Climatic signals recorded in snow avalanche-dominated colluvium in western Norway: depositional facies successions and pollen records

This paper is focused on the palaeoclimatic significance of snow-avalanche (snow-flow) activity and palaeobotanical data derived from postglacial colluvium. It is based on a series of case studies from western Norway, with emphasis on sedimentary facies and radiocarbon dating of the deposits. The present data demonstrate that the colluvial sedimentary successions bear important palaeoclimatic signals related to extreme weather events, and the record of snow avalanches reveals, in particular, a detailed pattern of winter climatic conditions (snow fall rates and the frequency of winter storms). The Late Weichselian record shows some snow avalanching around 12 300-12 000 BP, suggesting occurrence of major winter storms at that time. An overwhelming amount of data demonstrates the dramatic increase in snow-flow processes during the Younger Dryas chronozone, documenting that this phase was characterized by severe winter storms with distinct snowfall rates. The Holocene record of snow avalanching shows in general a good correspondence with the regional record of glacier fluctuations, indicating that both the avalanche and the glacier records were controlled primarily by winter climatic conditions. Snow-flow sedimentation has been recorded from c. 7000 BP (7800 cal. BP), but the records show highly fluctuating winter climatic conditions throughout the Holocene period. Both the avalanche activity and the pollen data suggest a clear deterioration in the winter climate from c. 4700 BP (5400 cal. BP), corresponding with other palaeoclimatic records in Scandinavia. The period between 3900 and 3100 BP (4350-3350 cal. BP) seems to have been one of the most severe phases in the Holocene, documented by a dramatic increase in snow-flow processes-resulting in opening of the local forests. Two phases of snow avalanching have been identified during the 'Little Ice Age' (AD 1450-1920). These data further indicate that episodes characterized by heavy snowfall and strong winds were more frequent in some phases of the Holocene than they are today.