 318, Escutia et al., 2006).

In parallel with ocean drilling activities, a different drilling approach based on a closed-circulation, continuous wireline coring system used by the minerals industry, was being applied from land and sea-ice platforms in the McMurdo Sound region. The first glacimarine Pleistocene records were recovered by the DVDP (McGinnis, 1981) from the deglaciated mouth of the Taylor Valley with significantly better core recovery (~90%) than ocean drilling. The first attempts at drilling from sea ice were made in the late 1970s with limited success and core recoveries of 50-60% (DVDP-15; Barrett and Treves, 1981; MSSTS-1; Barrett, 1986). During the 1980s and 1990s progressively more sophisticated sea-riser technology was employed by the CIROS (Barrett, 1989) and the Cape Roberts Project (CRP; CRST, 1999) that enabled the first deep Cenozoic records to be recovered (95-98%). Nevertheless, Pliocene-Pleistocene strata remained poorly sampled due to glacial erosion and non-deposition at these sites.

A region that had experienced high rates of Pliocene-Pleistocene basin subsidence and sedimentation was identified beneath the McMurdo Ice Shelf in the centre of the Victoria Land Basin (Naish et al., 2006), and was drilled by the ANDRILL Programme's McMurdo Ice Shelf (MIS) Project in the austral summer of 2006-2007 (Naish et al., 2007). The 1,285 m long AND-1B drill core recovered by the MIS Project provides the most complete (98% core recovery) and continuous sedimentary record of Pliocene-Pleistocene climate and glacial history from the Antarctic continental margin to date (Naish et al., 2008) and is reviewed in more detail below.

11.2.1. Weddell Sea

Pliocene-Pleistocene sediments were recovered in the Weddell Sea during ODP Leg 113 from three distinctly different deep-water environments

Figure 11.1: (A) Location of ice cores from the Antarctic interior and ocean drill cores (DSDP, IODP, ODP) from the continental margin and rise discussed in text. (B) Location of sea-ice and ice-shelf based geological drill core records from the Victoria Land Basin (VLB) in McMurdo Sound. (C) Geological cross section (A-A', Fig. 11.1B) across the VLB shows the stratigraphic relationships and age of strata recovered by drill cores in

Southern McMurdo Sound.

(Fig. 11.1): hemipelagic and biogenic sedimentation on the South Orkney microcontinent influenced by the WAIS (Site 697), a turbiditic sequence in the deep Weddell Basin influenced by both WAIS and the East Antarctic Ice Sheet (EAIS) (Site 694) and hemipelagic and terrigenous sedimentation on the east Antarctic continental margin (site 693). From these sites, Kennett and Barker (1990) were the first to identify a cooling step during the Late Pliocene that they associated with development of the present cold polar thermal regime of the Antarctic Ice Sheets. This is marked between 3 and 2.5 Ma, by a regional reduction in sedimentation rates and diatom abundances, a deterioration in microfossil preservation and a dominance of the sea-ice diatom Eucampia antarctica. At this time cooling led to expansion of WAIS and EAIS to the edge of the continental shelf coincident with the development of the NH continental ice sheets.

Leg 113 cores also showed that Quaternary expansions of the Antarctic Ice Sheet displayed pronounced orbital-scale, G-I sedimentary cyclicity characterised by relatively low siliceous biogenic components and higher average particle size during glacial deposition, alternating with larger silt-sized grains and higher diatom abundances during interglacials. Thus, glacial periods were characterised by increased sea-ice cover, lower productivity and stronger bottom water flows (Pudsey et al., 1988; Pudesy, 1990). The converse was true for interglacial periods. With the exception of the G-I cycles in the upper 20 m of Site 697 core, which were correlated with Marine Isotope Stages 10-1, the chronostratigraphy did not allow accurate dating of the Early and Middle Pleistocene, G-I cycles.

An enigmatic, 2.5 m thick interval of foraminiferal ooze, bearing coccolith assemblages occurred in a number of Leg 113 cores, and implied an interval of warming and depression of the lysocline. While the age of this calcareous unit remains poorly constrained in the Weddell Sea region, it may correlate with a similar unit reported from Prydz Bay and Ross Sea sediment cores and dated at ~ 1 Ma (see below).

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