Sub‐annual moraine formation at an active temperate Icelandic glacier

Benjamin M.P Chandler,Marek W Ewertowski,Harold Lovell,David H Roberts,Aleksandra M Tomczyk,David J.A Evans,Samuel J.P Chandler,Martin Schaefer

doi:10.1002/esp.4835

Abstract

AbstractThis paper presents detailed geomorphological and sedimentological investigations of small recessional moraines at Fjallsjökull, an active temperate outlet of Öræfajökull, southeast Iceland. The moraines are characterized by striking sawtooth or hairpin planforms, which are locally superimposed, giving rise to a complex spatial pattern. We recognize two distinct populations of moraines, namely a group of relatively prominent moraine ridges (mean height ~1.2 m) and a group of comparatively low‐relief moraines (mean height ~0.4 m). These two groups often occur in sets/systems, comprising one pronounced outer ridge and several inset smaller moraines. Using a representative subsample of the moraines, we establish that they form by either (i) submarginal deformation and squeezing of subglacial till or (ii) pushing of extruded tills. Locally, proglacial (glaciofluvial) sediments are also incorporated within the moraines during pushing. For the first time, to our knowledge, we demonstrate categorically that these moraines formed sub‐annually using repeat uncrewed aerial vehicle (UAV) imagery. We present a conceptual model for sub‐annual moraine formation at Fjallsjökull that proposes the sawtooth moraine sequence comprises (i) sets of small squeeze moraines formed during melt‐driven squeeze events and (ii) larger push moraines formed during winter re‐advances. We suggest the development of this process‐form regime is linked to a combination of elevated temperatures, high surface meltwater fluxes to the bed and emerging basal topography (a depositional overdeepening). These factors result in highly saturated subglacial sediments and high porewater pressures, which induces submarginal deformation and ice‐marginal squeezing during the melt season. Strong glacier recession during the summer, driven by elevated temperatures, allows several squeeze moraines to be emplaced. This process‐form regime may be characteristic of active temperate glaciers receding into overdeepenings during phases of elevated temperatures, especially where their englacial drainage systems allow efficient transfer of surface meltwater to the glacier bed near the snout margin. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd

Highlights

Moraines formed by seasonally driven processes at a glacier margin are important terrestrial archives, since the processes contributing to their formation can be clearly linked to glaciological and climatic conditions active in a given year (e.g. Bradwell, 2004a; Beedle et al, 2009; Lukas, 2012; Chandler et al, 2016a)
We suggest that sub-annual moraine formation at Fjallsjökull is a process-form regime linked to glacier retreat into an overdeepening during a period of elevated summer temperatures
We examined a sequence of small recessional moraines (‘minor moraines’) at the southern margin of Fjallsjökull, an active temperate Icelandic glacier, using a combination of geomorphological mapping, sedimentological analysis, remote sensing and repeat uncrewed aerial vehicle (UAV) surveys

Summary

Introduction

Moraines formed by seasonally driven processes at a glacier margin are important terrestrial archives, since the processes contributing to their formation can be clearly linked to glaciological and climatic conditions active in a given year (e.g. Bradwell, 2004a; Beedle et al, 2009; Lukas, 2012; Chandler et al, 2016a). Moraines formed by seasonally driven processes at a glacier margin are important terrestrial archives, since the processes contributing to their formation can be clearly linked to glaciological and climatic conditions active in a given year Younger, inset annual moraines can form where glacier retreat during the summer exceeds successive winter re-advances over a number of years Sharp, 1984; Boulton, 1986; Krüger, 1995; Evans and Hiemstra, 2005; Lukas, 2012; Reinardy et al, 2013; Chandler et al, 2016a; Wyshnytzky, 2017). The sedimentological end products (ice-marginal moraines) in ancient glacial environments can be used to reconstruct past glacier dynamics (e.g. Ham and Attig, 2001; Sutherland et al, 2019)

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