Last Glacial Maximum the Holocene to the Present Day 200 ka BP

Regardless of ice-sheet geometry at the last interglacial, there is persuasive evidence from the geological record to indicate that the Antarctic Ice Sheet was larger than present around the time of the global sea level lowstand at ~20ka BP, although the extent of this expansion is well constrained at only a few sites around the continental margin. Evidence of former ice surface elevations is particularly sparse, in part due to the lack of sites at which such information can be preserved, but also due to the difficulty in accessing remote inland mountain ranges where this evidence might occur. In the next sections we discuss glacial history by dividing Antarctica into seven sectors, based on lines of longitude (Fig. 12.2), which are approximately based on ice divides but centred on the geographical pole.

12.3.3.1. 0-60°E: Queen Maud land/Enderby land

Schirmacher Oasis (11.5°E) is a 34 km2 area located behind Novolazar-evskaya Ice Shelf. Infra-red stimulated luminescence dating of lake floor sediments yielded burial ages of ~52ka BP in Lake Glubokoye (Krause et al., 1997), which was in part corroborated by AMS 14C ages of 35 ka BP from 2m above the glacial diamict at the base of the lake sediments. On the basis of these data, Schirmacher Oasis was ice free throughout the LGM. On the northern side of Fimbulheimen, the mountain range to the south of Shirmacher Oasis, deposition of mumijo (proventricular ejecta of the snow petrel, Pagodroma nivea) throughout the LGM indicates that ice thickening in

Figure 12.2: Locations of the sectors discussed in this chapter; 1: 0-60oE Queen Maud/Enderby Land. 2: 60-750E Mac.Robertson Land/Lambert Gl/Prydz Bay. 3: 75-1500E Princess Elizabeth/Queen Mary/Wilkes Land. 4: 1500E-1500W Transantarctic Mountains/Ross Sea Embayment. 5: 150-850W Marie Byrd Land/Ellsworth Land and the Amundsen and Bellingshausen Seas. 6: 85-600W Antarctic Peninsula/Bellingshausen Sea.

7: 60-00W Weddell Sea/Filchner Ice Shelf/Coats Land.

Figure 12.2: Locations of the sectors discussed in this chapter; 1: 0-60oE Queen Maud/Enderby Land. 2: 60-750E Mac.Robertson Land/Lambert Gl/Prydz Bay. 3: 75-1500E Princess Elizabeth/Queen Mary/Wilkes Land. 4: 1500E-1500W Transantarctic Mountains/Ross Sea Embayment. 5: 150-850W Marie Byrd Land/Ellsworth Land and the Amundsen and Bellingshausen Seas. 6: 85-600W Antarctic Peninsula/Bellingshausen Sea.

7: 60-00W Weddell Sea/Filchner Ice Shelf/Coats Land.

this sector of the ice-sheet was also limited, with <80 m thickening occurring at the Insel Range (720S, 1TE) during this time (Hiller et al., 1995).

The well preserved 46-30 ka BP shorelines at Lutzow-Holm Bay (37^E) indicate that this site probably remained ice-free through the LGM, constraining the expansion of the ice-sheet in this area (Igarashi et al., 1995; Igarashi et al., 1998). Holocene shorelines are also present at this site, reaching at least 15 m above present sea level and dating from 8 ka to the present day (Maemoku et al., 1997; Miura et al., 1998), suggesting a decrease in regional ice load in this area since the LGM (Nakada et al, 2000).

Despite an abundance of small ice free areas that are suitable for Quaternary studies, including Riiser-Larsen (50.70E), 0ygarden Group (57.50E) and Stillwell Hills (59.30E), there is a paucity of information regarding the geometry of this sector of the ice-sheet during or following the LGM.

