Ross

The pioneering coring expedition of DSDP Leg 28 in the Ross Sea drilled four sites on the continental shelf: Sites 270-272 in the south eastern quadrant of the Ross continental shelf and Site 273 in the west-central portion of the shelf (Hayes and Frakes et al., 1975) (Fig. 11.1). While core recovery was low (~30%), a composite Pliocene-Pleistocene record of >200m was sampled at sites 270-272 comprising terrigenous silts and biosiliceous-bearing muds and diatomaceous ooze with ice-rafted debris throughout. A cyclostratigraphic interpretation was not reported for these sites, but a major glacial unconformity of inferred Late Pliocene age identified in Ross Sea seismic data, and correlated into Site 270, is interpreted as widespread westward expansion of the WAIS ice streams into the Ross Sea (Bart, 2004). Rebesco et al. (2006) have correlated this event with evidence elsewhere for continentwide development of a marine-based ice margin on the continental shelf, associated with development of a thicker, cold ice sheet similar to the present-day thermal regime.

Immediately prior to the coring of Leg 28 the Dry Valley Drilling Project (DVDP) recovered the first Pliocene-Pleistocene glacimarine sediment cores from the continent (McGinnis, 1981). A ~200m thick composite section based on DVDP Sites 10 and 11 in the Lower Taylor Valley, comprised a succession of ice-proximal Late Pliocene glacimarine diamictons, sands and gravels passing up-section into fluvio-delatic and beach sands of Late Pleistocene age (McKelvey, 1981; Powell, 1981). Lateral facies relationships and correlations between the two sites indicate that the down valley Site 10 was most proximal to the grounding line. This relationship together with the occurrence of McMurdo volcanic clasts in diamictons implied an eastern ice source during glacial advances associated with expansions of the WAIS (Porter and Beget, 1981). McMurdo volcanic-bearing diamictons occur above an unconformity at 203 m depth and 146 m depth in DVDP 11 and 10, respectively, corresponding to the proximity of the Gauss-Gilbert polarity transition (2.6 Ma) (Elston and Bressler, 1981). Thus, although some uncertainties surround the chronology of these cores remain, expansion of WAIS-sourced ice into the Dry Valleys is consistent with timing of Late Pliocene ice expansion elsewhere. Barrett and Hambrey (1992) report a Late Pliocene-Pleistocene succession of McMurdo volcanic-bearing tills in the upper 100 m of the CIROS-2 core from Ferrar Fiord. They interpret the succession to represent the onset of ice advance from the Ross/WAIS Ice Sheet at around 3-2.5 Ma.

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