The Polar Ice Sheet Early Pliocene Through Quaternary

This period includes seismic sequences RSS-7 and -8 (Fig. RS-3) and seismic units 1-7 of Alonso et al. (1992) and Anderson and Bartek (1992) (Fig. RS-4a and b). Pliocene sediment has been recovered at drill sites in the Taylor Valley (DVDP-10 and -11) and on the continental shelf at DSDP Sites 271 and 273. DVDP drill cores contain sediments deposited by the Taylor Glacier, while the sparsely sampled Pliocene deposit at DSDP 271 and 273 contains diatomaceous glaciomarine strata (Hayes and Frakes, 1975). These rocks imply that warmer interglacial conditions than today existed at that time (Anderson and Ashley, 1991).

Seismic sequences of inferred Pliocene through Quaternary age occur in the EAIS drainage in the Northern basin, where they are up to 800 m thick in the till delta fan system (Cooper et al., 1995). In the Eastern basin, Pliocene through Quaternary age strata lie within the WAIS drainage and are more than 1,000m thick (Cooper et al., 1995). Detailed seismic stratigraphic analyses from the Eastern basin margin (Fig. RS-4a; Alonso et al., 1992; Fig. RS-4b; Anderson and Bartek, 1992), recognize a major change in the seismic character of the Pliocene deposits. Up-section, the seismic unit thicknesses decrease, the geometry of the sequences changes from principally progradational to aggradational, and numerous widespread glacial erosion surfaces are seen. These features indicate more frequent grounding events on the continental shelf and increased subglacial till deposition relative to basal transport of sediments to the grounding line. Bart et al. (2000) and Anderson et al. (1992) suggest that on at least eight occasions during Pliocene to Quaternary times, the East and West Antarctic Ice Sheets were much larger than today. The frequent and extensive grounding events on the outer continental shelf contradict the widely held view that the land-based EAIS was relatively stable and the largely marine-based WAIS was relatively dynamic (Bart and Anderson, 2000).

The last glacial maximum (LGM) in the Ross Sea has been studied using seafloor cores, subbottom and swath bathymetry data (Thomas and Bentley, 1978; Kellogg et al., 1979; Denton et al., 1989; Leventer et al., 1993; Brambati et al., 1994; Hilfinger et al., 1995; Kellogg et al., 1996; Licht et al.,

A

P re-Pliocene RSU-2

P re-Pliocene RSU-2

Figure RS-4: Seismic-reflection profiles across the Eastern basin (A) and the Northern basin (B), illustrating the Neogene stratigraphic sections (from Anderson and Bartek, 1992). The shelf margin delta fan complex shown is a common feature in the seismic data from the continental margin and characterizes deposition close to the ice-sheet grounding line. See Fig. RS-1

for location.

1996; Cunningham et al., 1999; Domack et al., 1999; Licht et al., 1999; Shipp et al., 1999; Alley and Bindschadler, 2001). Radiocarbon dates from diamictons and sediment composition, indicate that ice-free conditions existed on the inner shelf at times during the period from 60 to 10 ka, and that the ice sheet was present between 26.5 and 19.5ka. (Domack et al., 1999). The maximum ice-sheet expansion in the LGM is still debated: Kellogg et al. (1996) place the grounding line at the continental shelf edge, Domack et al. (1999) put it just north of the Coulman Island and Licht et al. (1999) interpret that it was about 100 km south of Coulman Island. The last retreat of the grounding line occurred in the western Ross Sea around 11 ka at a rate of about 100m/year, and the grounding line reached its present position about 6ka (Domack et al., 1999; Shipp et al., 1999).

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