LATE WEICHSELIAN-EARLY HOLOCENE GLACIAL HISTORY OF NORTHWEST ICELAND, CONSTRAINED BY 36CL COSMOGENIC EXPOSURE AGES

We present twenty-four new cosmogenic isotope (³⁶Cl) surface exposure ages from erratic boulders, moraine boulders, and glacially eroded bedrock that constrain the late Weichselian to Holocene glacial history of northwest Iceland. The results suggest a topographically controlled ice sheet over the Vestfirðir (Westfjords) peninsula during the last glaciation. Cold based non-erosive sectors of the ice sheet covered most of the mountains while fjords and valleys were occupied with erosive, warm-based ice.

High ³⁶Cl exposure ages from uplands and mountain plateaux in combination with younger erratic boulders superimposed on such surfaces suggest the presence of non-erosive ice over uplands and plateaux in the Vestfirðir peninsula during the last glaciation. Glacially scoured terrain and erratic boulders yielding younger exposure ages in the lowland areas indicate that the valleys and fjords of the Vestfirðir peninsula were occupied by warm-based, dynamic ice during the last glaciation.

A 26.2 ka deglaciation time of Mt. Leirufjall indicate ice thinning and deglaciation of some mountain tops and plateaux that preceded any considerable lateral retreat of the ice sheet. Subsequently this initial ice thinning was followed by break-up of the shelf based ice sheet off Vestfirðir about 15 ka BP. Hence, the new exposure ages suggest a stepwise asynchronous deglaciation on land, following the shelf break-up with some valleys and most of the uplands, deglaciated about 14-15 ka BP.

The outermost moraine at the mouth of Leirufjörður was dated to 9.3 ka BP, and we suggest the moraine to be formed by a glacier re-advance in response to a cooler climate forced by the reduced Atlantic Meridional Overturning Circulation at around 9.3 ka BP. A system of moraines proximal to the 9.3 ka moraine in Leirufjörður and a 9.4 ka deglaciation age in the coastal area of Reykjarfjörður suggest that an extensive ice cap was preserved over the eastern Vestfirðir peninsula at least until c. 9 ka BP.