In his Comment, Rob Larter (2007) notes that the main change in stratal geometry within the Pacific margin of the Antarctic Peninsula (PMAP) occurred at the S2–S3 boundary, earlier than we estimated in our 2006 Geology article. Moreover, he argues that a synchronous, continent-wide change in basal ice conditions on Antarctica is unlikely.
We agree with Larter that a change in stratal geometry on the continental shelf of the PMAP occurred between 6.12 and 7.94 Ma at the S2–S3 boundary, which separates aggrading sequences with gently curved paleo-shelf breaks (below) from prograding sequences with sharp paleo-shelf breaks (above). This change, identified and defined by Larter and Barker (1989, 1991), was interpreted by Larter et al. (1997) as the transition from “pre-glacial” to glacial-marine deposition (the so-called onset of glacial sedimentation on the PMAP). This interpretation, however, was later discounted by the results of Ocean Drilling Program (ODP) Leg 178 drilling (Barker et al., 2002), which showed the glacio-marine nature of both S2 and S3 sequences.
Conversely, the architectural change we considered on the continental shelf is a major unconformity between sequences of similar geometry (sequences with sharp paleo-shelf breaks above and below). Therefore, we cannot share Larter's inference that the change in stratal geometry at the unconformity that we highlighted is typical of the S2–S3 boundary, or that the actual S1–S2 boundary, identified in Fig. 2 of our article, is to be in a higher stratigraphic position. In fact, not to argue the assumption that gently curved paleo-shelf breaks are limited to S3, we note that gentler “transitional” paleo-shelf breaks identical to those ascribed in our article to S2 were already ascribed to S2 by Larter et al. (1997) and Larter and Barker (1991). Therefore, the interpretation of the S1–S2 and S2–S3 boundaries in Fig. 2 of our article is consistent with that of seismic Line AMG845–03 of Larter et al. (1997; see their fold-out Fig. 5)
The relevance of the S1–S2 unconformity becomes much more evident in front of glacial troughs, where the erosion cuts into the base of the two underlying sequences, S2 and S3 (see Figure 1 of our article). One of the points made in support of our interpretation is that, unlike the S2–S3 boundary, the S1–S2 boundary is of margin-wide significance on the PMAP as it correlates with other pronounced changes in the continental slope (regional downlap surface) and rise (decrease in accumulation rate).
We are confident that the unconformity we highlighted is indeed the S1–S2 boundary. This interpretation is supported by careful correlation within the available seismic grid across the entire margin. This boundary is recognized in most profiles crossing the outer shelf, where the four seismic sequences S1 to S4 established by Larter and Barker (1989, 1991) are preserved. Larter et al. (1997) defined S1 to S4 ~150 km northeast of the profiles that we presented, and named A1 to A4 the comparable (but not directly correlatable) sequences identified over 300 km southward. The results of the ODP Leg 178 drilling and the seismic profiles that we discussed permit us to tie the S1–S2 boundary to ODP Site 1101 and the A1–A2 boundary to ODP Site 1096. The two unconformities are indeed coeval.
In regard to less specific considerations, we agree with Larter's Comment that the past ice sheet was probably heterogeneous in ice flow and basal regime. This is what we suggested in distinguishing the conditions “at the bases of a few large ice streams” from those on the adjacent banks. Moreover, we did not explicitly define as synchronous the inferred continent-wide change in basal ice conditions. What we suggested is that similar changes in the architecture of the margin—beneath the continental shelf, slope, and rise—are observable in the Pliocene in the key sectors of the West and East Antarctic margin, and dated to nearly 3 Ma in the PMAP. A recent stratigraphic review by Volpi et al. (2007) based on seismic-aided log-to-log correlations following the drilling of ODP Leg 188 in Prydz Bay (Cooper et al., 2004) refines the constraints for the change we observed on the Prydz Bay margin, suggesting that it may well have an age comparable to that on the PMAP. We suggested that the changes in the architecture of the margin occurred at about the same time as the climatic change that generated the Northern Hemisphere glaciation. Such climatic change is not particularly abrupt, as it results from the stable isotope curves (e.g., Zachos et al., 2001) or from the pre-dating of the Northern Hemisphere ice sheet proposed by Moran et al. (2006) following IODP drilling in the Arctic Ocean.