During Leg 178, an almost complete upper Miocene to Quaternary sedimentary sequence with a thickness of 554 m was recovered at Site 1095 on the distal flank of Drift 7. A complete and high-resolution lower Pliocene to Quaternary sequence with a thickness of 673 m was recovered at Site 1096 near the crest of this drift (Fig. F1; Table T1). At both sites, the recovered sediments generally consist of thin-bedded, bioturbated to massive, diatom-bearing muds alternating with thick-bedded, terrigenous, mainly fine-grained laminites (Barker, Camerlenghi, Acton, et al., 1999). The intervals with higher biogenic contents were interpreted as hemipelagic sediments deposited during interglacial periods, whereas the laminites were interpreted as distal turbidites and contourites deposited during glacial periods (Barker, Camerlenghi, Acton, et al., 1999). At distal Site 1095, turbidites are more abundant than at drift-crest Site 1096. The presence of ice-rafted debris (IRD) throughout the Neogene and Quaternary sediments indicates that they were deposited in a glaciomarine environment.
At Sites 1095 and 1096, multiple holes offset in depth were drilled to ensure recovery of a continuous sedimentary sequence (Table T1). All depth information (in meters below sea floor [mbsf]) given for the holes drilled at Sites 1095 and 1096 were recovery corrected and converted to meters composite depth (mcd) using the splices given by Barker, Camerlenghi, Acton, et al. (1999). We note that use of the revised composite depths given for Sites 1095 and 1096 by Barker (Chap. 6, this volume) does not influence our results significantly. Clay mineral assemblages were analyzed on 192 samples from Site 1095 and 119 samples from Site 1096 (Table T1), yielding an average sampling interval of 2.5 m for Site 1095 and 5.0 m for Site 1096. Corresponding average temporal resolutions of our sample sets are 48 k.y for Site 1095 and 39 k.y for Site 1096. We also analyzed 20 samples from Site 1097 in Marguerite Trough, from which a spot-cored Pliocene sequence of diamictons and glaciomarine muds was recovered (Fig. F1; Table T1) (Barker, Camerlenghi, Acton, et al., 1999).
For characterization of regional clay mineral distribution patterns in surface sediments west of the Antarctic Peninsula, we used published data from Petschick et al. (1996) and Diekmann et al. (2000). Clay mineral data from gravity core PS1565, located at the seaward termination of Drift 3 (Fig. F1), are presented to characterize changes in clay mineral assemblages during the last climatic cycle. These data have previously been published by Hillenbrand (2000).
All raw data presented in this study are available from the PANGAEA data bank of the Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany (http://www.pangaea.de).
Sample preparation and clay mineral analyses followed methods given in Ehrmann et al. (1992) and Petschick et al. (1996). Bulk-sediment samples were oxidized and disaggregated by means of a 3%-10% H2O2 solution. After sieving the samples through a 2-mm and a 63-µm mesh, the clay fraction (<2 µm) was separated from the silt fraction in settling tubes (settling time based on Stokes' Law). Dried gravel, sand, silt, and clay fractions were weighed, and then the relative proportion of the clay fraction was calculated. Clay (40 mg) was dispersed in an ultrasonic bath and mixed with 1 mL of an internal standard consisting of a 1% MoS2 suspension. The samples were mounted as texturally oriented aggregates by rapidly filtering the suspension through a membrane filter of 0.15-µm pore width. The filter cakes were dried at 60°C and mounted on aluminum tiles. They were exposed to ethylene glycol vapor at a temperature of 60°C for ~18 hr immediately before the X-ray analyses. The measurements were conducted on an automated powder diffractometer system Philips PW1700 with CoK
radiation (40 kV, 40 mA). The samples were X-rayed in the range 2°-40°2
with a scan speed of 0.02°2/s. Additionally, the range 28°-30.5°2 was measured with a step size of 0.005°2 in order to better resolve the (002) kaolinite peak and the (004) chlorite peak. The X-ray diffractograms were evaluated on an Apple Macintosh personal computer using the "MacDiff" (version 4.0.5) software (freeware available from http://servermac.geologie.uni-frankfurt.de/HomePage.html).
This study concentrates on the main clay mineral groups smectite, illite, chlorite, and kaolinite. These clay minerals were identified by their basal reflections at ~17 Å (smectite), 10 and 5 Å (illite), 14.2, 7, 4.72, and 3.54 Å (chlorite), and 7 and 3.57 Å (kaolinite). Semiquantitative evaluations of the mineral assemblages were made on the integrated peak areas. The relative percentages of smectite, illite, chlorite, and kaolinite were determined using empirically estimated weighting factors (Biscaye, 1964, 1965; Brindley and Brown, 1980). No effort was made to quantify mixed-layer clay minerals.
Age models for Sites 1095 and 1096 are based on the magnetostratigraphic datums published in Barker, Camerlenghi, Acton, et al. (1999) using the timescale and polarity history nomenclature of Berggren et al. (1995). Ages for the individual samples were calculated by linear interpolation between the datum levels. Computed linear sedimentation rates slightly differ from the sedimentation rates given by Barker, Camerlenghi, Acton, et al. (1999), because we refer the paleomagnetic ages to recovery-corrected composite depths. The age model for Site 1096 was refined by including the Reunion Event (Chron 2r.1n), which was identified in Core 178-1096C-2H (G. Acton, pers. comm., 1999). At Site 1095, we considered a hiatus within the upper Pliocene sedimentary sequence as suggested in Barker, Camerlenghi, Acton, et al. (1999). The shipboard paleomagnetic data indicate that this hiatus spans the earliest Matuyama Chron (C2r) and the onset of the Olduvai Event (C2n). Assuming nondeposition or erosion for the corresponding time period between 2.581 and 1.770 k.y., we calculated a linear sedimentation rate of 5.0 cm/k.y. for the late Gauss Chron (C2An.1n). This value agrees well with the sedimentation rate of 4.8 cm/k.y. calculated for the sediments deposited during the late Matuyama Chron.
