Figures 2 and 3 respectively show the bathymetry and the distribution of the main depositional features along the Antarctic Peninsula margin. On the shelf, a mid-shelf high running continuously along the margin and discontinuous mid-shelf basins inside it are relics of subduction and ridge crest collision (Fig. 3). The volcanics and plutonics of the central spine of the Peninsula have been dissected by glacial erosion. At present, the ice cover on the Peninsula is thin (a few hundred meters at most), and the grounding line lies at the heads of the numerous overdeepened fjords. Around glacial maxima, the ice was grounded over most or all of the continental shelf, and shallow troughs draining the interior transported basal till to four depositional lobes, L1-4 (trough mouth fans), that have extended the shelf edge.
Sites APRIS-01A and 02A
The present slope of the depositional lobes L1 to 4 (Fig. 7) is steep (15°-20°) and is assumed to be at the limit of stability. A GLORIA survey of the northeastern area of the rise (Tomlinson et al., 1992; Rebesco et al., 1996) and deep-tow boomer examination of the upper slope (Vanneste and Larter 1995) show small-scale dissection of the upper slope and a dendritic pattern of channels at the base of slope, which feed major channels heading northwestward toward the lower continental rise (Figs. 2, 3 and 6). Between the major channels lie large depositional mounds rising more than 1 km above the channel floors. These mounds are thought to have formed as the result of ambient bottom-current entrainment of suspended fines from the many small-scale turbidity currents that "drain" the continental slope via the channels (Rebesco et al., 1994, 1996; McGinnis and Hayes, 1994, 1995). The mounds are fine-grained hemipelagic sediment drifts. Those within the GLORIA survey area (Drifts D1-D4; Fig. 3) are clearly separated from the margin by tributary channels, and the larger drifts to the southwest are probably the same. It is doubtful if direct deposition of turbidites has contributed to the drift deposits, except at their distal extremity. Seismic reflection profiles show a remarkable similarity in reflection character of the separate drifts over distances of 400 km or more.
The drifts are likely to provide a fine-grained equivalent of the slope foreset record of glacial history, provided that the residence time of the unstable component of upper slope deposition is short compared with the glacial cycle, and individual slope turbidites are small. Recent gravity coring on Drift 7 (Camerlenghi et al., in press) confirms these conditions: a biogenic mud at the core top (rich in diatoms and radiolaria with benthic and planktonic foraminifers) overlies barren, laminated glacial silty clays above another biogenic mud (interglacial Stage 5). Average sedimentation rate is 3.5 to 5 cm/k.y. All the evidence indicates that the drifts provide a viable high-resolution record of glacial history. Leg 178 will drill one drift at two sites (Sites APRIS-01A and 02A on Drift 7). Site APRIS-01A is at the southeast end of the drift, proximal to the margin, and Site APRIS-02A is a distal offset site designed to penetrate deeper in the section, where it is thinner, in the event that silica diagenesis at the prime site eliminates biostratigraphic control (there is a silica diagenetic bottom simulating reflector [BSR] at about 600 m within Drift 7).
In summary, these seven sites will sample the 6-10 m.y. of Antarctic Peninsula glacial history in all three primary depositional environments (shelf topsets, slope foresets, rise drift). The rise sites (Sites APRIS-01A and 02A) will provide a high-resolution record that will serve as a reference for the shelf sites (Sites APSHE-01A to 05A), allowing correlation of the major changes in prograding wedge geometry and assisting in the interpretation of all three records in terms of climate change. The result will be a high-resolution record of glaciation through a period when sea level change has been (for Antarctica) imposed from outside (i.e., Northern Hemisphere glaciation over the past 0.8 and/or 2.5 m.y.), to a preceding regime in which the main contribution to sea level change was from Antarctic glaciation itself, and when the isotopic signal of insolation change was of lower amplitude and shorter period (Tiedemann et al., 1994; Shackleton et al., 1995). A major global sea-level drop occurred at 10 Ma, and there is controversy over the climate of the warm Pliocene in the Antarctic and over the depth of the preceding late Miocene cool period. Through most of the past 10 m.y., the contribution of ice-volume change to the isotopic signal is unknown. This will be the first Antarctic high-resolution record of any kind. In addition, the core recovered from Leg 178 will greatly improve our understanding of the potential and limitations of the main glacial depositional environments.
Sites APSHE-01A to 04A
Figure 7 shows a section through Lobe 1, along our prime drilling transect that consists of proposed Sites APSHE-01A to 04A. Seismic sequence groups S1 and S2 (Larter and Barker, 1989, 1991b) are considered to have been produced by ice-stream transport during ice-sheet grounding to the shelf edge over the past 5 m.y. or so. Glacial deposition is largely confined to the lobes and is discontinuous between them, so strict correlation between sequence groups cannot be made. However, sequence group geometries virtually identical to S1 and S2 are seen within Lobes 2 to 4. S1 is moderately progradational, with minor versions of the features that dominate and distinguish S2 -- the erosional truncation of foresets at their upper boundary. The main aim of the drilling transect is to date the major components of this characteristic geometry: the beginning of S2, the beginning and end of the episode of truncation with which it ended, and the assumed continuous foreset deposition in S1. It should also be possible to characterize quite fully the lateral coherence and degree of discontinuity of topset deposition within both S1 and S2.
Beneath sequence group S2 is sequence group S3 (Fig. 7) that, except in the northeast, is clearly post-collisional, but is different from S1 and S2 as it is much more continuous along the margin, is parallel-bedded down-dip (lacking a clear paleoshelf break), and either pinches out or is truncated at its down-dip end. In many of its characteristics, it resembles a sequence found elsewhere beneath the glacial prograded wedge (the Type IIA of Cooper et al., 1991). We have called S3 "pre-glacial," but that is a shorthand term as it most probably reflects an earlier or transitional stage of glacial deposition before ice sheets regularly extended to the shelf break. This sequence will be sampled at proposed Site APSHE-05A, between Lobes 3 and 4 (Fig. 3), where it is more accessible and clearly separated from collisional tectonics. Sequence group S4 is pre- and syncollisional, and in general its erosionally truncated upper boundary reflects collision-related uplift. If the S3/S4 boundary were to be sampled in the northeast, it would show pronounced collision-related unconformity; but here there is the possibility that S3 sediment represents a period sufficiently long that its basal sediments are more clearly pre-glacial and conformable on S4. The full depth of penetration at Site APSHE-05A is uncertain.
Proposed Site APSHE-13A (Proposal 502 by E. Domack) is located in the Palmer Deep on the inner continental shelf directly south of Anvers Island (Leventer et al., in press). It lies in one of three linked basins that contain an ultra-high-resolution Holocene record of Antarctic Peninsula climate. Short piston cores from this basin show a pronounced 200-300 yr periodicity in paleoproductivity that is also seen in some Antarctic Peninsula fjords. This region is particularly interesting because of its current apparent sensitivity to climate change. The expanded, presumed pelagic section may be compared with recently-acquired records from low and intermediate latitudes (Santa Barbara Basin, Saanich Inlet, Cariaco Basin) and ice-core records from Greenland and Antarctica, to examine decadal and millennial variability on a global scale. This record may provide opportunities to examine magnetic secular variation and, for the inshore environment, the time variability of the 14C "reservoir effect," which is large but uncertain for waters south of the Polar Front.
To 178 Drilling Strategy, Logging, Underway Geophysics, and Sampling Plan
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