Numerical modelling of subglacial ribs, drumlins, herringbones, and mega‐scale glacial lineations reveals their developmental trajectories and transitions

Abstract

Initially a matter of intellectual curiosity, but now important for understanding ice‐sheet dynamics, the formation of subglacial bedforms has been a subject of scientific enquiry for over a century. Here, we use a numerical model of the coupled flow of ice, water, and subglacial sediment to explore the formation of subglacial ribs (i.e., ribbed moraine), drumlins and mega‐scale glacial lineations (MSGLs). The model produces instabilities at the ice–bed interface, which result in landforms resembling subglacial ribs and drumlins. We find that a behavioural trajectory is present. Initially subglacial ribs form, which can either develop into fields of organized drumlins, or herringbone‐type structures misaligned with ice flow. We present potential examples of these misaligned bedforms in deglaciated landscapes, the presence of which means caution should be taken when interpreting cross‐cutting bedforms to reconstruct ice flow directions. Under unvarying ice flow parameters, MSGLs failed to appear in our experiments. However, drumlin fields can elongate into MSGLs in our model if low ice–bed coupling conditions are imposed. The conditions under which drumlins elongate into MSGLs are analogous to those found beneath contemporary ice streams, providing the first mechanism, rather than just an association, for linking MSGLs with ice stream flow. We conclude that the instability theory, as realized in this numerical model, is sufficient to explain the fundamental mechanics and process‐interactions that lead to the initiation of subglacial bedforms, the development of the distinctive types of bedform patterns, and their evolutionary trajectories. We therefore suggest that the first part of the longstanding ‘drumlin problem’ – how and why they come into existence – is now solved. However, much remains to be discovered regarding the exact sedimentary and hydrological processes involved.

Conceptual model for the formation of bedforms along subglacial meltwater corridors (SMCs) by variable ice‐water‐bed interactions

“…b) (Rattas & Piotrowski, 2003). The combination of these hypotheses would indicate that ice-bed coupling and deformation on top of bedforms could co-exist with meltwater flow inbetween bedforms, suggesting incomplete ice-bed coupling (Ely et al, 2023;Fannon et al, 2017).…”

“…Ribbed and streamlined bedforms are primarily thought to result from ice‐water‐bed interactions and bed deformation (e.g. Boulton, 1987; Ely et al, 2023; Fannon et al, 2017), modulated by factors such as grain size, bed thickness, topography, ice thickness, ice surface slope and/or ice velocity. Significantly, the spatiotemporal evolution of subglacial drainage systems, notably of hydraulic connectivity and water pressure, acts as an important controlling factor responsible for variations in ice‐bed coupling and subglacial bed deformation (Boulton et al, 2001; Lewington et al, 2020).…”

“…Their stubby appearance could reflect differences in till rheology (influencing bed stiffness and deformability) or the influence of increased erosion in‐between bedforms through the development of meltwater channels in response to high effective pressures in the till and more efficient drainage (Figures 11a‐b) (Rattas & Piotrowski, 2003). The combination of these hypotheses would indicate that ice‐bed coupling and deformation on top of bedforms could co‐exist with meltwater flow in‐between bedforms, suggesting incomplete ice‐bed coupling (Ely et al, 2023; Fannon et al, 2017).…”

Assessing ice sheet models against the landform record: The Likelihood of Accordant Lineations Analysis (LALA) tool

“…Often, these sources of uncertainty are potentially more difficult to quantify. From a data perspective, an uncertainty remains regarding the mechanics of lineation formation, which are still debated (for a recent summary, see Ely et al., 2022). Though we suggest reasonable conditions under which lineations are formed and preserved, ultimately users of LALA may wish to alter these according to any further insights gained in this field.…”

“…Lineations are typically on the order of 100–1000 m in length, though they can reach several kilometres in the case of MSGL (Spagnolo et al., 2014). The origin of subglacial lineations, especially drumlins, has garnered much scientific debate and is an active field of research (see, e.g., Ely et al., 2022). However, general agreement can be found amongst geomorphologists on two factors: (i) Lineations are streamlined in the direction of ice flow, and thus represent former ice flow direction at a point in time during the lifecycle of an ice sheet; and (ii) lineations form under warm‐based conditions, conducive to the transport of subglacial material.…”