Lake sediment analysis of the Nedre Glomsjø outburst flood event, southeastern Norway

Abstract

Sediment cores from lakes and mires can provide sedimentary archives for palaeoflood reconstruction. By analyzing lake sediments we can better our understanding of the nature and impact of large flood events. This study aims to improve our understanding of the outburst flood from the glacial lake Nedre Glomsjø at the end of the last Ice Age. Sediment cores from three lake basins along Glomma valley has been examined for grain size distribution, total organic carbon (TOC) and macrofossil content. The sediment cores has been analyzed by its geochemical properties, and macrofossils found above and under the flood layer have been dated to constrain the timimng of the flood event. A flood layer has been identified within all three lakes, and related to the drainage of the glacial lake Nedre Glomsjø based on its stratigrapgic properties. The thickness of the flood layer is 11-14 cm, and stands out as a light, fine-grained layer, increasing in finer material upwards. Sharp boundaries at the top and bottom separates the layer from the rest of the material, and the sterile conditions within the flood layer is indicated by an absence of macrofossils and low total organic carbon. The analyzed lakes indicates a run-up height of 60-44 m in the study area, and radiocarbon dates times the flood event to have occurred after 11 080 - 11 580 cal yr BP and before 9780 - 9530 cal yr BP. The results obtained from the lake studies has been correlated with previous studies from the area further downstream. A linear line has been constructed based on the identified silt bed along the valley, suggesting a tentative run-up height along the Glomma valley.