Deglaciation of the continental shelf, coastal margin and continental interior
- This page is part of the topic The last million years
One key to understanding the response of glacial systems to the evolving Antarctic climate during Termination 1 is knowledge of how the ice sheet retreated from the continental shelf, not least because that retreat heralded the arrival of current interglacial conditions. The Antarctic cryosphere consists of three main components, which together contain enough ice to raise sea level 57 metres (IPCC, 2007, Chapter 4). The largest is the East Antarctic Ice Sheet (EAIS), which presently is mostly land-based and is therefore considered to be the most stable. The second largest is the West Antarctic Ice Sheet (WAIS), which is largely a marine-based ice sheet and, as such, is generally considered to be unstable because it is potentially subject to direct melting and grounding line instability by a warming ocean. The third component is the Antarctic Peninsula, which today is characterized by ice caps, outlet glaciers and valley glaciers, but during the LGM was covered by a small ice sheet that extended to the edge of the continental shelf. While the Antarctic Peninsula Ice Sheet (APIS) has, according to some models, been a relatively minor (1-2 m) contributor to post-LGM sea level rise, it has been the most sensitive of the Antarctic ice sheets to climate change and sea level rise (Domack et al., 2003a).
The configuration of the Antarctic Ice Sheet during the LGM (c. 21 ka BP) was modelled in the CLIMAP reconstruction by Stuiver et al. (1981) and later revised by Denton et al. (1991). The grounding line was placed near the continental shelf edge, based on an ice surface profile reconstructed from the Ross Sea (Stuiver et al., 1981). Subsequently, numerous studies have addressed the questions of how large the ice sheets were during the LGM, and when they began their retreat.
There is unambiguous evidence that ice sheets grounded on the continental shelf all around Antarctica during the Late Pleistocene. Sediments from piston cores from all the continental shelves studied to date consist of glacially deposited tills overlain by glacial-marine sediments (Anderson, 1999). Seismic records from these areas show subglacial facies resting on regional glacial erosion surfaces, along with geomorphic features typical of grounded ice sheets. Swath bathymetry surveys of the continental shelf yield spectacular subglacial geomorphic features that have been collectively referred to as the “Death Mask of the Ice Sheet” (Wellner et al., 2006). More detail follows below, based on recent reviews by Anderson (1999); Anderson et al. (2002); and Bentley (1999).
East Antarctic Ice Sheet expansion and retreat
To date, results from studies in East Antarctica have yielded mixed results with regard to the size of the ice sheet during the LGM (21 ka BP) and its subsequent retreat history. Several onshore coastal locales apparently experienced only limited or no ice cover during the LGM, including the Bunger Hills (Gore et al., 2001) Larsemann Hills (Burgess et al., 1994; Hodgson et al., 2001), the Lützow-Holm Bay area (Igarashi et al., 1995). Colhoun (1991) summarizes other lines of evidence for a thinner (< 300 m) EAIS in coastal areas than had been previously hypothesized. These results are supported by marine geological data from the eastern Weddell Sea, where glaciomarine sediments that directly overly tills have yielded radiocarbon ages older than 20 Ka, indicating that the ice sheet retreated from the continental shelf prior to the LGM (Anderson and Andrews, 1999; Anderson et al., 2002), and the George V Coast (Presti et al., 2005).
Studies from other parts of the East Antarctic sector show ice sheets grounding on the continental shelf during the LGM, followed by retreat of the ice sheet from the shelf. Off Wilkes Land, continental shelf swath bathymetry data show lineations that extend across the shelf, and grounding zone wedges, that record the retreat of the ice sheet from the shelf (McMullen et al., 2006). Radiocarbon dates of glacial-marine sediment that directly overlies till indicate that the transition from subglacial to glaciomarine sedimentation on this shelf occurred prior to ~ 9 ka BP, and that the retreat of the grounding line to its present coastal position was complete by ~ 2 ka BP (Domack et al., 1989; Domack et al., 1991). These results are consistent with those from the Windmill Islands, west of the George V Coast between 110°E and 111°E, where de-glaciation occurred between 8 and 5 ka BP (Goodwin, 1993).
In Prydz Bay, O’Brien (1994), O’Brien and Harris (1996), O’Brien and Leitchenkov (1997), and Domack et al. (1998) used bottom profiler data, seismic data and sediment cores to reconstruct the Late Pleistocene ice sheet configuration. The reconstruction of Domack et al. (1998) shows that during the LGM the ice sheet was grounded on the shelf, except in the deeper portions of troughs. Radiocarbon dates indicate that the ice sheet retreated from Prydz Bay sometime around 11.5 ka BP. A mid-Holocene re-advance of the Lambert/Amery system occurred between 7 and 4 ka BP (Verleyen et al., 2005).
