The Marine Record of the West Antarctic and Antarctic Peninsula Ice Sheets

The late Cenozoic glacial history of West Antarctica is archived in the sedimentary sequences of its continental margins. During the last 20 years, multichannel seismic data were acquired from the margins of the Weddell Sea, the Antarctic Peninsula, the Bellingshausen and Amundsen seas, and the eastern Ross Sea. DSDP Legs 28 (Ross Sea margin) and 35 (Bellingshausen Sea) and ODP Legs 113 (Weddell Sea) and 178 (Pacific margin of the Antarctic Peninsula) recovered sedimentary sequences spanning the Neogene to Quaternary. In the following, we summarize the main results of these studies and their implications for the history of WAIS and Antarctic Peninsula Ice Sheet (APIS) from the Middle Miocene to Pliocene.

Seismostratigraphic studies suggest that an early stage of glaciation characterized by presence of local ice caps and tidewater glaciers had affected the Ross Sea region already during the Late Oligocene and the Early Miocene (De Santis et al., 1995, 1999; Anderson and Shipp, 2001). Glacial and glaciomarine sediments interbedded with fluvial to deep-water mud-stones were deposited on the innermost part of the shelf (CIROS-1 drill site), while open-marine sedimentation dominated on the outer shelf (Abreu and Anderson, 1998; Anderson and Shipp, 2001). This depositional pattern points to cool temperate interglacial conditions, but gives no direct evidence for presence of the WAIS (Abreu and Anderson, 1998; Anderson and Shipp, 2001). Based on seismic-stratigraphic investigations and sedimentary records drilled at DSDP Sites 270 and 272, De Santis et al. (1999) inferred that a glacial transitional regime developed in the Ross Sea region during the Early and Middle Miocene. Most of the subglacial erosion and deposition still took place on the inner shelf, but occasionally local ice caps expanded onto the outer shelf. Around this time, the first significant amounts of IRD were deposited in the Bellingshausen Sea (DSDP Leg 35, Sites 323 and 325) and the Weddell Sea (ODP Leg 113, Sites 693 and 694) (Abreu and Anderson, 1998). The onset of IRD sedimentation at Site 325 points to an establishment of West Antarctic ice caps outside the Ross Sea region during the Early to Middle Miocene (Hollister et al., 1976; Abreu and Anderson, 1998; Barker et al., 2002).

By analysing Mg/Ca ratios of calcareous benthic and planktonic foraminifera tests, Lear et al. (2000) and Shevenell et al. (2004) demonstrated that ice-sheet build-up in Antarctica between 14.2 and 13.8 Ma contributed to the pronounced, global increase in 818O ratios in benthic foraminifera tests observed by Zachos et al. (2001), even though the related eustatic sea-level fall was rather indistinct (Abreu and Anderson, 1998; Miller et al.,

2005). While most of the Middle Miocene ice-volume increase may have occurred in East Antarctica and possibly the Arctic (Moran et al., 2006), seismic studies also point to an expansion of ice caps in West Antarctica or their coalescence to a WAIS smaller than present (Abreu and Anderson, 1998; Anderson and Shipp, 2001). In the Ross Sea region, seismic data and sedimentary records reveal that grounded ice advanced at least five times across the shelf (Chow and Bart, 2003). A comparison with foraminiferal oxygen isotope and sea-level data shows that at least two of these grounding events fall into the early part of the Middle Miocene (Bart, 2003). Bart and Anderson (1995) proposed on the basis of seismic profiles from the western Antarctic Peninsula outer shelf that glacial erosional surfaces formed during the Middle to Late Miocene, when huge sediments drifts started to grow on the adjacent continental rise as indicated by seismic investigations (Rebesco et al., 1997, 2002). Bart et al. (2005) combined seismic data from the western Antarctic Peninsula shelf with sedimentological data from ODP Leg 178 Site 1097 drilled on the shelf, and concluded that the earliest grounding events on the Antarctic Peninsula shelf are of Late Miocene age. Seismic studies of the sedimentary development of the Crary Fan in the southeastern Weddell Sea indicate a minimum of 5 long-term and 14 small-scale ice-sheet expansions across the adjacent shelf from the Middle Miocene to the Pleistocene (Anderson and Shipp, 2001, and references therein). However, present-day ice-drainage patterns raise the question of whether or not WAIS expansion made a major contribution to the inferred ice advances (cf. Bart et al., 1999). Despite episodic ice expansion in parts of West Antarctica during the Middle Miocene, benthic and neritic planktonic diatoms, which were reworked from the Antarctic shelf and deposited in the central Weddell Sea (Site 694), point to the absence of a major WAIS or APIS from the Middle Miocene to the early part of the Late Miocene (Kennett and Barker, 1990; cf. Chow and Bart, 2003).

