Holocene flood variability and glacier fluctuations in the Central Alps revealed by lacustrine sediments
Global climate change is projected to significantly modify recent climate conditions. Especially the increased occurrence of large-scale intense precipitation causing severe flooding, would highly affect the Alpine region and its foreland, since floods represent a major natural hazard in the Alps. In order to assess natural flood variability and its forcing factors that could improve accurate projections of these climate extremes, we established a 10’000 year-long flood reconstruction of the Central Alps based on 10 Northern and 5 Southern Alpine lacustrine sedimentary archives. Lakes with a certain relief in the respective catchment record individual flood events by distinct sediment layers (flood turbidites). These layers contain high amounts of terrigenous material that was mobilized during intense precipitation events and transported to the next downstream lake. Therefore, individual lake sediment successions uncover the local flood history. To reconstruct a regional flood pattern and thus rather a synoptic Alpine intense rainfall signal, we studied multiple Alpine lake records.
The accuracy of the lacustrine flood chronology is verified by a good correlation with historical flood reconstructions from the Northern Alpine region covering the last 500 years. Regarding Northern Alpine flood variability over the past 2500 years, a coincidence between high flood frequencies and low summer temperatures is apparent, which we interpret in terms of changing North Atlantic atmospheric circulations accompanied by varying summer mean temperatures. In particular, we propose a weaker (stronger) expression of the subtropical highpressure zone accompanied by lower (higher) summer mean temperatures, which favors (impede) the occurrence of Atlantic storm tracks affecting the Central Alpine region and thus enhance (decrease) the occurrence of intense precipitation. Over the past 10’000 years, the overall pattern of the Northern and Southern Alpine flood frequency anti-correlate with variations in total solar irradiance that is considered to affect air temperatures. Thus, we assume that variations in North Atlantic circulation controlled flood frequency in the entire Central Alpine region throughout the Holocene. Nevertheless, during distinct periods a partial discrepancy in Northern and Southern Alpine flood variability is apparent, in which flood frequency is only enhanced in the Southern Alps. We explain this pattern by exceptional weak expressions of the subtropical high-pressure zone accompanied by distinct cool conditions shifting the Atlantic storm tracks to Mediterranean latitudes and thus favoring heavy precipitation at the southern slope of the Alps.
We further established a 2250 year-long reconstruction of mass accumulation rate (MAR) of Lake Trüebsee (Central Alps) at an annual resolution. Enhanced sub-glacial erosion during increased glacial cover in the catchment cause enhanced MAR and vice versa, mirroring continuously fluctuations of the upstream-located Titlis Glacier. We assume that our data reflect an overall Central Alpine glacial pattern considering the strong correspondences with other independent Alpine glacier reconstructions. Furthermore, MAR values strongly anticorrelate with a summer mean temperatures reconstruction over the past 2250 years, which indicate that Alpine glacier fluctuations were strongly controlled by varying melting rates of glaciers in summer. As the MAR reconstruction is based on individual varve thicknesses, detailed characterization of the annual laminae is crucial. Thus, we performed textural and mineralogical analyzes of selected laminae with an application of scanning electron microscopy (SEM) at micron resolution. The SEM-analyses were combined with visual core description (thin sections / smear slides / high-resolution core photographs) throughout the entire laminated succession.
In terms of global climate change and based on paleoclimatic evidence only, we expect in the Central Alpine region no upward-trend in the frequency of large-scale intense precipitation (intensity was not reconstructed). We assume that a poleward expansion of the subtropical dry zone accompanied by projected increasing summer mean temperatures will impede the occurrence of Atlantic storm tracks causing floods at Central Alpine latitudes. In contrast, as mean summer temperatures controlled Alpine glacier fluctuations over the past 2250 years, we propose that projected increases in summer temperatures will enhance melting rates of glaciated areas during summer and thus intensify Alpine glacier retreat under global climate change.