Sediment mounds on the continental rise west of the Antarctic Peninsula receive fine-grained sediment and ice-rafted debris (IRD) directly from the continent, thus indirectly recording the history of West Antarctic glaciation. The objective of this study is to interpret this record at Site 1101, which contains a nearly continuous sedimentary section extending from the late Pliocene to Holocene. The terrigenous lithofacies at this site are controlled by variable sedimentary processes and rates that are determined by the proximity of the glacial source to the continental shelf edge. IRD is identified as the coarse-grained component ranging from >250 µm to pebbles. The presence of IRD indicates periods when glaciers extended to sea level and distributed sediment by icebergs.
Site 1101 is located distally within Drift 4 on the continental rise ~94 km from the shelf edge (Fig. F1) (Rebesco et al., 1997, 1998). This asymmetrically shaped sediment drift is flanked on both sides by turbidite channels that convey sediment originating from the continental shelf to the Palmer deep-sea fan. On the continental shelf, troughs were carved by thick, 100-km-wide fast-flowing ice streams that were grounded at the shelf edge during glacial maxima (Pope and Anderson, 1992; Bart and Anderson, 1995; Pudsey et al., 1994). The trough inshore of Drift 4 is flanked by two sediment lobes (Lobes 2 and 3 in Fig. F1), which were deposited under thin, slowly moving ice (Rebesco et al., 1998). Significant volumes of coarse-grained, unsorted sediment were moved through the troughs and were deposited on the continental slope when the ice streams were near the shelf edge. Failure of this unstable sediment and meltwater plumes discharging from beneath ice streams resulted in high sediment accumulation rates on the drifts. Once the ice sheet became unstable because of rising sea level and a negative glacial mass balance, rapid disintegration by iceberg calving is predicted because of the few pinning points on the continental shelf beyond the northwest coastline of the large islands (Larter and Cunningham, 1993). During interglacials, this terrigenous sediment supply to the continental rise is cut off because (1) the shelf is too deep to be reworked by wind-generated currents or storms and (2) the shelf deepens toward the continent.
The sediments at Site 1101 were divided into three lithostratigraphic units (see Barker, Camerlenghi, Acton et al., 1999, for core photographs and descriptions). Unit I (0-53.3 meters below sea floor [mbsf]) and Unit II (53.3-142.7 mbsf) are composed of biogenic-bearing homogeneous muds alternating with predominantly laminated terrigenous muds that are interpreted to have been deposited during interglacial and glacial periods, respectively. A major difference between Units I and II is the composition of the biogenic component. Unit I contains diatom-bearing interglacial layers, whereas Unit II contains foraminifer-bearing layers. Unit III (142.7-217.7 mbsf) lacks the regular alternation of biogenic and terrigenous intervals observed above. However, it shows a strong glacial influence in the deposition of ice-rafted diamicton.
This study uses the IRD record from the Antarctic Peninsula as one aspect of the glacial signal since 3.0 Ma. IRD deposition is controlled by many factors, including oceanic conditions, ice thermal regime, and bedrock erodibility, as well as the ice volume on the continent (Denton et al., 1991). This study follows an approach similar to that used by Grobe and Mackensen (1992) and heeds Anderson's (1999) recent warning that detailed sedimentologic investigations are necessary to interpret the IRD flux with respect to depositional processes and sedimentation rates that vary greatly. Site 1101 provides an opportunity to interpret the IRD mass accumulation rate (MAR) with respect to other sedimentary processes, because its location near the continent resulted in high sedimentation rates and a high-resolution record with an identifiable glacial-interglacial cyclicity.