12.3.3.2. 60-75°E: Mac.Robertson Land/Lambert Glacier-Amery Ice Shelf/Prydz Bay

10Be and 26Al cosmogenic isotope exposure ages on glacial erratics show that the ice-sheet thickness at Framnes Mountains (62.5°E) was ~350m greater than present during the LGM, had begun lowering at 13 ka BP, and had reached the modern ice margin by 6ka BP (Mackintosh et al., 2007). This evidence agrees reasonably well with side-scan sonar/piston core data, which indicate that ice had retreated from the mid-outer continental shelf in Nielsen Basin ~ 80 km to the east of Framnes Mountains by ~ 11 cal ka BP and had reached the inner shelf by 6 cal ka BP (Harris and O'Brien, 1998).

Former ice advances of the Lambert Glacier-Amery Ice Shelf system (70°E) into Prydz Bay (Fig. 12.3) are relatively well constrained through geophysical investigations and marine sediment cores (Domack et al., 1998; Taylor and McMinn, 2002), and by drilling through the ice shelf (Hemer and Harris, 2003). The sedimentary record indicates that ice was grounded on shallow (<500m water depth) banks in Prydz Bay and across most of the area currently occupied by the Amery Ice Shelf at the LGM. However, the base of an uninterrupted sequence of glacial marine and biogenic sediments containing a relative abundance of organic material has been AMS radiocarbon dated to at least 30 ka BP within the Prydz Channel, a 700 m deep trench that cuts across the continental shelf in Prydz Bay (Domack et al., 1998). This suggests that grounded ice may not have extended all the way to the continental shelf at the LGM. Ages from acid-insoluble organic matter offshore from Amery Ice Shelf suggest a retreat timing of 11.5 cal ka BP for the Lambert Glacier-Amery Ice Shelf System (Domack et al., 1998).

Investigations into glacial and lake sediments deposited on mountains flanking the major outlet glaciers (Wagner et al., 2004; White and Hermichen, in press) also constrain the thickness of the ice sheet at the LGM. With the exception of the area around the southern tip of the modern Amery Ice Shelf, glacial sediments dating from the LGM are restricted to < 200 m above the modern ice margin. Also, there are no emergent shorelines around epishelf Beaver Lake (Adamson et al., 1997), but there are subaerially deposited postglacial sediments on the lake bottom at 60 m below modern sea level (Wagner et al., 2007). These data support the results from the continental shelf, and indicate that both thickening and expansion of ice in this region at the LGM was limited. One reason for this may be the deep basin along which the outlet glaciers flow, which reduces the ability of the ice-sheet to advance to the continental shelf edge (Taylor et al., 2004).

12.3.3.3. 75-150°E: Princess Elizabeth Land/Queen Mary Land/Wilkes Land

Larsemann Hills (76.2°E) hosts lake sediments dated to 14C background (>42ka BP), and lake sediment stratigraphy which has been used to infer continual exposure since the last interglacial (Hodgson et al., 2005).

An optically stimulated luminescence age of 21 ka BP was obtained from glaciofluvial sediments within 500 m of the ice margin (Hodgson et al., 2001), supporting interpretations of continual exposure through the LGM. Emergent shorelines are <3m asl, consistent with relatively minor former ice loading.

Vestfold Hills (78°E) has emergent marine shorelines to < 10 m asl (Zwartz et al., 1998), and cosmogenic 10Be analyses reveal that the ice margin had retreated to within 5 km of the present margin by 12.5-9 ka BP (Fabel et al., 1997). There has been a minor (<4km) readvance of ice along the flank of Sorsdal Glacier during the late Holocene (Adamson and Pickard, 1983; Gore, 1997a).

Gaussberg (89.2°E) is a 370 m high, glacially striated volcano on the coast. The benched morphology and presence of palagonite encrusted pillow lavas indicates that this eruption occurred in a water filled subglacial vault, and that the ice-sheet has since retreated to its present position since the eruption at 56 ka BP (Tingey et al., 1983).

