Lake sediments from southern Norway capture Holocene variations in flood seasonality

Abstract

Owing to the combined impact of human encroachment and global climate change, river flood hazards have increased around the world. Over the past decades, rapid warming has augmented atmospheric moisture fluxes. The resulting intensification of the hydrological cycle, as evident from instrumental records, is linked to more frequent and severe flooding. However, this observational baseline is too short to adequately assess long-term flood variability under different climate conditions. Geological archives can fill this critical knowledge gap by providing long-term (millennia) empirical constraints on the magnitude and frequency of floods. To reconstruct the frequency of flood events over the last >8000 years, this study investigated the Holocene lacustrine sediments from Lake Lygne in southwestern Norway. The precipitation in this area is sourced from the North Atlantic Ocean and transported by the prevalent westerly winds. As such, reconstructions of flood frequency variability may shed light on past moisture flux and flood generating weather systems in this part of northwestern Europe. We applied a multiproxy approach to robustly fingerprint the flood sediment signatures using multiple independent lines of evidence. This approach included analysing the sediments using X-ray fluorescence (XRF), computed tomography (CT scanning), and magnetic susceptibility (MS). We identified two types of event beds that differed significantly from background sedimentation throughout the sedimentary archive, which we interpreted as river flood deposits: one type is minerogenic and coarse silt-dominated; the other is a composite of minerogenic sediments and macrofossils. By comparing the sediment record with instrumental water discharge data from the last 100 years, we argue that these differing flood signals represent two flood regimes: rainfall-triggered floods during autumn and snowmelt-triggered floods during spring. Conducting a rate of change (RoC) analysis, we found significant millennial- and centennial-scale variability in the frequency of flood events. Moreover, comparing our findings with palaeoclimate reconstructions in the North Atlantic region, we found that the warmer than present early-mid-Holocene was dominated by extreme precipitation-triggered floods, shifting towards more snowmelt-triggered floods in the late Holocene.