THE LAST STAND OF THE GERLACHE-BOYD PALEO-ICE STREAM AND A PALEO-MEGA FJORD?

Ice drainage from the modern Antarctic ice sheets is largely via fast-flowing ice streams, making the study of paleo-ice stream existence and retreat crucial in understanding present and future ice sheet disintegration. The Gerlache Strait, located off of the western coast of the Antarctic Peninsula along the Danco Coast, was a paleo-megafjord formed by the occupation and retreat of the Gerlache-Boyd Ice Stream (GBIS) during the Last Glacial Maximum through the early Holocene. Previous studies have analyzed the subglacial morphology and sedimentology created by this ice stream in the Gerlache and Boyd Straits, but new bathymetric, backscatter, chirp and sedimentological data exposed the configuration and timing of the GBIS during its final stages of retreat as it was grounded in the southern Gerlache Strait.

Bathymetric data collected on cruise NBP 10-01 and backscatter data collected on the Araon 2013 cruise revealed a detailed image of the grounding line and the location of ponded sediment deposited when the GBIS was grounded. Chirp imaging was employed on cruise LMG 12-11 near the grounding line to strategically target a sediment core that could recover the basal unit of ponded reflectors with a 12 meter jumbo piston core. The ponded reflectors signified alternating turbidite and silt-clay couplets deposited by basal melt plumes as the ice was grounded, either as a tidewater glacier front or ice shelf system. Postglacial diatom mud and ooze as part of a sediment drift lay above the ponded sediment. This sediment was denoted by drape reflectors in the chirp imagery, which indicated an open marine environment. The deglacial transition between these units was radiocarbon dated at 8090 +/- 60 cal yr BP, which is in agreement with a cosmogenic exposure date of 8620 +/- 190 years at Duthier’s Point and a deglacial age in Paradise Harbour of 8400 cal yr BP, both located southeast of the grounding line. The GBIS likely persisted through the early Holocene due to constriction and protection of the ice stream by islands to the west and the Antarctic Peninsula to the east of the Gerlache Strait. Its ultimate collapse, however, could have been caused by factors such as a rise in global eustasy during the early Holocene.