In the western Ross Sea region, the most widespread high-elevation moraine unit is the Ross Sea Drift; it is perched 240 to 610 m above present sea level (Stuiver et al., 1981). These moraines merge with the inland ice plateau near modern glacier heads, indicating that the inland EAIS stood at about the same elevation as today (Denton et al., 1991). Radiocarbon dates on Late Pleistocene deposits confirm that the LGM occurred here between 21.2 and 17 ka BP (Stuiver et al., 1981), which is consistent with the date we use here for the LGM (21 ka BP) and the initial warming at 19 ka BP. Swath bathymetry records show glacial lineations that extend to the outer continental shelf (Shipp et al., 1999). These observations confirm earlier results from sedimentological and petrographic studies of sediment cores that indicated the EAIS was grounded on the continental shelf. The ice sheet begun its retreat from the shelf shortly after the LGM, and retreated from the outer shelf at a fairly constant rate, with the grounding line being located somewhere near the present ice shelf front by about 7 ka ago (Licht et al., 1996; Domack et al., 1999; Licht et al. 1999). Radiocarbon dates show that the recession of the Holocene grounding line was completed between ~ 7 and 5 ka BP in this region (Denton et al., 1991).
West Antarctic Ice Sheet expansion and retreat
Swath bathymetry records from the eastern Ross Sea show glacial lineations that extend to the shelf break and confirm that the WAIS was grounded on the shelf in the past (Mosola and Anderson, 2006). Radiocarbon dates from sediment cores indicate that the ice sheet was grounded there during the LGM. While the resolution of these dates is not sufficient to determine the exact retreat history of the ice sheet from the shelf, the distribution of grounding zone wedges on the shelf indicates that the grounding line retreated in a step-wise fashion.
The extent of the grounded ice sheet on the southern Weddell Sea continental shelf is poorly constrained. Piston cores from the Crary Trough show that an ice sheet was grounded in the trough, but attempts to date the glacial-marine sediments that overly these tills have been hampered by a lack of carbonate material (Bentley and Anderson, 1998). Marine geological studies off Marie Byrd Land, including Pine Island Bay, indicate that the ice sheet extended to the edge of the continental shelf (Evans et al., 2006) and that the ice sheet was in its final phase of retreat from Pine Island Bay by 10 ka cal BP (Lowe and Anderson, 2002).
Antarctic Peninsula Ice Sheet Expansion and Retreat
The Antarctic Peninsula Ice Sheet (APIS) advanced to the edge of the continental shelf during the LGM. The expanded ice sheet there was more than double the size of the landmass (Ó Cofaigh et al., 2002; Dowdeswell et al., 2004; Heroy and Anderson, 2005; Ó Cofaigh et al., 2005; Bentley et al., 2006). The retreat of the APIS from the shelf occurred progressively from the outer, middle, and inner continental shelf regions, as well as progressively from the north to the south. Retreat began on the outer shelf of the northern peninsula by ~18,000 cal yr BP and continued southward by ~14 ka cal yr BP on the outer shelf off Marguerite Bay (Heroy and Anderson, 2005). Steps in the data occur at ~14 and possibly 11 ka cal yr BP, coincidental with global melt water pulses MWP 1a and 1b, and show that rapidly rising sea level at those times may have destabilised the marine ice sheet (Heroy and Anderson, 2005).
By ~10 ka cal BP the APIS grounding line reached the inner shelf. From that time on the retreat of the ice sheet was diachronous. Again, this is not unexpected as the highly irregular bedrock relief on the inner shelf was undoubtedly an important factor regulating grounding line retreat (Heroy and Anderson, 2005). Also, as the ice sheet retreated the glacial system evolved into discrete outlet and valley glaciers that responded differently to different forcing mechanisms, a result of their different drainage basin size, elevation, and climate setting.
While the retreat history of the ice sheet was largely in phase with Northern Hemisphere deglaciation, it is not clear if climate warming or other factors caused the ice sheet to retreat. Marine geological data provide clear evidence that the expanded ice sheet was drained by large ice streams, at least during the final stages of the advance. These ice streams would have contributed to thinning of the ice sheet, thereby rendering it more sensitive to global sea level rise. Hence, the retreat of the Antarctic Ice Sheet was probably a response to a combination of climate warming driven by insolation and rising CO2, changing sea level, and other dynamical processes.
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