Holocene glacier history of Bjornbreen and climatic reconstruction in central Jotunheimen, Norway, based on proximal glaciofluvial stream-bank mires

Holocene variations of Bjornbreen, Smorstabbtinden massif, west-central Jotunheimen are reconstructed from the lithostratigraphy of two alpine stream-bank mires flooded episodically by meltwater. The approach uses multiple sedimentological indicators (weight loss-on-ignition, mean grain size, grain-size fractions, bulk density, moisture content and magnetic susceptibility), an a priori model of overbank deposition of suspended glaciofluvial sediments, a detailed chronology based on 56 radiocarbon dates, and a Little Ice Age sedimentological analogue. Rapid, late-Preboreal deglaciation was indicated by immigration of Betula pubeseens by 9700 cal. BP. An interval of at least 3000 years in the early Holocene when glaciers were absent was interrupted by two abrupt episodes of, glacier expansion around the time of the Finse Event, the first at ca 8270-7900 cal. BP (Bjornbreen I Event) and the second at ca 7770-7540 cal. BP (Bjornbreen II Event). Neoglaciation began shortly before ca 5730 cal. BP with gradual build-up to the maximum of the Bjgrnbreen III Event at ca 4420 cal. BP. Later maxima occurred at ca 2750 cal. BP (Bjernbreen IV Event) and at 1300, 1260, 1060 and 790 cal. BP (all within the Bjornbreen V Event). Glaciers were smaller than today and possibly melted away on several occasions in the late Holocene (ca 3950, 1410 and 750 cal. BP). Minor maxima also occurred at ca 660 and 540 cal. BP, within the late Mediaeval Warm Period and the early Little Ice Age, respectively. The Little Ice Age maximum was dated to 213 25 BP (ca 205 cal. BP). The relative magnitudes of the main glacier maxima were determined: Erdalen Event > Little Ice Age Event (Bjornbreen VI) > Bjornbreen I (Finse Event) approximate to Bjgrnbreen II > Bjornbreen V greater than or equal to Bjornbreen IV > Bjornbreen III. These episodic events of varying magnitude and abruptness were used in conjunction with an independent summer-temperature proxy to reconstruct variations in equilibrium-line altitude (ELA) and a Holocene record of winter precipitation. Since the Preboreal, ELA varied within a range of about 390 m, and winter precipitation ranged between 40 and 160% of modern values. Winter precipitation variations appear to have been the main cause of these century- to millennial-scale Holocene glacier variations. (C) 2004 Elsevier Ltd. All rights reserved.