Widespread glacial unconformities assumed to be Late Miocene in age are observed in seismic profiles from the eastern Ross Sea margin (Cooper et al., 1991; De Santis et al., 1995; Anderson and Shipp, 2001). The grounded ice advances were accompanied by the deposition of glacial and glaciomarine strata (DSDP Leg 28 Site 270 and CIROS-1), which might exhibit an interior West Antarctic provenance (Anderson and Shipp, 2001) and which are interbedded with meltwater deposits and diatom oozes documenting temperate interglacial episodes (Abreu and Anderson, 1998). De Santis et al. (1999) correlated seismic data with sedimentary records drilled at DSDP Leg 28 sites and proposed an onset of fully glacial conditions in the Ross Sea region during the latest Miocene and Early Pliocene. A large and thick WAIS advanced up to the shelf edge (Accaino et al., 2005) and gave the

Ross Sea shelf its typical modern seabed morphology, which is characterized by landward dipping, overdeepening and troughs carved by grounded ice streams (De Santis et al., 1999). Shelf progradation observed in seismic profiles west of the Antarctic Peninsula indicates a general expansion of APIS assumed to be Late Miocene to Early Pliocene in age (Larter and Barker, 1991; Larter and Cunningham, 1993; Bart and Anderson, 1995; Larter et al., 1997; Barker et al., 2002; Bart et al., 2005). Similar shelf progradation is reported from seismic profiles elsewhere on the continental margin in the Bellingshausen and Amundsen seas although the age of the prograding sequences in these areas is unconstrained (Nitsche et al., 1997, 2000; Scheuer et al., 2006). Episodic advance and retreat of grounded ice across the eastern and western Antarctic Peninsula shelf throughout the latest Miocene and Early Pliocene are evident from seismic unconformities (Sloan et al., 1995; Larter et al., 1997; Bart and Anderson, 2000; Bart, 2001; Bart et al., 2005) and the onset of shelf overdeepening west of the Antarctic Peninsula (Camerlenghi et al., 2002). Recovery of sediments deposited in subglacial and glaciomarine settings from the western Antarctic Peninsula shelf (ODP Leg 178 Sites 1097 and 1103) confirm episodic APIS grounding events at least since the Latest Miocene to Early Pliocene (Barker et al., 1999a, 2002; Eyles et al., 2001). The general APIS expansion during the Late Miocene to Early Pliocene coincided with a northward shift of the Antarctic Circumpolar Current (ACC) (Hernandez-Molina et al., 2006), pronounced growth of large sediment drifts (Rebesco et al., 1997), clay-mineral fluctuations (Hillenbrand and Ehrmann, 2005) and distinct maxima in accumulation rates of terrigenous detritus (Grutzner et al., 2005) on the western Antarctic Peninsula rise. The sedimentary development of the drifts suggests episodic, enhanced supply of glacial-sourced sediments from the adjacent shelf from ca. 9.4 Ma onwards. Considerable growth of APIS and WAIS during the Late Miocene is also indicated by the onset of IRD deposition and sedimentation of a clay mineral assemblage pointing to the dominance of physical weathering in the South Orkney region (Site 696) and of rapid turbidite deposition with a West Antarctic provenance in the central Weddell Sea (Site 694) (Kennett and Barker, 1990). Anderson and Shipp (2001) infer that during the Late Miocene, the WAIS reached its modern size and, at times, was even larger than today. The seismic and core data from the western Antarctic Peninsula margin mentioned above suggest a similar scenario for the APIS.

