Synchronizing Antarctic and Northern Hemisphere ice retreat
Weber et al. examine the timing of the retreat of the Antarctic and Northern Hemisphere ice sheets at the end of the Last Glacial Maximum (LGM):
A long-standing hypothesis for ice-sheet synchronization invokes sea-level forcing of Antarctic grounding lines driven by fluctuations of NH ice sheets (41, 42), but until now the chronology of the Antarctic ice sheets has been too limited to evaluate this hypothesis, other than for the deglaciation where existing arguments for a 4- to 5-ky lag relative to the start of deglacial sea-level rise (5, 8, 23) would appear to contradict it. Where dating constraints for onset of the [local Last Glacial Maximum] exist, however, they support a sea-level forcing in placing the associated Antarctic margins at their maximum extent when global sea level was approaching or first reached its LGM lowstand (Fig. 3). In particular, we suggest that NH ice-sheet growth that occurred in response to decreases in insolation and Pacific SSTs (9, 30) caused the global mean sea level to fall, allowing Antarctic ice margins to advance across the continental shelf and reach their maximum extent. At the same time, the reduction in NADW formation (Fig. 3) and attendant heat flux would further contribute to advance of Antarctic marine margins.
The subsequent onset of NH deglaciation ~19 ka in response to boreal summer insolation forcing caused an initial rapid global mean sea-level rise of ~5 to 10 m (Fig. 3) (9, 43, 44). Although this sea-level forcing may explain the contemporaneous retreat of Antarctic grounding lines in the Weddell Sea and, perhaps, Amundsen Sea regions, the lack of a response at other dated Antarctic marine margins appears inconsistent with this hypothesis. This spatial variability in response may reflect different geometries of ice shelves, variations in subshelf pinning points, or differences in sedimentary wedges (size and stiffness) that stabilize grounding lines to a rapid sea-level rise of this magnitude (45). In addition, sea-level calculations indicate that gravitational, deformation, and rotational effects associated with the initial melting of NH ice ~19 ka caused enhanced sea-level rise around the Weddell and Amundsen Seas relative to eustatic, whereas it was equal to or less than eustatic around the Ross Sea and Mac Robertson Land margins (Fig. 4) (SOM). This regional enhancement may have been sufficient to overcome the stabilizing effect provided by sedimentary wedges at grounding lines in the Weddell and Amundsen Seas (45), causing early retreat, whereas grounding lines elsewhere remained immune to the lesser sea-level rise.
