Stephanie Shipp

The Antarctic Ice Sheet during the Last Glacial Maximum and its subsequent retreat history: a review

Abstract An emerging body of evidence from studies of the last glacial/interglacial cycle suggests that the East and West Antarctic ice sheets have not advanced and retreated in concert. The West Antarctic ice sheet advanced to the outer shelf in most regions during the Last Glacial Maximum (LGM). The retreat from the shelf commenced shortly after the LGM and continued into the late Holocene. The West Antarctic Ice Sheet in Ross and Amundsen seas slid across a deforming bed, at least during the final phases of extended glaciation. This implies that at this time the ice sheet had a low profile. Differences in the number and locations of grounding-zone wedges and smaller grounding zone features from trough to trough imply that individual West Antarctic Ice Sheet ice streams retreated episodically. Details concerning the expansion, retreat, and behavior of the East Antarctic Ice Sheet (EAIS) are more sparse. The picture emerging is that the EAIS did not expand to the continental shelf edge during the LGM; rather, it achieved a maximum extent of a mid-shelf position in some locations, while in other areas the ice terminus was situated near its present location. The timing of retreat along sectors within the EAIS appears diachronous, and in places occurred prior to the LGM. The Antarctic Peninsula shelf contained considerably more ice during the LGM than previously proposed. The results presented in this paper support more recent published ice-sheet models that call for greater contributions of melting ice from West Antarctica, including the Antarctic Peninsula, to the post-glacial rise in sea level.

Late Pleistocene–Holocene retreat of the West Antarctic Ice-Sheet system in the Ross Sea: Part 2—Sedimentologic and stratigraphic signature

Sedimentologic, geotechnical, geochemical, and accelerator mass spectrometer (AMS) radiocarbon data from two marine geologic cruises in the Ross Sea have allowed us to constrain facies relationships and temporal changes in the West Antarctic Ice Sheet. The selection of core sites was facilitated by the use of multibeam bottom imagery and a good quality (CHIRP) subbottom reflection system. A complex but consistent succession of facies documents a number of environments from subglacial to open marine. Massive, mud-rich diamictons have low water contents, contain (in places) a calcareous microfossil assemblage, show minimal textural variation, and contain low and uniform total organic carbon values. This reflects a subglacial setting. This unit passes upward into a stratified, thin, granulated facies consisting of pelletized, sandy, muddy gravel that is loosely compacted and contains a variable water content and concentrated horizons of pebble-sized clasts. This facies reflects the lift-off zone or thin water film between the basal debris and sea floor. Overlying this unit are silty clays that contain a well-sorted, very fine-grained sand component. There are no coarse grains within this unit. This facies is, in part, laminated and reflects deposition beneath an ice shelf, near and away from the grounding-line zone. The ice-shelf facies passes upward into a siliceous mud and ooze unit that represents deposition in an open-marine setting. A sandy, volcaniclastic-rich subfacies marks the transition from ice shelf to open-marine environments found at the calving line. The 86 AMS radiocarbon dates on organic matter provide an accurate chronology for 19 cores. Ice-shelf conditions were established in the outer Drygalski Trough by 11 ± 0.25 ka and perhaps earlier. This transition took place in the JOIDES Basin by 10–8 ka. The calving front of the Ross Ice Shelf passed over the Drygalski Trough at 74°S by 9.5 ± 0.25 ka. The timing of deglaciation in the Ross Sea calls into question current models for the contribution of Antarctic glacial ice to Holocene sea-level rise and suggests that recession was relatively gradual and more closely aligned with Northern Hemisphere deglaciation and its associated eustatic pulse.

Retreat signature of a polar ice stream: sub-glacial geomorphic features and sediments from the Ross Sea, Antarctica

Abstract Three research expeditions to the Ross Sea, Antarctica resulted in collection of a dataset of more than 270 km of side-scan and chirp-sonar data, more than 330 km of swath bathymetry and 3.5 kHz data, and 24 cores within a glacially-carved trough. The former ice-stream flow path is divided into six zones, covering a distance of approximately 370 km, distinguished by unique stratigraphic signatures and geomorphic features. An erosional surface with thin, patchy lodgement till characterizes Zone 1. This region is interpreted as having experienced relatively high basal shear stress conditions. Zones 2, 3, and 4 are characterized by an erosional surface and thin, time-transgressive subglacial and grounding-line proximal deposits that include back-stepping moraines, flutes, transverse moraines, and corrugation moraines. These zones represent the transition between erosional and depositional regimes under the expanded LGM ice sheet; material eroded from the inner shelf was transported toward the outer shelf, possibly as a thin deforming till layer. The two outer zones are depositional and include maximum grounding-line (Zone 5) and pro-glacial deposits that were overridden subsequently by the ice sheet (Zone 6). Surface features include mega-scale glacial lineations, corrugation moraines, and iceberg furrows. Ice in these zones is interpreted as having experienced relatively lower basal shear stress, an extensional regime, and faster flow. This advance may have destabilized the ice sheet, initiating local draw-down and production of icebergs that furrowed the sea floor. Corrugation moraines are thought to represent annual retreat moraines, constraining the retreat rate of the ice sheet across the continental shelf to a consistent 40 to 100 m a−1.