Bunger Hills (101 °E) has emergent marine shorelines to <11m asl (Colhoun and Adamson, 1992; Colhoun et al., 1992). Optically stimulated luminescence ages from glacial lake shorelines and glaciofluvial sediments indicate that deglaciation commenced ~40-30ka BP (Gore et al., 2001), with the area largely deglaciated by ~25ka BP. Like Vestfold Hills, the oasis attained its present form around 11 ka BP. These data are corroborated by 14C ages indicating exposure through the LGM (Hedges et al., 1996, cited in Krause et al., 1997).

Windmill Islands (110.3°E) has emergent marine shorelines to 35 m asl (Goodwin, 1993; Goodwin and Zweck, 2000), with deglaciation of the southern islands by 11 cal ka BP and the northern peninsulas by 8 cal ka BP (Kirkup et al., 2002).

Offshore from the large Mertz and Ninnis glaciers (145-150°E) of George V Land swath bathymetry has been used to identify mega-scale glacial

Figure 12.3: (A) LGM ice-sheet surface reconstruction in Mac.Robertson Land and eastern Princess Elizabeth Land. Grey lines indicate present icesheet contours (in m asl), brown polygons indicate ice-free areas, and grey areas indicating ice-sheet flow >300 ma-1 (Joughin, 2002). Solid green lines indicate LGM ice-sheet contours derived from field studies, with investigated sites indicated by red circles (Domack et al., 1998; Harris and O'Brien, 1998; Zwartz et al., 1998; Hodgson et al., 2001; Taylor and McMinn, 2002; Whitehead et al., 2003; Leventer et al., 2006; K. Lilly, Pers. Comm., 2007; White and Hermichen, in press). Dashed lines indicate areas where heights or grounding lines are poorly known. (B) Profile through the cross-section AB.

lineations within the Mertz Trough which indicate LGM expansion of grounded ice to the outer continental shelf in this part of the ice-sheet (McMullen et al., 2006). Successive grounding zone wedge deposits have also been imaged in this region which record pauses in the retreat of the ice-sheet across the continental shelf, these however are yet to be conclusively dated (McMullen et al., 2006).

12.3.3.4. 150°E-150°W: Transantarctic Mountains/Ross Sea Embayment

During the 1990s a comprehensive geophysical and sedimentological dataset was compiled across the Ross Sea using seismic profiles, swath bathymetry and side-scan sonar imagery of the sea-floor sediments in front of the present day Ross Ice Shelf (Anderson,1999; Domack et al., 1999; Shipp et al., 1999). Lineations, drumlins and large-scale grooves provide a firm basis for the identification of former fast-flowing, marine based ice streams situated in bathymetric troughs which extend right to the edge of the continental shelf in places but only to the mid-outer parts of the shelf in others (Fig. 12.4). Sediment cores taken from these mega-scale lineations have retrieved

165Ë 170E 17SE 180 175W 170W 1ÊSW 160W

165Ë 170E 17SE 180 175W 170W 1ÊSW 160W

16SE 170E 175E 180 175W 170W 165W 160W

Figure 12.4: LGM paleodrainage map for the Ross Sea based on geomorphic features on the shelf and including data from Shipp et al. (1999). Arrows are flow directions based on lineations and the dashed line is the maximum grounding line position. Taken from Mosola and Anderson (2006), and reproduced with permission from Elsevier.

16SE 170E 175E 180 175W 170W 165W 160W

Figure 12.4: LGM paleodrainage map for the Ross Sea based on geomorphic features on the shelf and including data from Shipp et al. (1999). Arrows are flow directions based on lineations and the dashed line is the maximum grounding line position. Taken from Mosola and Anderson (2006), and reproduced with permission from Elsevier.

unconsolidated Plio-Pleistocene strata that are interpreted to have composed the readily deformable bed which allowed such extended ice streams to develop (Mosola and Anderson, 2006).

