Southwestern Greenland Ice Sheet Retreat from the Outer Continental Shelf by 18 ka

Abstract:

Records of southwestern Greenland ice sheet retreat from the last glacial maximum are primarily limited to the late deglaciation, when the ice margin withdrew inland of its modern coastline. Therefore, southwestern ice sheet response to earlier deglacial retreat forcings is largely unknown. We use grain size, oxygen isotope, radiocarbon, and sedimentation rate data from southeastern Davis Strait core HU87033-008 to reconstruct southwestern Greenland ice margin response to early deglacial forcings. We calculate peak sediment fluxes of ~100 cm ka^-1 during the global last glacial maximum, suggesting an ice margin position at or near the continental shelf break. By ~18 ka, sedimentation rate drops to ~40 cm ka^-1, concurrent with a decrease in planktic oxygen isotopes to near interglacial values. We interpret this reduction in sediment supply to be a result of early retreat from the outermost continental shelf. A further decrease in sedimentation rate to ~25 cm ka^-1 at ~17 ka suggests a middle to inner shelf position of the ice margin, where it remained until 12-11 ka when our ¹⁰Be record from coastal boulders show ice-margin retreat off of the shelf. Regional air and ocean temperatures remained near glacial levels during the period of early deglacial outer shelf retreat. However, the first major deglacial rise in global mean sea level, due to meltwater pulse 19 ka, was coincident with our peak in sedimentation rate. We conclude that a change in relative sea level in southwestern Greenland initiated repositioning of the ice margin, followed by rapid ice retreat inland of the outer shelf. This reconstruction supports an ice margin that was highly sensitive to sea-level changes while on the outer shelf, but also raises the question of why later sea-level rise did not drive complete Greenland ice sheet retreat off of the continental shelf, which only occurred much later.