Footprint of the Expanded West Antarctic Ice Sheet: Ice Stream History and Behavior

Evaluating the stability of the West Antarctic Ice Sheet and its regulating role in global climate and eustasy hinges on our ability to understand the interaction of ice streams and the bed on which they rest. Rapid streaming of ice is enabled by flow across a deformable till bed produced by the incorporation of basal meltwater into unconsolidated sedimentary material. These ice streams are shown to have flowed across extensive deformable till beds characterized by megascale glacial lineations composed of soft deformation till. The onset of rapid ice discharge occurs at the transition from crystalline bedrock to seaward-dipping sedimentary strata. In most locations, the deformed bed extends tens of kilometers to the outer continental shelf, which implies a thin ice sheet margin. Furthermore, most of the lateral boundaries of these ancestral ice streams were not constrained geologically, and there is evidence that these boundaries migrated a few tens of kilometers. The extent of the deformable till bed, the nature of the boundaries, and the location of grounding-zone wedges, which record grounding-line positions of individual ice streams, vary from trough to trough, implying unique ice advance and retreat histories. These are all critical parameters in glaciological models and, therefore, predictions of the West Antarctic Ice Sheet’s stability.

Lineations on the Ross Sea Continental Shelf, Antarctica

Ross Sea is a broad embayment, approximately 1500 km wide and 900 km long, on the Antarctic coast (Fig. 1). Depths range from less than 300 m to greater than 1200 m and average in excess of 500 m. Bathymetry is dominated by a series of roughly northeast-southwest ridges and troughs. The continental shelf is foredeepened; the inner shelf is deeper than the outer shelf due to a combination of enhanced glacial scour and isostatic loading. Repeated expansion of the East and West Antarctic ice sheets is interpreted to have modified the continental shelf. The records of the most recent glacial expansions are preserved in the surficial features and sedimentary deposits of the Ross Sea floor.

Grounding Zone Wedges on the Antarctic Continental Shelf, Ross Sea

Ross Sea is a broad embayment, approximately 1500 km wide and 900 km long, on the Antarctic coast (Fig. 1). Water depths range from less than 300 m to greater than 1200 m and average in excess of 500 m. A series of roughly northeast-southwest ridges and troughs dominate the bathymetry. The continental shelf is foredeepened; the inner shelf is deeper than the outer shelf due to a combination of enhanced glacial scour and isostatic loading. Repeated expansion of the East and West Antarctic ice sheets is interpreted to have modified the continental shelf. The records of the most recent glacial expansions are preserved in the surficial features and sedimentary deposits of the Ross Sea floor.

Drumlin Field on the Ross Sea Continental Shelf, Antarctica

Ross Sea is a broad embayment, approximately 1500 km wide and 900 km long, on the Antarctic coast (Fig. 1). Water depths range from less than 300 m to greater than 1200 m and average in excess of 500 m. The regional bathymetry is dominated by a series of roughly northeast-southwest ridges and troughs. The continental shelf is foredeepened; the inner shelf is deeper than the outer shelf due to a combination of glacial scour and isostatic loading. Repeated expansion of the East and West Antarctic ice sheets is interpreted to have modified the continental shelf. The records of the most recent glacial expansions are preserved in the surficial features and sedimentary deposits of the Ross Sea floor.

Till Sheets on the Ross Sea Continental Shelf, Antarctica

Ross Sea is a broad embayment, approximately 1500 km wide and 900 km long, on the Antarctic coast (Fig. 1). Water depths range from less than 300 m to greater than 1200 m and average in excess of 500 m. Bathymetry is dominated by a series of roughly northeast-southwest ridges and troughs. The continental shelf is foredeepened; the inner shelf is deeper than the outer shelf due to a combination of enhanced glacial scour and isostatic loading. Repeated expansion of the East and West Antarctic ice sheets is interpreted to have modified the continental shelf. The records of the most recent glacial expansions are preserved in the surficial features and sedimentary deposits of the Ross Sea floor.

Paleo-Ice Streams and Ice Stream Boundaries, Ross Sea, Antarctica

During the RVIB Nathaniel B. Palmer 1994–01 and 1995–01 cruises, approximately 7141 km of 50 in3 airgun seismic profiles and 12,150 km of 3.5 kHz trackline data were collected from the Ross Sea continental shelf. Ross Sea is a broad embayment, approximately 1500 km wide and 900 km long, on the Antarctic coast (Fig. 1). Water depths range from less than 300 m to greater than 1200 m and average in excess of 500 m. Bathymetry is dominated by a series of roughly northeast-southwest ridges and troughs. The continental shelf is foredeepened; the inner shelf is deeper than the outer shelf due to a combination of enhanced glacial scour and isostatic loading. Repeated expansion of the East and West Antarctic ice sheets is interpreted to have modified the continental shelf. The records of the most recent glacial expansions are preserved in the surficial features and sedimentary deposits of the Ross Sea floor.