High Relative Sea Level during the Bolling Interstadial in Western Iceland: A Reflection of Ice-sheet Collapse and Extremely Rapid Glacial Unloading

Abstract

New geologic data from western Iceland reveal raised marine terraces at elevations 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 the high marine levels a Bølling Interstadial age of ca. 12.6 ± 0.1 ka BP. Inferred gradient for raised Bølling beaches in the lower Borgarfjördur area is close to 2.3 m km⁻¹. The marine terraces indicate a Bølling glacio-isostatic crustal depression of up to 250 ± 20 m, reflecting ice thickness of up to 840 ± 150 m. This is close to previously estimated Last Glacial Maximum (LGM) ice thickness. The Icelandic crust responds rapidly to changes in ice-load. Gradual thinning and retreat of the Icelandic ice sheet would have been concurrently compensated for by isostatic rebound, inhibiting formation of raised shorelines reflecting LGM isostatic crustal depression. The B0lling shorelines, together with recent marine and geophysical data, indicate a very rapid deglaciation of western Iceland shelf and coastal areas around ca. 12.6 ka BP. The rapid deglaciation coincides with a period of rapid eustatic sea-level rise, which destabilized the western part of the Icelandic ice sheet and caused it to collapse.