Lateral moraines deposited by the many glaciers flowing through the Transantarctic Mountains and into the Ross Sea during the LGM have very similar profiles. These moraines are located high above the present day ice surface at the feet of these glaciers but merge toward the modern-day surface of the ice-sheet when traced upstream. LGM thickening indicated by these moraines therefore increases towards the marine margin, indicative of a thicker, grounded Ross Sea ice-sheet whereas the interior of the ice sheet in East Antarctica appears to have maintained a relatively constant thickness (Broecker and Denton, 1990).

Deglaciation of the Ross Sea is thought to have begun first at the western margin and proceeded like a 'swinging gate' hinged at the eastern side of the basin, with the LGM Ross Sea ice-sheet largely composed of extensions of the present day Siple Coast ice streams (Conway et al., 1999). The contribution of ice from East Antarctic glaciers, flowing through the Transantarctic Mountains, to the Ross Sea ice-sheet remains contentious. The mineralogical composition of glacial diamicts from the floor of the eastern Ross Sea is similar to that of source areas on the Siple Coast, whereas diamicts from the western Ross Sea match with sources in the Transantarctic Mountains (Licht et al., 2005), indicating more complicated glacial flow patterns at the LGM than a simple expansion of the present day Siple Coast ice streams (Fig. 12.5).

Recent model investigations, which have attempted to reconstruct the late Holocene change in the thickness of Siple Dome from the depth-age relationship of the Siple Dome ice core and plausible scenarios for the changes in accumulation rate since the LGM, have led to estimates of surface height increase of 200-400 m. For the ice-sheet to have extended ~ 1000 km to the continental shelf edge, the required ice surface gradient would have been much shallower than that of present day ice streams, and thus only possible by invoking a very slippery bed (Waddington et al., 2005). This supports the interpretation that ice from the Transantarctic Mountains must have contributed significantly to the Ross Sea ice sheet.

The timing of ice retreat in the eastern and central Ross Sea is poorly understood, but reworked foraminifera in glacial diamict on the outer shelf indicate that the maximum ice extent occurred after 16.5 cal ka BP. In contrast, there are numerous lines of both onshore and offshore evidence that constrain the timing of deglaciation in the western half of the Ross Sea. A recent review of the evidence from the marine record concluded that the maximum ice extent occurred after ~ 17 cal ka BP (Licht, 2004),

Figure 12.5: Flow lines for the Ross Ice Sheet during the last glaciation proposed by Licht et al. (2005) based on Ross Sea, East and West Antarctic till mineralogy and lithology. Dashed lines represent inferred flow due to lack of sample coverage. Reproduced with permission from Elsevier.

Figure 12.5: Flow lines for the Ross Ice Sheet during the last glaciation proposed by Licht et al. (2005) based on Ross Sea, East and West Antarctic till mineralogy and lithology. Dashed lines represent inferred flow due to lack of sample coverage. Reproduced with permission from Elsevier.

with deglaciation beginning by 14cal ka BP (Domack et al., 1999; Shipp et al., 1999). The grounding line retreated relatively slowly along the western margin of the Ross Sea, reaching the modern ice shelf edge by 7cal ka BP (Domack et al., 1999). This marine-based deglacial chronology is supported by evidence from terrestrial studies, including the timing of penguin recolonisation along the coast (Baroni and Orombelli, 1994), the altitude of large proglacial lakes in the McMurdo Dry Valleys dammed by an expanded Ross Sea ice sheet (Hall et al., 2002), cosmogenic exposure ages from moraines deposited by ice in the McMurdo Dry Valleys (Brook et al., 1995) and moraines and proglacial lake sediments on the Hatherton Glacier (Bockheim et al., 1989).

12.3.3.5. 150-85° W: Marie Byrd Land/Ellsworth Land and the Amundsen and Bellingshausen Seas

Exposure age dated moraines and recessional deposits indicate that ice thickness in Marie Byrd Land was around 45 m greater than present when deglaciation began around 10ka BP (Ackert et al., 1999). Further surface exposure dating studies support the view that ice in this region was significantly thicker at the LGM and that surface lowering has continued steadily throughout the Holocene (Sugden et al., 2006). Deglaciation in Marie Byrd Land therefore happened much later than the retreat of the Ross Sea grounding line and may still be in progress (Stone et al., 2003).

