Late Quaternary ice sheet thinning and retreat in southern Victoria Land, Antarctica

Abstract

Retreat of the Antarctic ice sheets since the Last Glacial Maximum (LGM) has been associated with sea-level rise and ocean warming on the ice sheet margins, but the significance and relative contribution to eustatic sea-level rise since this time has been difficult to quantify. This thesis presents new constraints for the timing and retreat of the Skelton Glacier in the Ross Embayment and grounded ice in the Ross Sea, the Ross Sea Ice Sheet (RSIS).

Using two nunataks, Escalade and Tate peaks as a gauge for past ice sheet levels, glacial geologic evidence and ¹⁰Be and ²⁶Al cosmogenic-nuclide exposure ages provide new and direct constraints on the past extent and timing of retreat of the Skelton Névé over the Late Quaternary. Glacial geological and geochronological evidence from Escalade and Tate peaks show that between 288 ka and 40.3 ka, the ice surface experienced slow deflation, lowering from ≥1431 to 1363 metres above sea level (masl). Ice in the southern Skelton Névé lowered by ~50 m between 40.3 ka and ~13.6 ka. Records from the eastern margin of Escalade Peak indicate the ice surface of the Skelton Névé was between 50 and 106 m higher than present during the LGM. The ice surface elevation remained close to its maximum ice level prior to 17.2 ka and has thinned by at least 50 m to the present-day level since ~13.6 ka. Thinning continued after 8.7 ka, and likely reached the present-day ice level ~2 - 3 ka. This lateglacial-Holocene ice-surface lowering is asynchronous from other sites in the Transantarctic Mountains where increased snow accumulation has been reported to have caused thickening up glacier in the early to mid-Holocene. ¹⁰Be exposure ages from large (>1 m) boulders in southern McMurdo Sound show that the RSIS had an ice surface elevation ~520 masl on the eastern side of Mount Discovery during the LGM and the onset of deglaciation was ~13.1 ka. The ice surface lowered from ~520 to 234 masl between 13.6 ka and 9.3 ka; and from 234 masl to the present ice shelf between 9.3 ka and 6.6 ka. This late-glacial and Holocene chronology from southern McMurdo Sound is consistent with other records in the Ross Embayment, and implies the RSIS experienced rapid retreat during the early to middle Holocene. These results suggest that the majority of ice sheet thinning and retreat in the Skelton Névé and in southern McMurdo Sound began just after meltwater pulse 1A (MWP-1A), a period of abrupt sea-level rise of up to 20 m that occurred between ~14.7 ka and 14.3 ka. Thus, it is unlikely that the RSIS and outlet glaciers from the East Antarctic Ice Sheet (EAIS) that drain into the Ross Embayment made a significant contribution to eustatic sea-level rise at this time. From the distribution and petrography of glacial deposits and the retreat chronology in southern McMurdo Sound a two-stage ice flow model for McMurdo Sound was reconstructed: (1) Prior to ~18 ka an expanded Koettlitz Glacier lobe of ice owed north and northeast through the Brown Saddle during the LGM and coalesced with northward owing ice from the Ross Sea. (2) Retreat of the Koettlitz Glacier and perhaps other outlet glaciers then accommodated westward and northward ice flow north of Brown Peninsula, fed from grounded ice in the Ross Sea. These findings reveal that components of both the EAIS and West Antarctic Ice Sheet (WAIS) that drained into the Ross Sea contributed to lateglacial-Holocene sea-level rise. However, it is likely to be in response to warming of the Southern Ocean and sea-level rise from the retreat of the Northern Hemisphere ice sheets and the outer margins of the Antarctic.