Radiocarbon chronology of Holocene colluvial (debris-flow) events at Sletthamn, Jotunheimen, southern Norway: a window on the changing frequency of extreme climatic events and their landscape impact

摘要

The history of colluvial events over the last 8500 years is reconstructed in three Norwegian alpine slope-foot mires fed by three independent debris-flow systems. Chronologies for each site are constructed based on 155 radiocarbon-dated peat samples. At the multimillennial scale, debris-flow activity was greatest during the transition from the Holocene Thermal Maximum to the late Holocene (c. 4300-2800 cal. BP) when debris-flow events occurred with a frequency of 1 in 33 years: two peaks in activity, characterized by 1 event in 14 years and 1 event in 25 years, were reached at the beginning and end of this interval, respectively. Least activity occurred during the Holocene Thermal Maximum from c. 8000 to 7100 cal. BP, with one event in 900 years. Eight distinct century- to millennial-scale phases of high debris-flow frequency (>3 events per 100 years) are identified at c. 8300-8000, 7100-7000, 4300-3700, 3200-2800, 2200-1900, 1500-1200, 800-700 and 300-0 cal. BP. Similarities in the records from the three sites suggest underlying climatic variations linked to the frequency of intense summer and autumn rainfall events (the primary meteorological trigger of slope failure in the source areas). Differences between the records reflect local site sensitivity to the initiation of debris-flow activity and the triggering of each subsequent debris-flow event. It is also inferred that each debris-flow system passes through multimillennial stages of at first increasing and later decreasing sensitivity as its source area expands. The climatic signals in the debris-flow record appear to differ in various respects from those derived from other precipitation-sensitive proxies from southern Norway (river floods, snow avalanches and glacier variations). Debris-flow records provide, therefore, complementary information relating to extreme climatic events and demonstrate instabilities in the Holocene landscape but provide little or no support for the concept of an increasing landscape impact of debris flows in response to global warming.