At the Miocene/Pliocene boundary, the deposition of IRD at Site 325 increased (Hollister et al., 1976), while the first significant deposition of IRD started at Site 322 near Drake Passage (Abreu and Anderson, 1998). Kennett and Barker (1990) attributed a shift from pelagic to hemipelagic deposition at Site 696 and from turbiditic to hemipelagic sedimentation at Site 694 during the earliest part of the Pliocene to the development of a stable APIS and WAIS or climatic warming in West Antarctica. Seismic stratigraphy combined with sedimentological data from DSDP Leg 28 drill cores indicates that the eastern Ross Sea shelf reached a late fully glacial stage with an overdeepened topography, which was mainly influenced by glacial processes of erosion and deposition resulting in relatively low sedimentation rates (De Santis et al., 1999). Throughout the Early Pliocene, high-frequency grounding events are observed in seismic profiles from the eastern Ross Sea shelf and the western Antarctic Peninsula shelf (Alonso et al., 1992; Larter and Cunningham, 1993; Larter et al., 1997; Bart and Anderson, 2000; Bart, 2001). Cyclicity in drift sedimentation recorded in drill cores from the Antarctic Peninsula rise confirms that ice streams repeatedly advanced to the shelf break throughout the Pliocene (Barker et al., 2002; Hillenbrand and Ehrmann, 2005; Hepp et al., 2006). During the Early Pliocene, sedimentation rates increased in the deep Bellingshausen Sea (DSDP Leg 35 Sites 322, 323 and 325) and the Weddell Sea (ODP Leg 113 Sites, 693 and 697) (Barker, 1995). On the western Antarctic Peninsula continental rise, down-slope transport exceeded along-slope transport throughout the Pliocene (Hernandez-Molina et al., 2006), and the sediment supply from the shelf enhanced and maintained the sediment drifts (Rebesco et al., 1997, 2002). Seismic studies of the continental margin in the western Bellingshausen and Amundsen seas revealed truncations of foreset reflectors and extensive progradation, probably during the Pliocene (Nitsche et al., 1997, 2000; Scheuer et al., 2006), suggesting that grounded ice eroded the shelf and transported the terrigenous detritus across the shelf edge. According to Scheuer et al. (2006), the accumulation rate of terrigenous sediment on the adjacent continental rise reached its maximum during the Pliocene and Quaternary, which may indicate a relatively late establishment of WAIS in the adjacent hinterland (cf. Nitsche et al., 1997). In contrast, sedimentation rates on other parts of the West Antarctic margin reached distinct maxima during the Late Miocene (Site 1095; Barker et al., 2002) or during the Early Pliocene (Barker, 1995).

During the Late Pliocene, sedimentation rates decreased at the drifts on the Antarctic Peninsula rise (ODP Leg 178, Sites 1095, 1096 and 1101; Barker et al., 1999a, 2002), in the deep Bellingshausen Sea (DSDP Leg 35, Sites 322, 323 and 325; Barker, 1995) and in the Weddell Sea (ODP Leg 113, Sites 694 and 697; Kennett and Barker, 1990; Barker, 1995). A concomitant transition from progradation to aggradation is observed in seismic profiles from elsewhere on the West Antarctic margin, from the Antarctic Peninsula to the eastern Ross Sea (Larter and Barker, 1991; Larter and Cunningham, 1993;

De Santis et al., 1995; Larter et al., 1997; Nitsche et al., 1997, 2000). In contrast to these findings, Rebesco et al. (2006) compared changes in the geometries of seismic reflection profiles and concluded that enhanced progradation of the continental shelf and slope started almost all-around Antarctica at ca. 3 Ma. These authors suggest that APIS, WAIS and EAIS switched from a predominantly polythermal to a predominantly cold-based mode at this time of major ice-sheet build-up in the Northern Hemisphere (cf. Barker, 1995).

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