Glacier outburst floods reconstructed from lake sediments and their implications for Holocene variations of the plateau glacier Folgefonna in western Norway

Abstract

A sudden release of large volumes of water during a glacier outburst flood (GLOF) is a major hazard worldwide. Here, we identify the sedimentary signature of glacial and non‐glacial processes, including GLOFs, based on lacustrine sediments from the distal glacier‐fed Lake Buarvatnet in western Norway. Historically documented GLOFs in 2002 CE and during the 1980s CE are identified in the 210Pb‐ and 14C‐dated sediments. These events have the same sedimentary signature as 12 earlier events throughout the Holocene interpreted to represent previous GLOFs in the catchment. The GLOFs are interpreted to have occurred during periods when the glacier extent was similar to the modern positions, and the events are thus used to pinpoint past positions of the glacier terminus and, hence, the equilibrium line altitudes (ELAs). The results indicate that the glacier Svartenutbreen, located at the eastern part of Folgefonna, had a similar size in 2002 CE as c. 8200–8300 cal. a BP, corresponding to the 8.2 ka event in the North Atlantic region. The regrowth of Sørfonna after the Holocene Thermal Optimum occurred at c. 6900 cal. a BP and Svartenutbreen was at modern size and extent in the periods c. 6400, c. 5450, c. 4850, c. 3850, c. 3550 and c. 1650 cal. a BP. Since 1650 cal. a BP, we infer that the glacier was larger than the 2002 CE glacier extent until 1910 CE when a GLOF occurred. Svartenutbreen has been retreating since 1910 CE, which led to the ice damming of the two historical GLOFs in the 1980s and 2002 CE separated by a glacier advance in the 1990s CE. The findings are discussed and compared to other regional glacier reconstructions in Norway, and emphasize the value of identifying and utilizing GLOFs as an indicator of past ELA variability.

Signature of modern glacial lake outburst floods in fjord sediments (Baker River, southern Chile)

“…Boes et al, 2018;Røthe et al, 2019;Piret et al, 2021). In such settings, GLOF deposits were also represented as deposits that are finer than the background sediments (Røthe et al, 2019).…”

“…Although previous studies have investigated GLOF deposits in lake settings (e.g. Xu et al, 2015;Røthe et al, 2019), little is known about how GLOFs are recorded in fjord sediments. The only GLOF deposits that have been described in marine sediments are those of Disenchantment Bay (Alaska) (Willems et al, 2011), which are composed of stratified mud, sand and diamicton, formed by sediment gravity flows and hypopycnal plume sedimentation.…”

“…In Rundvatnet Lake in Norway, for example, GLOFs are recorded as clear sand layers embedded in a finer-grained background matrix (Xu et al, 2015). In Buarvatnet Lake (Norway), however, GLOFs are recorded as fine-grained deposits, which was explained by sediment filtration in lakes located along the course of the river (Røthe et al, 2019). Aside from intermediate lakes, which are able to trap sediments, the nature of the glacial lake dam (i.e.…”

High‐resolution fjord sediment record of a receding glacier with growing intermediate proglacial lake (Steffen Fjord, Chilean Patagonia)

“…they preferentially trap larger particles (i.e. the bedload), leaving only the finer suspended fraction of the sediment for downstream export (Gjerde et al, 2016; Røthe et al, 2019).…”

Wintertime extreme events recorded by lake sediments in Arctic Norway

“…Sediments deposited in lakes have high preservation potential, and previous studies have shown how valuable lake sediment studies are to reconstruct the frequency of past natural hazards in Scandinavia, such as floods (Bøe et al, 2006; Nesje et al, 2001; Støren et al, 2010, 2016), glacier outburst floods (Røthe et al, 2019; Xu et al, 2015) and mass-wasting events (Nesje et al, 2007; Nielsen et al, 2016; Sletten et al, 2003; Vasskog et al, 2011). Recent developments in technical and analytical capacity allow us to use high-resolution x-ray computed tomography (CT) to visualise the lacustrine sediment sequence and quantify the volume of material found in the lake.…”