Identifying flood deposits in lake sediments: Changing frequencies and potential links to long-term climate change

This thesis consists of an introduction and three individual papers that investigate the possibility for identifying the sedimentary imprint of catchment processes in lake sediments, emphasising on extreme events and in particular those deposited by river floods. Three individual lake sediment basins in southern Norway have been studied and changes in the frequency of such events are reconstructed for the last c. 10,000 years. In Paper I, the sediments of the glacier-fed Lake Russvatnet in eastern Jotunheimen (61′ N 8′ E) were studied. The record comprises a combination of glacier-derived material produced by the glacier Blackwellbreen and also several episodic processes in the catchment area such as floods and debris flows. In order to distinguish late- Holocene (last 4000 years) river floods and mass movements from glacier fluctuations, the sedimentary record from Russvatnet was analysed for grain-size distribution and minerogenic content, which allowed for discrete mass-movement and river-flood deposits to be recognized. Twenty-two such episodic events were identified; 11 mass movement events and 11 river-flood events. Enhanced river-flood and colluvial activity are observed at 4000–3400, 2900–2500, 2000–1400 and 1000– 500 cal. years BP, suggesting a decreasing trend over the last 4000 years. At c. 2300 cal. years BP a shift in sedimentation regime from a paraglacial to a glacially dominated regime was observed, followed by a Neoglacial expansion period after 2300 cal. years BP. In Paper II we examine the possibility for objectively identifying flood deposits in lake sediments and hence construct Holocene flood records that may reveal changes in the long-term frequency of river floods. The method for identifying flood deposits was successfully applied to a high-resolution lake sediment core retrieved from Meringsdalsvatnet in eastern Jotunheimen (61′ N 9′ E) resulting in a detailed record of river-flood activity covering the last c. 10,000 years, including floods that have also been recorded by instrumental and historical data. The minimum number of individual floods recorded for this period is c. 100. On centennial timescales significant change in flood frequency is observed that arguably can be attributed to large-scale climatic changes such as the varying amount of winter precipitation and number of summer rainstorms. The flood frequency was low during the early Holocene (9770–7700 cal. years BP), and was even lower for the period that followed, lasting until 5500 cal. years BP. For the next 2500 years, a modest increase in flood activity followed. This trend was truncated at 2500 cal. years BP by a sudden shift towards increased flooding frequency. With the exception of a short interval around 1000 cal. years BP, when the number of floods was again low, this tendency of increased flood activity prevailed until the present day; including Stor-Ofsen, a large flood that occurred in AD 1789, and also three other historically documented river floods. In Parer III we compared the record from Jotunheimen to a second continuous, highresolution palaeoflood record from Butjønna (62′ N 10′ E) and found that both the frequency and distribution of flood events over southern Norway has changed significantly during the Holocene. The present regional-discharge regime is dominated by spring–summer snowmelt, and results indicate that the changing flood frequency cannot be explained by local conditions associated with the respective catchments of the two lakes, but rather by long-term variations of solid winter precipitation and related snowmelt. Applying available instrumental winterprecipitation data and associated sea-level pressure re-analysis data as a modern analogue, we document that atmospheric-circulation anomalies, significantly different from the North Atlantic Oscillation (NAO), have some potential in explaining the variability of the two different palaeoflood records. Centennial-scale patterns in shifting flood frequency might be indicative of shifts in atmospheric circulation and can shed light on past pressure variations in the North Atlantic region, in areas not dominated by the NAO. Major shifts were found at about 2300, 1200 and 200 cal. years BP. This thesis presents and applies approaches to detect rapid geological events in lake sediment archives, and indicates that there have been significant changes in the frequency of floods and possibly debris flows over the Holocene. These changes may partly be linked to climatic, particularly precipitation, fluctuations, but also to other mechanisms such as land uplift, changing seasons and vegetation changes