Large scale troughs have been identified offshore from all the major present day outlet glaciers of this sector of West Antarctica, extending across the shelf of the Amundsen Sea (Anderson and Shipp, 2001) and the Bellingshausen Sea (O'Cofaigh et al., 2005b). Swath bathymetry has been used to determine that the grounded ice-sheet reached the shelf edge in outer Pine Island Bay (Lowe and Anderson, 2002; Evans et al., 2006, Fig. 12.6) where elongate subglacial bedforms in the soft sediments deposited at the base of a palaeo-ice stream end in gullies incised seaward of the shelf break (Dowdeswell et al., 2006). Likewise the Belgica Trough, which extends the full width of the continental shelf in the Bellingshausen Sea, exhibits geomorphological evidence of an ice stream which is thought to have drained much of Ellsworth Land as well as some of the southern part of the the Antarctic Peninsula Ice Sheet at the LGM (O'Cofaigh et al., 2005b).

AMS dating of foraminifera near the Getz Ice Shelf (Marie Byrd Land) indicates that an ice stream in this trough retreated across a mid-shelf position by ~ 13.4 cal ka BP (Anderson et al., 2002). The Pine Island Bay ice stream retreated from the mid-outer shelf after 17.5 74 cal ka BP, and had reached a position near the present day grounding line by 10.2 7 0.5 cal ka BP (Lowe and Anderson, 2002). Retreat from the shelf edge in the Bellingshausen Sea probably occurred at around the same time with a mid-shelf position reached by B14 cal ka BP at the latest (Pope and Anderson, 1992; Pudsey et al., 1994; Heroy and Anderson, 2005). The ice edge had retreated to the inner shelf at Palmer Deep by 13 cal ka BP (Domack et al., 2001), and there is no evidence for the presence of grounded ice on the mid-outer continental shelf after this time.

12.3.3.6. 85-60° W: Antarctic Peninsula

The late Pleistocene extent of grounded ice in the Antarctic Peninsula has received a significant amount of attention during the past decade, partly due to ice-sheet models that predict a significant increase in ice volume during this period (e.g., Huybrechts, 1990). Geomorphic evidence including glacial erratics and striations on nunatak summits along the length of the peninsula indicate that the ice sheet in this region thickened significantly during the LGM. The centre of the ice-sheet thickened by up to 500 m in places,

Figure 12.6: Geomorphological evidence for the extent and configuration of the West Antarctic Ice Sheet in Pine Island Bay during the last glaciation (Evans et al., 2006). Also shown are the locations of postglacial iceberg scours (Lowe and Anderson, 2002; Evans et al., 2006). Reproduced by permission of Elsevier.

Figure 12.6: Geomorphological evidence for the extent and configuration of the West Antarctic Ice Sheet in Pine Island Bay during the last glaciation (Evans et al., 2006). Also shown are the locations of postglacial iceberg scours (Lowe and Anderson, 2002; Evans et al., 2006). Reproduced by permission of Elsevier.

reaching a maximum height of 2350 m asl at Mt Jackson (72°S), and there is evidence for at least two distinct ice domes along the spine of the southern part of the peninsula (Bentley et al., 2000, 2006; Ingolfsson, 2004). The striation orientations in the south-western part of the peninsula also suggest that ice flow was deflected by an expanded West Antarctic Ice Sheet in the Weddell Sea, indicating that ice advance in these two regions was synchronous (Bentley et al., 2006).

