Implications of a rapidly thinning ice-margin for annual moraine formation at Gornergletscher, Switzerland

Abstract: This study examines genetic processes and climatological significance of annual moraine formation in the foreland of Gornergletscher, a large alpine-valley glacier located in the southern Swiss Alps. A particular focus is set on moraine ridges that have been forming between 2007 and 2019, a period when the glacier has been subject to accelerated retreat and pronounced frontal thinning. These moraines exhibit varying geomorphological and sedimentological characteristics, ranging from minor moraines composed primarily of subglacial traction till up to more prominent ridges that comprise deformed glaciofluvial and glaciolacustrine deposits. Based on representative sections through five of these annual moraines, the dominant mechanisms of their formation could be constrained. They include: (1) Freeze-on of submarginal sediments to the advancing glacier front; (2) the formation of ice-cored, controlled moraines from the isolation of ice-marginal debris cones; and (3) bulldozing and deformation of pre-existing proglacial sediments, such as fluvial outwash, by the advancing glacier margin. It is found that the largest and most well-defined moraine ridges are genetically linked to bulldozing processes along a sufficiently steep glacier front. Moraines formed at the present, thin ice-margin are oftentimes affected by post-depositional alteration such as the melt-out of buried ice and hence the preservation potential of these landforms over longer time scales is largely limited. Comparing the spacings between series of annual moraines to climatic records from close-by weather stations reveals that the rates of frontal retreat at Gornergletscher after 2006 are closely correlated to mean annual temperatures. This is in contrast to observations from previous decades, when climatic control on moraine formation was predominantly restricted to temperatures during the accumulation season. This could imply that summer ablation will become increasingly important for governing glacier retreat at a gradually thinning ice-margin. However, as the glacier recedes over a topographically complex foreland, the possibility of non-climatic factors modulating the rates of marginal retreat is high. This study highlights significant challenges that are connected to applying annual moraines as a geomorphological proxy of frontal variations at alpine valley-glaciers over longer timescales.