Glacial remobilization cycles as revealed by lateral moraine sediment, Bodalsbreen glacier foreland, western Norway

摘要

The proglacial area of Bodalsbreen glacier in western Norway contains nine moraine ridges formed at the maximum of the 'Little Ice Age' (ad 1755) and during the subsequent glacier retreat (ad 1767-2000). The frontal moraines are composed of a sandy diamicton, whereas the lateral ones consist of only boulder-sized clasts without any matrix. These lateral moraines seemingly are composed of a very high proportion of mature clasts, a notion supported by detailed clast roundness and shape analyses. Furthermore, there is evidence for a decline in clast maturity between the outer (older) three and the inner (younger) three moraine ridges. This decline is interpreted as a change from recycled to freshly plucked clasts. Thus, the more mature clasts in the outer moraines are thought to consist of sediment that has been recycled in the glacial system, whereas the less mature clasts on the inner moraine ridges comprise younger glacially, freshly plucked material. The Holocene glacial history of the study area with a considerably fluctuating ice margin allowed much of the sediment in the catchment to undergo several cycles of erosion, transport and deposition. Moreover, an undulating subglacial topography with several major depressions may have acted as efficient sediment traps during deglaciation phases and as sediment sources during glacial advances. The concept of glacial sediment recycling may thus be applicable to many glaciers around the world, particularly those with a history of intensive glacier fluctuations.