Refining the history of Younger Dryas and Early Holocene glacier oscillations in the Borgarfjörður region, western Iceland

The lower Borgarfjörður region, western Iceland, has been central to the reconstructions of the dynamics and collapse of the Icelandic Ice Sheet during the deglaciation. Here, extensive stratigraphical sections and landforms provide a rare opportunity to study past glacier dynamics in this part of Iceland. Previous studies reveal that a large outlet glacier in Borgarfjörður advanced during the Late Weichselian resulting in large-scale deformation of glaciomarine sediments and the formation of a series of ice-marginal moraines. However, the events recorded by these sediments and landforms are poorly constrained in time. We present and discuss 22 new radiocarbon dates in the context of recent reconstructions of the regional glacier dynamics in order to constrain the timing of the glacier oscillations. The results show that a dynamic, marine-terminating glacier advanced out of Borgarfjörður sometime after c. 13.0 cal. ka BP, resulting in the formation of an extensive moraine complex. The timing indicates that the advance occurred during climate cooling and widespread glacier expansions within the Younger Dryas. Followed by the first initial advance, the glacier exhibited at least five re-advances punctuated by phases of retreat. Each re-advance terminated proximal (within 5 km) to the outermost moraine complex although the extent of periodic retreat and the exact timing of these oscillations are unknown. All these phases of re-advance occurred prior to the onset of the Holocene (around 11.7 cal. ka BP), during which marine fauna re-colonized the area and the Borgarfjörður glacier retreated from the moraines. During the Early Holocene (sometime after c. 11.3 cal. ka BP), the Borgarfjörður glacier re-advanced to a position within ~5 km of the YD ice limit. This is the first recorded Early Holocene large-scale glacier advance in western Iceland and suggests that glacier expansion in this region coincided with widespread advances elsewhere in Iceland