Rebesco et al. (2006) present multichannel seismic (MCS) reflection profiles showing a regional change in the style of margin accretion on the Pacific margin of the Antarctic Peninsula (PMAP), and estimate an age of ca. 3 Ma for this change by correlation with Ocean Drilling Program (ODP) Site 1101. They also review seismic stratigraphy and drilling results from other Antarctic margins and suggest that changes in several places were associated with a transition from wet- to dry-based ice regimes that affected the whole ice sheet synchronously. The prominent shelf-slope unconformity on the PMAP highlighted by Rebesco et al., however, is significantly older than they estimate, and glaciological considerations make a synchronous, continent-wide change in basal ice conditions unlikely.
By tracing seismic reflections more than 100 km beneath the upper continental rise and slope, Rebesco et al. correlate sediments at ODP Site 1101 (aged 2.9 ± 0.2 Ma) with a continental shelf and slope unconformity that marks a regional change in the style of margin accretion. They interpret this unconformity as the boundary between sequence groups S1 and S2 in the stratigraphic scheme of Larter and Barker (1989, 1991). Regionally, however, the main change in stratal geometry on the PMAP occurred earlier, at the S2–S3 boundary (Larter et al., 1997; Hernández-Molina et al., 2006). Moreover, the change in stratal geometry at the unconformity highlighted by Rebesco et al. is typical of the S2–S3 boundary: aggrading sequences with paleo-shelf breaks that are gently curved in profile exist immediately below the unconformity, while above it, the sequences prograde, have sharp paleo-shelf breaks, and have forset reflections that downlap onto the unconformity. S1 to S3 were originally defined on a network of MCS profiles centered ~150 km northeast of the profiles presented by Rebesco et al. S1 pinches out at the seafloor when traced along shelf seismic profiles running southwest from that area; so it may be thin or absent on the shelf in the area in question. Indeed, the sequence boundary with a paleo-shelf edge 1 km landward of the modern shelf break on Line 281 of Rebesco et al. exhibits characteristics typical of S1–S2.
If the unconformity Rebesco et al. highlight is indeed the S2–S3 boundary, results from continental shelf ODP sites 1097 and 1103 indicate a late Miocene age. At Site 1097, ~200 km southwest of the MCS lines presented by Rebesco et al., the change in stratal geometry Larter et al. (1997) interpreted as corresponding to S2–S3 (unconformity 3.13 of Bart and Anderson, 1995) was drilled at 310 mbsf (Bart et al., 2005). Diatom zones identified at Site 1097 (Iwai and Winter, 2002) constrain the age of this boundary to be between 6.12 and 7.94 Ma. At Site 1103, in the area where S1 to S3 were originally defined, diatom zones (Iwai and Winter, 2002) and Sr isotope dating of barnacle fragments (Lavelle et al., 2002) constrain the youngest S3 sediments to be within the same age range, although S2 is absent at this site. The discrepancy between these ages and the age derived by Rebesco et al. through seismic correlation to Site 1101 may in part be due to the inescapable ambiguity in seismic correlation through the slope, resulting from extensive downlap of reflectors onto the unconformity in question. The discrepancy may also partly reflect the difficulty in tracing sequence boundaries through the zone of major facies changes at the base of the continental slope and beneath the erosionally dissected upper rise.
The occurrence of melting at the base of an ice sheet depends on basal temperature and pressure, which in turn are affected by a range of parameters including ice thickness, surface temperature, accumulation rate, geothermal heat flow, and amount of heating from internal strain and basal sliding. The variations in these parameters across Antarctica make it unlikely that there was ever a synchronous, continent-wide change in basal ice conditions. Thus, while climatic changes were probably the ultimate cause of changes in patterns and rates of sedimentation around Antarctica in the late Miocene and late Pliocene, a continent-wide change in basal ice conditions is an improbable mechanism. Observations of the modern ice sheet show much heterogeneity in flow (e.g., Bamber et al., 2000) and therefore in basal regimes. Thus, the past ice sheet was probably similarly heterogeneous. The discovery of a large number of paleo-ice streams on Antarctic margins in recent years supports this view.