Ice-sheet scale distribution and morphometry of triangular-shaped hummocks (murtoos): a subglacial landform produced during rapid retreat of the Scandinavian Ice Sheet

Antti E K Ojala,Gustaf Peterson,Christian Öhrling,Joni Mäkinen,Mark D Johnson,Elina Ahokangas,Jukka-Pekka Palmu,Kari Kajuutti

doi:10.1017/aog.2019.34

Abstract

AbstractHigh-resolution digital elevation models of Finland and Sweden based on LiDAR (Light Detection and Ranging) reveal subglacial landforms in great detail. We describe the ice-sheet scale distribution and morphometric characteristics of a glacial landform that is distinctive in morphology and occurs commonly in the central parts of the former Scandinavian Ice Sheet, especially up-ice of the Younger Dryas end moraine zone. We refer to these triangular or V-shaped landforms as murtoos (singular, ‘murtoo’). Murtoos are typically 30–200 m in length and 30–200 m in width with a relief of commonly <5 m. Murtoos have straight and steep edges, a triangular tip oriented parallel to ice-flow direction, and an asymmetric longitudinal profile with a shorter, but steeper down-ice slope. The spatial distribution of murtoos and their geomorphic relation to other landforms indicate that they formed subglacially during times of climate warming and rapid retreat of the Scandinavian Ice Sheet when large amounts of meltwater were delivered to the bed. Murtoos are formed under warm-based ice and may be associated with a non-channelized subglacial hydraulic system that evacuated large discharges of subglacial water.

Highlights

The increased resolution provided by digital elevation models (DEM) based on LiDAR (Light Detection and Ranging) reveals well-known glacial landforms in greater detail but landforms new to science
Compared with other glacial landforms, murtoos are most similar to glacial hummocks, which are commonly interpreted to be formed by stagnant ice
Murtoo fields that were identified in Sweden and Finland have, in some cases, earlier been mapped as ‘hummocky moraines’ or ‘dead-ice moraines’ (e.g. Virkkala, 1962; Perttunen and others, 1984; Malmberg Persson and Persson, 2011), which is not surprising because the distinctive morphological characteristics of murtoos in these heavily forested terrains is hard to discern without LiDAR

Summary

Introduction

The increased resolution provided by digital elevation models (DEM) based on LiDAR (Light Detection and Ranging) reveals well-known glacial landforms in greater detail but landforms new to science. Recent studies describe a distinct triangular or V-shaped glacial landform in the Fennoscandian Shield area (Seppälä, 2016; Mäkinen and others, 2017; Peterson and others, 2017) (Fig. 1) We call these landforms ‘murtoos’ (singular ‘murtoo’). Examples of this landform were found near the village of Murtoo, Finland, and the word ‘murtoo’ in Finnish refers to fragmentary topography. These triangularshaped landforms have been undetected in Sweden and Finland or have been included in ‘hummocky moraine’ or ‘dead-ice moraine’ on many geological survey maps. We present a map of their icesheet scale distribution and show that it is related to major deglacial climatic events including the Bølling-Allerød warm period, Greenland Stadial 1 (when the prominent Younger Dryas end moraines were formed) and postglacial Holocene warming

Objectives

Methods

Results

Conclusion