The life history of sea ice is written in its basal morphology. Sea ice progresses through several stages during its lifecycle, including frazil, nilas and pancake ice, until it bonds into a near-continuous sheet known as first-year (FY) ice. If this ice survives the summer melt-season, it becomes multiyear (MY) ice (Weeks & Ackley 1986). Both FY and MY sea ice have significant morphological variability, ranging from undeformed, level ice to heavily deformed or ridged ice. These variations lead to a complex, highly heterogeneous ice topography. Profiling the underside of Arctic sea ice was first performed by nuclear submarines (e.g. Wadhams 1988, 2000); however, since the end of the Cold War, military submarine transits of the Arctic have become less frequent and are insufficient to understand climate-induced trends. Therefore, deployment of autonomous underwater vehicles (AUVs) equipped with upward-looking multibeam sonar represents an important innovation.
Description
Sea ice is not a homogeneous medium. It occurs in a wide variety of forms due to the various thermodynamic and dynamic forces it undergoes during its lifecycle of growth, decay and deformation. The greatest variability is associated with deformed ice in the form of ridges: long, quasi-linear features composed of blocks of ice, piled both above and below the relatively level ice surface. Ridge sails may be up to several metres in height above the sea surface and keels may reach depths of >40 m. These morphological variations lead to a complex, highly heterogeneous sea-ice topography that varies on scales from <1 metre to 100s of kilometres.
Several frequently occurring sea-ice keel types and morphological features imaged by an upward-looking multibeam system mounted on the Autosub-II AUV are described. These data were acquired from sea ice in the Belgica Bank region of the NE Greenland continental shelf (Fig. 1a). A series of AUV missions (up to 25 hours duration or about 150 km in length; AUV depth 40 m) were undertaken westwards from the Belgica Bank shorefast sea-ice edge towards the Greenland coast. A systematic survey of the underside of locally grown shorefast sea-ice, known as Norske Øer Ice Barrier, also took place (Wadhams et al. 2006).
Multibeam images and topographic profiles of the underside of sea ice showing different sea-ice keel morphologies. (a) Location of study area showing the region of the Autosub-II AUV missions on Belgica Bank, NE Greenland (red box; map from IBCAO v. 3.0). WT, Westwind Trough. (b) Oblique multibeam swath image showing the three-dimensional shape of the underside of a large, multiyear (MY) sea-ice keel; colour scale is sea-ice draft. (c) Plan view of the underside of an old MY ridged floe embedded within first year (FY) ice; colour scale is sea-ice draft. (d) Sea-ice draft profile across the features in (c). VE×10. (e) Plan view image of a young ridge (centre) and an older MY hummock within a field of well-rounded FY floes glued together by very young ice; colour scale is sea-ice draft. (f) Sea-ice draft profile across the ridge and hummock features in (e). VE×14. (b, c, e) Multibeam acquisition system Kongsberg EM2000. Frequency 200 kHz. Grid-cell size 1 m.
Examples of the morphological variations of sea-ice keels are shown in Figure 1b–f. The deepest ridge encountered during the survey had a draft (ice thickness below the sea surface) of 33 m (Fig. 1b). The ridge consists of small individual blocks of crushed sea-ice forming a relatively uniform triangle in cross-section, representing the angle of repose of a pile of buoyant ice. A smaller ridge is visible in the foreground. The undeformed ice on the nearside corner of the image is 1.75 m in draft, whereas between and behind the two ridges the ice is 4 m thick. In the region behind the giant MY ridge, the ice exhibits a ‘rubble field’ texture with drafts up to 10 m. Figure 1c, d shows a ridged floe of draft 3–5 m that is embedded in thinner ice with a draft of 1.8 m. The ridge, which occupies half the floe, has a maximum draft of 11 m and contains separate ice blocks of typical diameters 5–20 m (Fig. 1c, d); the edges of the floe are sharp and linear (Fig. 1c).
A third sea-ice keel morphology is shown in Figure 1e, f from Westwind Trough. Well-rounded, thin (draft 1.2 m) floes are glued together by very young ice (draft less than 0.25 m) (Fig. 1f). A narrow and linear ridge in the centre of the image and an old hummock, presumably eroded partly by summer melt processes, can be seen as well as a newly formed steep FY ridge. (Fig. 1e). Between the two is an ice floe of draft 1.85–2.25 m containing the hummock and the FY ridge.
Interpretation
It is likely that the giant ridge in Figure 1b is part of an embedded MY sea-ice floe that has drifted onto Belgica Bank from the Arctic Ocean (Fig. 1a). The thin, undeformed ice is probably FY ice, whereas the thicker ice between the ridges is probably MY ice. The ‘rubble field’ morphology derives from crushing processes that occurred sometime during the long history of the MY floe as it made contact with other floes.
The thicker MY ice floe and ridge in Figure 1c, d resemble floes observed in the marginal ice zone (MIZ) of Fram Strait (Wadhams 2000), where large sheets of advecting Arctic sea-ice are fractured by wave action and turned into floes of typical diameter 100 m that retain fragments of their original pressure ridges. The sharp edges of the floe are produced by fracturing that probably occurred just before embedding, when the floe was in a state of disintegration; observations in the MIZ often show such floes shedding ice blocks from their underside under the impact of waves (Wadhams 2000). The surrounding thinner ice is probably FY ice, with thicker small blocks embedded in it which were once small wave-broken cakes floating freely off the ice edge. The faint pattern of depressions in the underside is due to the presence of meltwater pools on the upper surface. These pools preferentially absorb incoming radiation, yielding a heat flux which enhances bottom melting and generates a bottom depression which mirrors the pool's position on the upper surface (Wadhams 2000).
The thin, well-rounded floes in Figure 1e are FY floes and the central ridge is a young (probably FY) feature with the hummock being an older, water-eroded MY feature. The narrow, linear morphology of the young ridge may indicate that it is a shear ridge rather than a pressure ridge, formed by motion between the FY ice and the older floe. The contrast between the sharpness of topography of the ridges and the hummock is dramatic (e.g. Fig. 1f), showing the effect of a number of years of ageing and partial melt in rounding off the blocky topography of the older hummock.
It can be seen from these results that multibeam imagery of the underside of sea ice provides valuable data on the processes involved in its deformation and melting. The significant depth of some sea-ice keels has the potential to impact upon the seafloor in areas of shallow water (<50 m), which were much more abundant during glacials when global sea-level was lower by around 120 m. Information contained within these mutibeam images is therefore of value both to industry for determining ice forces on fixed and floating engineering structures and in our understanding of ploughing of the sedimentary seafloor by sea-ice submarine ridges.
- © 2016 The Author(s). Published by The Geological Society of London. All rights reserved
