Subglacial melt channels and fracture in the floating part of Pine Island Glacier, Antarctica

Abstract

[1] A dense grid of ice-penetrating radar sections acquired over Pine Island Glacier, West Antarctica has revealed a network of sinuous subglacial channels, typically 500 m to 3 km wide, and up to 200 m high, in the ice-shelf base. These subglacial channels develop while the ice is floating and result from melting at the base of the ice shelf. Above the apex of most channels, the radar shows isolated reflections from within the ice shelf.Comparison of the radar data with acoustic data obtained using an autonomous submersible, confirms that these echoes arise from open basal crevasses 50-100 m wide aligned with the subglacial channels and penetrating up to 1/3 of the ice thickness. Analogous sets of surface crevasses appear on the ridges between the basal channels. We suggest that both sets of crevasses were formed during the melting of the subglacial channels as a response to vertical flexing of the ice shelf toward the hydrostatic condition. Finite element modeling of stresses produced after the formation of idealized basal channels indicates that the stresses generated have the correct pattern and, if the channels were formed sufficiently rapidly, would have sufficient magnitude to explain the formation of the observed basal and surface crevasse sets. We conclude that ice-shelf basal melting plays a role in determining patterns of surface and basal crevassing. Increased delivery of warm ocean water into the sub-ice shelf cavity may therefore cause not only thinning but also structural weakening of the ice shelf, perhaps, as a prelude to eventual collapse.

Interannual changes of the floating ice shelf of Petermann Gletscher, North Greenland, from 2000 to 2012

“…These undulations occur near the grounding line and do not have surface expressions. They appear similar in form to basal crevasses observed recently at Pine Island Glacier, Antarctica (Bindschadler and others, 2011;Vaughan and others, 2012), but have not previously been observed in North Greenland. Following Vaughan and others (2012), we hypothesize that stresses across these crevasses may contribute to structural weakening of the ice shelf.…”

Channelization of plumes beneath ice shelves

“…A number of ice shelves have been observed to have channel-like incisions at their base, oriented roughly in the direction of ice flow (Rignot & Steffen 2008;Bindschadler et al 2011;Vaughan et al 2012;LeBrocq et al 2013). Observationally inferred basal melt rates are enhanced within the channels compared to the neighbouring thicker ice (Rignot & Steffen 2008;Dutrieux et al 2013), and plumes containing a mixture of entrained warm deep waters and ice shelf melt have been observed to emerge from under Pine Island ice shelf in the vicinity of larger channels (Mankoff et al 2012).…”

“…Observationally inferred basal melt rates are enhanced within the channels compared to the neighbouring thicker ice (Rignot & Steffen 2008;Dutrieux et al 2013), and plumes containing a mixture of entrained warm deep waters and ice shelf melt have been observed to emerge from under Pine Island ice shelf in the vicinity of larger channels (Mankoff et al 2012). The existence of channels has an important but unclear effect on the long term evolution of an ice shelf; bending stresses resulting from uneven melting may lead to crevassing and eventual break-up (Vaughan et al 2012), while the focussing of buoyant flow may actually lower the average melt rate, thus protecting it from ocean warming to some extent (Gladish et al 2012;Millgate et al 2013). It is of interest to understand how such channels are created.…”

Complex network of channels beneath an Antarctic ice shelf

“…More recent studies found basal crevasses and channels of various sizes oriented both perpendicular and parallel to the main ice-flow direction at Larsen C, Amery and Getz, Pine Island Glacier, and Fimbul, as well as in Greenland [Luckman et al, 2012;McGrath et al, 2012;Vaughan et al, 2012;Mankoff et al, 2012;Humbert and Steinhage, 2011;Nicholls et al, 2006;Rignot and Steffen, 2008]. The majority of the basal channels reported to date are from ice shelves exposed to ocean waters with a high thermal potential for melting the overlying ice shelf, such as at Pine Island Glacier [Jacobs et al, 2011;Dutrieux et al, 2013].…”

“…The base of an ice shelf is a critical interface; basal roughness in the form of channels influences ice-shelf stability [Gladish et al, 2012;McGrath et al, 2012;Vaughan et al, 2012], affects ocean circulation close to the ice base, and plays an important role in modifying the exchange of heat and mass between the ocean and ice shelf [Stanton et al, 2013]. Mapping of the ice basal topography, in most cases, relies on ice surface elevations from satellite altimetry and the freeboard assumption to calculate ice thickness [Pritchard et al, 2012]; thus, basal topography on spatial scales similar or less than the local ice-shelf thickness remains largely unknown for most ice shelves.…”