Equifinality and preservation potential of complex eskers

Abstract

Eskers are useful for reconstructing meltwater drainage systems of glaciers and ice sheets. However, our process understanding of eskers suffers from a disconnect between sporadic detailed morpho-sedimentary investigations of abundant largescale ancient esker systems, and a small number of modern analogues where esker formation has been observed. This paper presents the results of detailed field and high-resolution remote sensing studies into two esker systems that have recently emerged at Hørbyebreen, Svalbard, and one at Breiðamerkurjökull, Iceland. Despite the different glaciological settings (polythermal valley glacier versus active temperate piedmont lobe), in all cases a distinctive planform morphology has developed, where ridges are orientated in two dominant directions corresponding to the direction of ice flow and the shape of the ice margin. These two orientations in combination form a cross-cutting and locally rectilinear pattern. One set of ridges at Hørbyebreen is a hybrid of eskers and geometric ridges formed during a surge and/or jökulhlaup event. The other sets of ridges are eskers formed time-transgressively at a retreating ice margin. The similar morphology of esker complexes formed in different ways on both glacier forelands implies equifinality, meaning that care should be taken when interpreting Quaternary esker patterns. The eskers at Hørbyebreen contain substantial ice cores with a high ice:sediment ratio, suggesting that they would be unlikely to survive after ice melt. The Breiðamerkurjökull eskers emerged from terrain characterised by buried ice which has melted out. Our observations lead us to conclude that eskers may reflect a wide range of processes at dynamic ice margins, including significant paraglacial adjustments. This work, as well as previous studies, confirm that constraints on esker morphology include: topographic setting (e.g. confined valley or broad plain); sediment and meltwater availability (including surges and jökulhlaups); position of formation (supraglacial, englacial or subglacial); and ice-marginal dynamics such as channel abandonment, the formation of outwash heads or the burial and/or exhumation of dead ice.

Subglacial meltwater routes of the Fennoscandian Ice Sheet

“…Traditionally, eskers are associated with the most recent deglaciation ( Storrar et al, 2014 ; Stroeven et al, 2016 ) because repeated proglacial fluvial and/or subsequent glacial erosion would have likely destroyed them ( Storrar et al, 2020 ). However, in some areas of Fennoscandia cross-cutting relationships ( Johansson & Kujansuu, 1995 ; see also Section 3.1), radiocarbon ages from associated kettle holes ( Lagerbäck & Robertsson, 1988 ) and stratigraphical inferences ( Sutinen & Middleton, 2021 ) indicate that some eskers formed prior to the last deglaciation.…”

“…They are often expressed as linear ridges but can also comprise beads, fans and enlargements (e.g. De Geer, 1897 , 1910 , 1940 ; Dewald et al, 2021 ; Gorrell & Shaw, 1991 ; Hebrand & Åmark, 1989 ; Lindström, 1993 ; Livingstone et al, 2019 ; Persson, 1974 ; Storrar et al, 2015 , 2020 ). Single ridges are the most common type and are typically several 10s m wide and metres to 10s m high.…”

Glacial and periglacial geomorphology of central Troms and Finnmark county, Arctic Norway

“…Classification is based on their sinuous planform; extensive length (with consideration of post depositional fragmentation); orientation parallel to sub-parallel with presumed ice flow direction; and oblique to features identified as end moraines (e.g. Brennand, 2000;Delaney, 2002;Price, 1966Price, , 1969Shilts et al, 1987;Storrar et al, 2014Storrar et al, , 2015Storrar et al, , 2020Warren & Ashley, 1994). They also appear as lighter-coloured landscape features due to the higher proportion of glaciofluvial material (e.g.…”

Glacial geomorphology of the central sector of the Cordilleran Ice Sheet, Northern British Columbia, Canada

“…Eskers are linear depositional ridges composed of glaciofluvial sand and gravel that are deposited by meltwater flowing through conduits beneath ice masses (Hebrand & Åmark, 1989;Shreve, 1985;Storrar et al, 2014). They exist as individual segments that often align to form networks up to 200 km in length and their morphology can vary from continuous single ridges to multiple ridged eskers to large esker complexes or deltas (Margold et al, 2011;Storrar et al, 2020). Within esker complexes it can be difficult to distinguish the individual esker ridges and where this occurs they are mapped as a polygon (Figure 5).…”