High relative sea level during the Bolling Interstadial in western Iceland: a reflection of ice-sheet collapse and extremely rapid glacial unloading

New geologic data from western Iceland reveal raised marine terraces at ele vations between 105 and 148 m a.s.l., 45-80 m above the late Younger Dryas (ca. 10.3 ka BP) raised beaches in the region. Radiocarbon dating assigns t he high marine levels a Bolling Interstadial age of ca. 12.6 +/- 0.1 ka BP. Inferred gradient for raised Bolling beaches in the lower Borgarfjordur ar ea is close to 2.3 m km(-1). The marine terraces indicate a Bolling glacio- isostatic crustal depression of up to 250 +/- 20 m, reflecting ice thicknes s of up to 840 +/- 150 m. This is close to previously estimated Last Glacia l Maximum (LGM) ice thickness. The Icelandic crust responds rapidly to chan ges in ice-load. Gradual thinning and retreat of thr Icelandic ice sheet wo uld have been concurrently compensated for by isostatic rebound, inhibiting formation of raised shorelines reflecting LGM isostatic crustal depression . The Balling shorelines, together with recent marine and geophysical data, indicate a very rapid deglaciation of western Iceland shelf and coastal ar eas around ca. 12.6 ka BP. The rapid deglaciation coincides with a period o f rapid eustatic sea-level rise, which destabilized the western part of the Icelandic ice sheet and caused it to collapse.