Geophysical investigations of the continental shelf around the Antarctic Peninsula using a combination of multibeam swath bathymetry, side-scan sonar and seismic surveys have revealed a comprehensive set of subglacial landforms. The sites of former ice streams that drained the expanded Antarctic Peneinsula Ice Sheet at the LGM have been located at Marguerite Bay (O'Cofaigh et al., 2002, 2005a; Dowdeswell et al., 2004), in the western Bransfield Basin to the north-west of the Peninsula (Canals et al., 2000, 2002; Evans et al., 2004) and the Robertson Trough on the eastern side of the Antarctic Peninsula (Evans et al., 2005) amongst others. In each case there is clear evidence that grounded ice streams extended all the way to the continental shelf break at the LGM.

Post LGM retreat of the grounding line from the shelf edge position in the Robertson Trough region on the eastern side of the Antarctic Peninsula left no grounding zone wedges to indicate stillstands and is therefore interpreted to have been continuous and perhaps rapid (Evans et al., 2005). Slightly further north however, around the northern margin of the Larsen Ice Shelf A/southern part of the Prince Gustav Channel, ice retreat appears to have been much more gradual. The transition from subglacial to glacimarine deposition here has been shown by AMS radiocarbon dating to have occurred before 12 14C ka BP (Evans et al., 2005). A somewhat more complicated picture has emerged of the retreat history in the Marguerite Bay area but final retreat across the continental shelf is known to have been underway by 13 14C ka BP (Pope and Anderson, 1992; O'Cofaigh et al., 2005a).

Terrestrial and marine records are in broad agreement that the various presently ice-free coastal and bay regions of the Antarctic Peninsula were mostly deglaciated between 8 and 6ka BP (Ingolfsson, 2004). The base of the glaciomarine sedimentary strata has been AMS radiocarbon dated to ~ 8ka BP in both the Gerlache Strait (Harden et al., 1992) and in Lallemand Fjord (Shevenell, et al., 1996). The terrestrial record includes fossil molluscs from raised marine deposits on King George Island that indicate deglaciation by around 9-8 ka BP (Mausbacher, 1991) and glaciomarine and sub-littoral deposits overlying till on James Ross Island which began to be deposited around 7.4ka BP (Hjort et al., 1997).

The timing of ice-sheet lowering inland is less well constrained, but has been attempted using cosmogenic exposure dating. At George VI Sound, at Moutonnee Valley (on Alexander Island) and in the Batterby Mountains on the western margin of the peninsula, ice downwasting may have begun as early as 25ka BP and been complete by 15-10ka BP (Bentley et al., 2006).

12.3.3.7. 60-0° W: Weddell Sea Embayment/Filchner-Ronne Ice ShelfjCoats Land

Despite the potential of the Weddell Sea sector to have contributed a similar magnitude of post-LGM sea-level rise to that of the Ross Sea embayment, neither the extent of the grounding line or the change in thickness of the inland ice is yet well constrained (see Fig. 12.7). Glacial trimlines more than 1000 m above the present ice surface in the Ellsworth Mountains (Denton et al., 1992) are probably much older than the LGM (Sugden et al., 2006;

Figure 12.7: Ice-sheet reconstruction based on field evidence from the Weddell Sea-Antarctic Peninsula region. Reproduced from Bentley et al. (2006) with permission from the GSA (see also Sugden et al., 2006).

Fogwill et al., 2007). Cosmogenic exposure-age dating of surface bedrock in the Shackleton Range has also been used to constrain the LGM thickening of the ice near the present day at grounding line of the Filchner Ice Shelf to a maximum of 750 m and a more probable limit of 340 m, consistent with limited LGM thickening (Fogwill et al., 2004).

Marine geological evidence for the location of the LGM grounding line in the Weddell Sea is perhaps even more sparse than terrestrial data of ice-sheet thickness. Glacial diamict sampled from the bed of Crary Trough, which underlies the Filchner-Ronne Ice Shelf and extends out towards the edge of the continental shelf east of Berkner Island, has a composition indicative of a source in West Antarctica (Anderson et al., 1991). Radiocarbon dating, however, has so far been unable to confirm the age of these deposits.

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