Borehole‐Based Characterization of Deep Mixed‐Mode Crevasses at a Greenlandic Outlet Glacier. Issue 2 (2nd June 2021)

• Record Type: Journal Article
• Authors: Hubbard, Bryn; Christoffersen, Poul; Doyle, Samuel H.; Chudley, Thomas R.; Schoonman, Charlotte M.; Law, Robert; Bougamont, Marion
• Is Part Of: AGU advances. Volume 2:Issue 2(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-06-02
• Subjects: borehole -- crevasse -- deep crevasse -- glacier -- Greenland Ice Sheet -- optical televiewer; Earth sciences -- Periodicals; Space sciences -- Periodicals; 550
• DOI: 10.1029/2020AV000291
• Languages: English
• ISSNs: 2576-604X

Abstract

Abstract: Optical televiewer borehole logging within a crevassed region of fast‐moving Store Glacier, Greenland, revealed the presence of 35 high‐angle planes that cut across the background primary stratification. These planes were composed of a bubble‐free layer of refrozen ice, most of which hosted thin laminae of bubble‐rich "last frozen" ice, consistent with the planes being the traces of former open crevasses. Several such last‐frozen laminae were observed in four traces, suggesting multiple episodes of crevasse reactivation. The frequency of crevasse traces generally decreased with depth, with the deepest detectable trace being 265 m below the surface. This is consistent with the extent of the warmer‐than‐modeled englacial ice layer in the area, which extends from the surface to a depth of ∼400 m. Crevasse trace orientation was strongly clustered around a dip of 63° and a strike that was offset by 71° from orthogonal to the local direction of principal extending strain. The traces' antecedent crevasses were therefore interpreted to have originated upglacier, probably ∼8 km distant involving mixed‐mode (I and III) formation. We conclude that deep crevassing is pervasive across Store Glacier, and therefore also at all dynamically similar outlet glaciers. Once healed, their traces represent planes of weakness subject to reactivation during subsequent advection through the glacier. Given their depth, it is highly likely that such traces—particularly those formed downglacier—survive surface ablation to reach the glacier terminus, where they may represent foci for fracture and iceberg calving. Plain Language Summary: Crevasses allow ice‐mass motion and the transfer of water and the heat it holds into the ice mass' interior. Crevasses extending deeper than some tens of meters have been inferred from indirect evidence such as radar scattering but have not been observed directly. Here, we evaluate the presence and properties of such deep crevasses by using an optical televiewer (OPTV) to record a continuous high‐resolution image of the complete wall of a borehole drilled to a depth of 325 m in Store Glacier, a heavily crevassed fast‐moving outlet glacier of the Greenland Ice Sheet. The OPTV image intersects several now‐closed crevasses to a depth of 265 m, many of which show evidence of multiple phases of opening and closing as they have been carried through the glacier to the borehole location. We infer that, at least in fast‐moving glaciers such as Store, deep crevasses are pervasive, may be regenerated several times, and are able to transfer water and heat from the surface to depths of at least 400 m. The crevasse traces we image are sufficiently deep to survive surface melting and reach the glacier terminus. Here, inherited traces could act as planes of weakness, enabling ice fracture and iceberg calving. Key Points: Borehole logging by optical televiewer reveals planes, interpreted to be crevasse traces, to a depth of 265 m at Store Glacier, Greenland Traces are nonvertical and offset in azimuth from principal extending strain, indicating formation by mixed‐mode fracture ∼8 km upglacier Crevasse traces show evidence of multiple reactivation phases, indicating that they represent planes of fracture weakness