The chronology and rate of ice-sheet margin retreat in the major fjords of Western Norway during the Early Holocene

Hardangerfjorden is up to 900 m deep and about 170 km long. The well defined Younger Dryas (YD) end moraine is located 40 km inside the fjord mouth. Lateral moraines show that the surface gradient near the ice front was steep and the glacier was grounded. These moraines can be traced up to 1000 m a.s.l. about 40 km inland, where the fjord is 600 m deep, indicating a total ice thickness of 1600 m. The YD margin terminated on a threshold and the fjord gets much deeper on the proximal side of the moraine. Therefore, the ice margin must have been floating as soon as retreat from the YD position began and calving was certainly an important process during deglaciation. The break up of the glacier ice in the fjord must also have led to a considerable draw down of the ice-sheet surface further inland. According to our dating results the retreat started at 11,500 cal yr BP (Bondevik and Mangerud 2002; Lohne 2007). We have used three methods to estimate the up-fjord retreat: 1) 14C dating of organic material deposited during or soon after deglaciation (Romundset et al., 2009), 2) age determination of ice-marginal deltas by using a shore-line diagram we constructed from data in Lohne et al (2007) and Romundset et al (2009), 3) 10Be exposure dating of a number of large ice-transported boulders. All three methods provide consistent ages in the interval 10,900-11,100 cal yr BP for the deglaciation of the fjord head, which yields a mean retreat rate of 220-330 m/year. The ages for Hardangerfjorden can also be used for the even longer (220 km) and deeper (1300 m) Sognefjorden, giving a retreat rate of 370-550 m/year, because the relevant YD and Early Holocene moraines have been mapped and correlated (Vorren and Mangerud 2008). Bondevik, S. and Mangerud, J., 2002. A calendar age estimate of a very late Younger Dryas ice sheet maximum in western Norway. Quaternary Science Reviews 21, 1661-1676. Lohne, Ø.S., 2007. Late Weichselian relative sea-level changes and glacial history in Hordaland, Western Norway. PhD, Dept. Earth Sc., Univ. Bergen. Lohne, Ø.S., Bondevik, S., Mangerud, J. and Svendsen, J.I., 2007. Sea-level fluctuations imply that the Younger Dryas ice-sheet expansion in western Norway commenced during the Allerød. Quaternary Science Reviews 26, 2128-2151. Romundset, A., Lohne, Ø., Mangerud, J. and Svendsen, J., 2009. The first Holocene relative sea-level curve from the middle part of Hardangerfjorden, western Norway. Boreas (in press). Vorren, T. and Mangerud, J., 2008. Glaciations come and go. Pleistocene, 2.6 million-11,500 years ago. In: I. Ramberg et al (Eds.). The making of a land - Geology of Norway, pp. 480-533. Norsk Geologisk Forening, Trondheim, Norway.