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

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.I denna studie undersöks bildningsprocesser och klimatologisk betydelse av hur av årsmoräner bildas vid Gornergletscher, en stor alpin dalglaciär i de södra schweiziska Alperna. Ett särskilt fokus ligger på moränryggar som har bildats mellan 2007 och 2019, en period då glaciären har retirerat snabbt och det har skett en tydlig förtunning av isfronten. Dessa moräner uppvisar olika geomorfologiska och sedimentologiska egenskaper, allt ifrån mindre moränryggar bestående huvudsakligen av subglaciala sediment till mer framträdande ryggar som innehåller deformerade glaciofluviala och glaciolakustrina avsättningar. Baserat på representativa skärningar genom fem av dessa årsmoräner, kan de dominerande mekanismerna för deras bildning bestämmas. De inkluderar: (1) Tillfrysning av submarginala sediment på den framåtryckande glaciärfronten; (2) bildning av kontrollerade iskärnemoräner från isolering av ismarginala smutskoner; och (3) hoptryckning och deformation av redan existerande proglaciala sediment, t.ex. isälvsmaterial, genom den iskantens framåtrörelse. Det har visat sig att de största och mest väldefinierade moränryggarna är genetiskt kopplade till hoptryckningsprocesser längs en tillräckligt brant glaciärfront. Moräner som bildas vid den nuvarande, tunna iskanten påverkas ofta av förändringar efter avsättning, som utsmältning av begravd is, och följaktligen är bevaringspotentialen för dessa landformer över längre tid till stor del begränsad. En jämförelse av avståndet mellan serier av årsmoräner ochklimatdata från intilliggande väderstationer visar att iskantens reträtthastighet vid Gornergletscher efter år 2006 är nära korrelerad till årsmedeltemperaturen. Detta kontrasterar med iakttagelser från tidigare årtionden, då den klimatiska påverkan på moränbildningen främst var begränsad till temperaturen under ackumulationssäsongen. Detta kan innebära att sommarablationen kommer att få allt större betydelse när det gäller vad som styr glaciärens reträtt med en successivt tunnare isfront. Eftersom glaciären drar sig tillbaka över ett topografiskt komplext landskap är emellertid möjligheten stor för att icke-klimatrelaterade faktorer kommer att påverka hastigheten. Denna studie belyser de betydande utmaningar som är kopplade till att använda årsmoräner som ett geomorfologiskt arkiv för att rekonstruera isfrontens variationer hos alpina dalglaciärer över längre tidsskalor.Annual moraines at Gornergletscher, Switzerland – markers of a retreating mountain glacier In a globally warming climate, mountain glaciers in the European Alps are rapidly shrinking, causing major economic and ecological problems. As we try to better estimate the future fate of Alpine glaciers, knowledge of how they reacted to climatic fluctuations in the past therefore becomes increasingly important. However, detailed documentation in the form of aerial photographs or historical maps is not always available for those time periods and therefore we rely on information stored in glacial sediments and landforms to reconstruct past variations in glacier extent. Annual moraines are a particular type of landform that can help to better constrain those glacial fluctuations and to understand the rates and drivers of glacier retreat. Essentially, annual moraines are small ridges of sediment that are formed at the margins of seasonally oscillating glaciers. Even if a glacier is in a state of overall retreat, lower temperatures and increased snowfall during the winter months can enable its front to slightly advance. Sediments that are lying in front of the glacier will thereby be pushed up into a ridge that marks the extent of the glacier at the end of the winter season – an annual moraine. If this happens over the course of several years or decades, longer sequences of these moraines can be preserved within the landscape, even at times when the glacier is long gone. This opens the exiting opportunity to trace the retreating glacier back in time and to understand which factors influenced its decline. In this study, we examined a sequence of annual moraine ridges that has been forming at the margin of Gornergletscher, a large mountain glacier located in the southern Swiss Alps. The goal was to understand the sedimentary processes behind their formation and to use the moraine record to quantify the rates and understand the drivers of recent retreat. We found that Gornergletscher has been subject to accelerated retreat and frontal thinning during the previous decade. Retreat rates, calculated from annual moraine ridges, show a significant correlation to annual temperatures, indicating that climatic fluctuations could be a major driver of glacial retreat even on annual time scales. However, we also found that annual moraines that have been deposited at the glacier margin during recent years are heavily influenced by the pronounced thinning of the snout. Some of these moraines contain cores of glacier ice that quickly melt out when temperatures in summer are high (Fig. 1). As this can lead to substantial modification and partly even a collapse of the moraine bodies, it becomes increasingly hard to reliably reconstruct the position of the glacier from the landscape record. This indicates major challenges that are connected with using annual moraines as markers of glacial retreat at a thinning mountain glacier