The Antarctic Peninsula has been a major active margin for at least 200 m.y., with subduction of Pacific oceanic lithosphere beneath continental lithosphere along the west coast of the peninsula. As a consequence of a series of spreading center-trench collisions, subduction ceased during the Cenozoic progressively northward, from ~50 Ma in the southern sector of the margin to ~4 Ma in the northern part (Barker, 1982). After cessation of subduction, magmatism continued with intraplate basaltic rocks erupted from centers scattered along the whole Antarctic Peninsula (e.g., Hole, 1990; Smellie, 1999). This long-lived igneous activity makes the Antarctic Peninsula a region where the magmatic record may be used to place timing constraints on paleoenvironmental evolution during the Cenozoic.
Analogous to other continental margins of the world, the Antarctic margin records a long-term history of Cenozoic oceanographic changes and sedimentary processes. Ocean Drilling Project (ODP) Leg 178 was the first drilling campaign of the JOIDES Resolution aimed at the reconstruction of the history of the Antarctic ice sheet, as proposed by ANTOSTRAT (Antarctic Offshore Acoustic Stratigraphy) initiative (Barker and Camerlenghi, 1999) (Fig. F1). However, the presence of a polar ice sheet as the main agent of sediment erosion, transport, and deposition introduces elements of uncertainty into the interpretation of the drilling record. The distribution of sedimentary bodies on the continental shelf is not primarily controlled by sea level, because the waxing and waning of the ice sheet has overdeepened the seafloor surrounding the continent to depths in general greater than 500 m, well outside the range of Cenozoic eustatic changes. The sediments do not generally contain a continuous record of microfossils. Periodic advances of grounded ice sheets alternate erosion to deposition and the sediments deposited are mostly unsorted terrigenous sediments (diamicts).
During ODP Leg 178, we drilled the Pacific margin of the Antarctic Peninsula based on seismic stratigraphic information collected over the years. It was recognized, as it turned out, that (1) large sediment drifts on the proximal continental rise contain a continuous and expanded sedimentary record sensitive, indirectly, to ice sheet fluctuations on the continental shelf; (2) the continental shelf contains a discontinuous sedimentary record as direct evidence of past ice sheet fluctuations; and (3) the inner shelf overdeepened sedimentary basins preserve an ultra high resolution paleoceanographic record since the last glacial maximum, at least (Barker, Camerlenghi, Acton, et al., 1999).
A drastic change in seismic unit geometry and internal configuration identified on the continental shelf was postulated to indicate the transition from preglacial to glacial sedimentation on this part of West Antarctica. The most recent progradational and aggradational units developed above widespread unconformities and formed a sedimentary wedge with extraordinarily steep continental slope were believed to represent the glacial evolution of the margin (Units S1 and S2 in Fig. F2) (Larter and Barker, 1991; Larter et al., 1997). The underlying lens-shaped Unit S3, downlapping on a preexisting erosional surface, displaying subparallel reflectors, moderate divergence, and overall aggradational pattern, was instead believed to represent preglacial conditions.
Drilling at Sites 1097 and 1103 revealed instead that most of Unit S3 (the base of S3 was not reached by drilling) is composed of glaciogenic sediments (Shipboard Scientific Party, 1999b, 1999c). This finding allows us to relocate at deeper stratigraphic levels the so-called "onset of glaciation" on the northern Antarctic Peninsula. An effort to understand sedimentological and glaciological causes of such a different aspect between glacial Units S3 and overlying S1/S2 and to confine in time Unit S3 as accurately as possible with the available information obtained by drilling has been triggered by this finding. The age control of S3 is dependent, with a high degree of uncertainty, on biostratigraphy (Shipboard Scientific Party, 1999a) and strontium isotopic ratios of barnacles at one level of Site 1103 (Lavelle et al., Chap. 27, this volume), which both place S3 in the late Miocene.
In this chapter, we present in detail an attempt to obtain 40Ar-39Ar ages from volcanic clasts identified in the matrix of diamictites drilled within Unit S3 at Sites 1097 and 1103. These fragments were detected in generally low percentages as "volcanic glass" via smear slide description on board the JOIDES Resolution. Under normal conditions such grains would not attract interest for isotopic dating. However, we hypothesized that concentrations of volcanic grains resembling glass in the matrix of subglacial or ice-proximal diamictons could represent volcanic activity contemporaneous to deposition at the ice front or near-contemporaneous to erosion, transport, and deposition if the eruption happened on the continent just prior to the ice sheet advance on the continental shelf. We will show that in spite of analytical difficulties, volcanic clasts not necessarily concentrated in ash layers can provide an additional tool for chronostratigraphic reconstruction in glaciogenic sediments from the Antarctic margin.