Shelf-edge glaciation offshore of northeast Greenland during the last glacial maximum and timing of initial ice-sheet retreat

This paper presents new marine geophysical data and radiocarbon dated sediment cores to reconstruct the maximum extent of the Northeast Greenland Ice Stream (NEGIS) during the last glaciation and the timing of its initial retreat from the continental shelf. The NEGIS is the largest ice stream to drain the Greenland Ice Sheet (GrIS) today, and holds a sea-level equivalent of 1.1–1.4 m. It has undergone recent retreat but the longer-term history of NEGIS on the adjoining continental shelf is still relatively poorly constrained. Two cross-shelf bathymetric troughs, Westwind and Norske troughs, acted as pathways for offshore-flowing ice during the last glaciation but little is known about the acoustic stratigraphy, sedimentology and chronology of ice sheet retreat in the outer shelf sections of both troughs. Multibeam swath bathymetry and acoustic data from both troughs show flow parallel and flow transverse glacial landforms in the outer shelf sections of both troughs. Mega-scale glacial lineations in Westwind Trough record former streaming flow towards the shelf-edge. Grounding-zone wedges record episodic stabilisation during retreat from the shelf-edge. Sediment cores recovered subglacial tills and grounding-zone proximal sediments overlain by glacimarine sediments. The slope beyond Norske Trough is characterised by glacigenic debris flows typical of submarine slopes offshore of shelf-edge terminating palaeo-ice streams. Radiocarbon dates indicate that initial retreat of the ancestral NEGIS from the northeast Greenland shelf-edge was underway by 21.5–21.6 cal ka BP in Norske Trough and c. ≥19.0 cal ka BP in Westwind Trough. Retreat rates across the outer shelf were slow at 19–23 m a⁻¹ but increased across the inner shelf. Our data provides the first direct chronological support for a shelf-edge terminating GrIS offshore of northeast Greenland during the last glacial maximum and demonstrates this sector of the GrIS underwent relatively early retreat from the shelf-edge.