What are the causes of repeated mass-transport deposits in perialpine Lake Brienz (Switzerland)?

Abstract

Mass transport deposits (MTDs) are common features in lakes that impose a significant natural hazard as they can be tsunamigenic (e.g., in Lake Geneva; Kremer et al., 2012). MTDs can be assigned to different categories, such as lateral-slope landslides, margin collapses, delta collapses and rockfalls (Sammartini et al., 2019). In 1996 AD, a MTD occurred in Lake Brienz, a 260 m deep Alpine lake at the frontal range of the Bernese Swiss Alps. The 1996 MTD extends over ∼8.5 km2 and has a total volume of 2.7 106 m3, which amounts to ∼8.7 years of the lake’s annual sediment input. The 1996 MTD had no specific trigger and occurred as a spontaneous failure due to sediment accumulation on the Aare Delta (Girardclos et al., 2007).However, the 1996 delta collapse was not a unique event: a dense grid of 240 km of seismic lines reveals at least four older massive MTDs in the lake basin with similar dimensions. Our study aims to identify the ages and the processes behind these large deposits. For this purpose, we focus on long sediment cores with a cumulative length of about 40 m from four coring locations. A combined analysis of seismic profiles and sediment cores, coupled with an age model combining radionuclides, radiocarbon, coal and steamboat slag, and historical flood events, allows us to explore the feedback between climate and anthropogenic impact acting upon these MTDs. The MTDs are analysed regarding their source areas, either the Aare or the Lütschine deltas, both occurring at the opposite ends of the elongated lake basin. Moreover, we investigate whether earthquakes, spontaneous failures or a shift of the Lütschine River could act as potential triggers, as well as the role of sediment loading in the deltas related to climate changes or anthropogenic impacts in the watershed. Altogether, these factors have likely affected the recurrence rates of these significant sedimentological events.