Retreat history of the Ross Ice Sheet (Shelf) since the Last Glacial Maximum from deep-basin sediment cores around Ross Island

Abstract

Radiocarbon-dated sediment cores from deep basins beneath the McMurdo Ice Shelf, and seasonally open water north of Ross Island, McMurdo Sound, display a characteristic succession of sedimentary facies that document the retreat of the Ross Ice Sheet at the Last Glacial Maximum to its present ice shelf configuration. The facies succession records a transition from a nearly-grounded ice sheet to open marine environments (north of Ross Island) that comprises in ascending stratigraphic order: (1) slightly consolidated, clast-rich muddy diamict dominated by basement clasts from the Transantarctic Mountains, and interpreted as melt-out from the basal layer debris proximal to a retreating grounding zone; (2) sparsely-fossiliferous (reworked diatom frustules) and non-bioturbated mud lacking lonestones, interpreted as a sub-ice shelf facies; and (3) diatom mud and diatom ooze indicative of open marine conditions with evidence of iceberg rafting. The succession in the open marine Lewis Basin north of Ross Island is similar, though the diamict is much sandier and sedimentation rates 1–2 orders of magnitude higher. We present a radiocarbon chronology from total organic carbon which implies that lift-off of grounded ice in the 900 m-deep marine basins surrounding Ross Island occurred by ~10,100 14C years BP. Following lift-off, an ice shelf was maintained to the north of Ross Island until ~8900 14C years BP. We identify a phase of accelerated retreat at that time between the Drygalski Trough and Ross Island, immediately preceding the timing of Meltwater Pulse 1b. At ~8900 14C years BP the calving line became pinned to Ross Island, significantly decoupling from the grounding line, and marking the transition from a retreating ice sheet to the development of the present ice shelf.