Early deglacial onset of southwestern Greenland ice-sheet retreat on the continental shelf

• SW Greenland ice sheet retreated from continental shelf break at 19–18 ka.
• Early deglacial retreat was likely driven by insolation and/or sea level rise.
• After initial retreat, the ice margin maintained a shelf position until 12–11 ka.

The Greenland ice sheet (GrIS) advanced onto the continental shelf during the last glacial period. While deglacial records for when the GrIS withdrew onto the modern coastline are relatively abundant, the timing of early GrIS retreat on the shelf is poorly constrained. Here we use planktic foraminiferal δ18O, sediment grain size, sedimentation rates, and 14C ages in southeastern Davis Strait core HU87033-008 to develop an early deglaciation chronology of the southwestern GrIS while on the continental shelf. Sedimentation rates, and especially silt and clay fractions, are high between ∼20.5 and ∼17.1 ka, suggesting that the southwestern GrIS margin was near or at the shelf break, where it released subglacially derived sediment-laden meltwater. A peak in sedimentation rates of ∼110 cm ka−1 between ∼19.3 ka and ∼18.6 ka, combined with an initial decrease in planktic δ18O of ∼0.5 per mil, suggests an early deglacial pull back of the GrIS margin from the shelf break with a concurrent increase in surface ocean meltwater discharge. A subsequent planktic δ18O decrease of ∼1.0 per mil combined with a drop in silt and clay sedimentation rates at 18–17 ka likely record further GrIS retreat inland from the shelf break. Terrestrial 10Be surface exposure ages indicate that the GrIS margin remained on the continental shelf until ∼11 ka, yet the cause of this subsequent ice-margin stability on the inner shelf is not known. Our new records provide the first evidence that the southwestern GrIS margin may have begun to deglaciate at the same time as other Northern Hemisphere ice sheets. As Labrador Sea water temperatures likely remained near glacial values until ∼15 ka, we suggest that initial southwestern GrIS retreat was in response to rising global sea level from retreat of other ice sheets and/or the initial deglacial rise in boreal summer insolation.