Onshore and offshore studies show that an expanded, grounded ice sheet occupied the Ross Sea
Embayment during the Last Glacial Maximum (LGM). Results from studies of till provenance and the
orientation of geomorphic features on the continental shelf show that more than half of the grounded ice
sheet consisted of East Antarctic ice flowing through Transantarctic Mountain (TAM) outlet glaciers; the
remainder came from West Antarctica. Terrestrial data indicate little or no thickening in the upper
catchment regions in both West and East Antarctica during the LGM. In contrast, evidence from the
mouths of the southern and central TAM outlet glaciers indicate surface elevations between 1000 m and
1100 m (above present-day sea level). Farther north along the western margin of the Ross Ice Sheet,
surface elevations reached 720 m on Ross Island, and 400 m at Terra Nova Bay. Evidence from Marie Byrd
Land at the eastern margin of the ice sheet indicates that the elevation near the present-day grounding
line was more than 800 m asl, while at Siple Dome in the central Ross Embayment, the surface elevation
was about 950 m asl. Farther north, evidence that the ice sheet was grounded on the middle and the
outer continental shelf during the LGM implies that surface elevations had to be at least 100 m above the
LGM sea level. The apparent low surface profile and implied low basal shear stress in the central and
eastern embayment suggests that although the ice streams may have slowed during the LGM, they
remained active.
Ice-sheet retreat from the western Ross Embayment during the Holocene is constrained by marine and
terrestrial data. Ages from marine sediments suggest that the grounding line had retreated from its LGM
outer shelf location only a few tens of kilometer to a location south of Coulman Island by w13 ka BP. The
ice sheet margin was located in the vicinity of the Drygalski Ice Tongue by w11 ka BP, just north of Ross
Island by w7.8 ka BP, and near Hatherton Glacier by w6.8 ka BP. Farther south, 10Be exposure ages from
glacial erratics on nunataks near the mouths of Reedy, Scott and Beardmore Glaciers indicate thinning
during the mid to late Holocene, but the grounding line did not reach its present position until 2 to 3 ka BP.
Marine dates, which are almost exclusively Acid Insoluble Organic (AIO) dates, are consistently older than
those derived from terrestrial data. However, even these ages indicate that the ice sheet experienced
significant retreat after w13 ka BP. Geomorphic features indicate that during the final stages of ice sheet
retreat ice flowing through the TAM remained grounded on the shallow western margin of Ross Sea.
The timing of retreat from the central Ross Sea remains unresolved; the simplest reconstruction is to
assume that the grounding line here started to retreat from the continental shelf more or less in step
with the retreat from the western and eastern sectors. An alternative hypothesis, which relies on the validity of radiocarbon ages from marine sediments, is that grounded ice had retreated from the outer
continental shelf prior to the LGM. More reliable ages from marine sediments in the central Ross
Embayment are needed to test and validate this hypothesis.