We report a new relative sea level curve from Inglefield Land, northwest Greenland, to investigate the transition from maximum to minimum loading across Nares Strait. We sampled marine bivalves and terrestrial macrofossils for radiocarbon dating from raised marine terraces in Rensselaer Valley, Inglefield Land (78.58°N, 70.71°W) to constrain relative sea level through the Holocene. The oldest terrestrial macrofossil of 9010–8650 cal yr BP provides a minimum-limiting constraint for the deglaciation. Sea level fell rapidly from the marine limit at 85 ± 4 m to 37.5 ± 4 m above sea level (m asl) between 9010–8650 and 7970–7790 cal yr BP at a rate of 49 m/ka. The rate of sea -level fall decreased to 11 m/ka between 7970–7790 and 5320–5060 cal yr BP, when it fell from 37.5 ± 4 to 9 ± 4 m asl. After 5,320–5,060 cal yr BP, we estimate sea level fell at a lower rate of 2 m/ka to modern sea level. The period of fastest emergence in Inglefield Land is earlier in time than in Hall Land, reflecting earlier deglaciation, and is steeper than in Hall Land and Washington Land. This sea-level history captures the transition from the style of emergence from Pituffik to Hall Land.

Petermann Gletscher drains ~4% of the Greenland ice sheet (GrIS) area, with ~80% of its mass loss occurring by basal melting of its ice shelf. We use a high-resolution coupled ocean and sea-ice model with a thermodynamic glacial ice shelf to diagnose ocean-controlled seasonality in basal melting of the Petermann ice shelf. Basal melt rates increase by ~20% in summer due to a seasonal shift in ocean circulation within Nares Strait that is associated with the transition from landfast sea ice to mobile sea ice. Under landfast ice, cold near-surface waters are maintained on the eastern side of the strait and within Petermann Fjord, reducing basal melt and insulating the ice shelf. Under mobile sea ice, warm waters are upwelled on the eastern side of the strait and, mediated by local instabilities and eddies, enter Petermann Fjord, enhancing basal melt down to depths of 200 m. The transition between these states occurs rapidly, and seasonal changes within Nares Strait are conveyed into the fjord within the same season. These results suggest that long-term changes in the length of the landfast sea-ice season will substantially alter the structure of Petermann ice shelf and its contribution to GrIS mass loss.