Interpreted SEM images of the six analyzed samples are presented in Plate P2. Strontium isotope results are summarized in Table T1 and plotted in Figure F1. Four barnacle fragments were analyzed from Sample 178-1103A-28R-5, 93-95 cm (262.63 mbsf). Samples 1-3 show homogeneous carbonate and the moderate preservation of sheet nacre (Pl. P2, figs. 1-3), which produced a mean age of 7.4 (+1.5/-0.9) Ma. Sample 4 shows very well-preserved sheet nacre and produced a significantly older age of 12.2 (+1.2/-1.1) Ma. Two individual barnacle fragments were analyzed from Sample 178-1103A-28R-5, 127-129 cm (262.98 mbsf). Samples 5 and 6 reveal sheet nacre and homogeneous carbonate, respectively (Pl. P2, figs. 5, 6) and produced a mean age of 7.8 (+1.5/-1.1) Ma.
The SEM images reveal three preservational groups: homogeneous microcrystalline carbonate (Samples 1 and 6), moderately preserved sheet nacre (Samples 2, 3, and 5), and well-preserved sheet nacre (Sample 4). All SEM images were taken from an oblique vertical angle. Support for the 7.4- and 7.8-Ma best-fit ages are provided by independent biostratigraphic evidence. The underlying sediments between 320 and 355 mbsf are placed in the Actinocyclus ingens var. ovalis diatom zone (8.68-5.89 Ma) (M. Iwai, unpubl. data).
A cautious approach is taken in linking measured age to depositional age. Although the diamict is interpreted as a sediment gravity-flow deposit (Shipboard Scientific Party, 1999), the preservation of multiple articulated plates suggests that postdepositional transport of the barnacle fragments was minimal (see Plate P1). Bearing in mind the likely proximal proglacial environment, both specimens likely originate from the same biocoenosis. Even were this not so, the high sedimentation rates associated with proglacial Antarctic margin settings often result in minimally reworked ages that are within the uncertainty of the true depositional age (Lavelle, 2000). The most likely interpretation of our data is that Samples 1-3 and 5-6 represent approximate mean depositional ages of ~7.4 and ~7.8 Ma for sediments at 262.63 and 262.98 mbsf, respectively, and that Sample 4 (~12.2 Ma) represents analytical error. Unless additional shell fragments are discovered in the lower half of Hole 1103A, no further specific conclusions may be drawn.