Stepwise deglaciation of the Nuup Kangerlua system, Southwest Greenland

Understanding past Greenland Ice Sheet behavior provides critical context for its future stability. This study reconstructs the deglaciation history of the Nuup Kangerlua system in Southwest Greenland using new high-resolution multibeam bathymetry, backscatter data, sediment core analyses and published multi-proxy records. Results reveal stepwise retreat of the Greenland Ice Sheet between c. 11000 and c. 10200 yrs BP (minimum-limiting ages), expressed by at least six distinct ice-margin still-stands and/or readvances. The absence of moraines from deep outer-to mid-fjord basins suggests rapid early retreat phases (c. 11000-10800 yrs BP), dominated by grounding-line migration through over-deepened basins. In contrast, numerous moraines in shallower, branching inner-fjord regions record slower, more punctuated retreat during later stages (c. 10800–10200 yrs BP), when topographic confinement and restricted inflows of warm Atlantic Water likely stabilized the ice margin. Geomorphic evidence suggests that fjord geometry - particularly basin depth, sill configuration, and tributary branching - was the primary control on retreat dynamics, with secondary modulation by oceanic forcing. Sediment core data suggest that following ice retreat, a transition from glacimarine to (non-glacial) marine conditions occurred by c. 8500 cal yrs BP, accompanied by turbidity flows and slope failures as freshly exposed terrain adjusted to deglaciation. These findings demonstrate that ice-sheet retreat through Nuup Kangerlua was not rapid and continuous but instead occurred in multiple stages with spatially variable retreat styles and speed controlled by regional topography. This provides a valuable analogue for modern and future responses of Greenland's marine-terminating glaciers to climate and ocean forcing.