Borehole-Based Characterization of Deep Mixed-Mode Crevasses at a Greenlandic Outlet Glacier

Funder: Aberystwyth University Capital Equipment FundAbstract: 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

Thermodynamics of a fast-moving Greenlandic outlet glacier revealed by fiber-optic distributed temperature sensing

Funding: This research was funded by the European Research Council as part of the RESPONDER project under the European Union’s Horizon 2020 research and innovation program (grant 683043). R.L. and T.R.C. were supported by Natural Environment Research Council Doctoral Training Partnership studentships (grant NE/ L002507/1). B.H. was supported by a HEFCW/Aberystwyth University Capital Equipment Grant.Measurements of ice temperature provide crucial constraints on ice viscosity and the thermodynamic processes occurring within a glacier. However, such measurements are presently limited by a small number of relatively coarse-spatial-resolution borehole records, especially for ice sheets. Here, we advance our understanding of glacier thermodynamics with an exceptionally high-vertical-resolution (~0.65 m), distributed-fiber-optic temperature-sensing profile from a 1043-m borehole drilled to the base of Sermeq Kujalleq (Store Glacier), Greenland. We report substantial but isolated strain heating within interglacial-phase ice at 208 to 242 m depth together with strongly heterogeneous ice deformation in glacial-phase ice below 889 m. We also observe a high-strain interface between glacial- and interglacial-phase ice and a 73-m-thick temperate basal layer, interpreted as locally formed and important for the glacier's fast motion. These findings demonstrate notable spatial heterogeneity, both vertically and at the catchment scale, in the conditions facilitating the fast motion of marine-terminating glaciers in Greenland.Publisher PDFPeer reviewe

External validation of computed tomography decision rules for minor head injury: Prospective, multicentre cohort study in the Netherlands

Objective To externally validate four commonly used rules in computed tomography (CT) for minor head injury. Design Prospective, multicentre cohort study. Setting Three university and six non-university hospitals in the Netherlands. Participants Consecutive adult patients aged 16 years and over who presented with minor head injury at the emergency department with a Glasgow coma scale score of 13-15 between March 2015 and December 2016. Main outcome measures The primary outcome was any intracrania

Thermodynamics of a fast-moving Greenlandic outlet glacier revealed by fiber-optic distributed temperature sensing.

Measurements of ice temperature provide crucial constraints on ice viscosity and the thermodynamic processes occurring within a glacier. However, such measurements are presently limited by a small number of relatively coarse-spatial-resolution borehole records, especially for ice sheets. Here, we advance our understanding of glacier thermodynamics with an exceptionally high-vertical-resolution (~0.65 m), distributed-fiber-optic temperature-sensing profile from a 1043-m borehole drilled to the base of Sermeq Kujalleq (Store Glacier), Greenland. We report substantial but isolated strain heating within interglacial-phase ice at 208 to 242 m depth together with strongly heterogeneous ice deformation in glacial-phase ice below 889 m. We also observe a high-strain interface between glacial- and interglacial-phase ice and a 73-m-thick temperate basal layer, interpreted as locally formed and important for the glacier's fast motion. These findings demonstrate notable spatial heterogeneity, both vertically and at the catchment scale, in the conditions facilitating the fast motion of marine-terminating glaciers in Greenland.European Union's Horizon 2020 (Grant 683043

Supraglacial lake drainage at a fast-flowing Greenlandic outlet glacier

Supraglacial lake drainage events influence Greenland Ice Sheet dynamics on hourly to interannual timescales. However, direct observations are rare, and, to date, no in situ studies exist from fast-flowing sectors of the ice sheet. Here, we present observations of a rapid lake drainage event at Store Glacier, west Greenland, in 2018. The drainage event transported 4.8 × 106 m3 of meltwater to the glacier bed in ∼5 h, reducing the lake to a third of its original volume. During drainage, the local ice surface rose by 0.55 m, and surface velocity increased from 2.0 m⋅d-1 to 5.3 m⋅d-1 Dynamic responses were greatest ∼4 km downstream from the lake, which we interpret as an area of transient water storage constrained by basal topography. Drainage initiated, without any precursory trigger, when the lake expanded and reactivated a preexisting fracture that had been responsible for a drainage event 1 y earlier. Since formation, this fracture had advected ∼500 m from the lake's deepest point, meaning the lake did not fully drain. Partial drainage events have previously been assumed to occur slowly via lake overtopping, with a comparatively small dynamic influence. In contrast, our findings show that partial drainage events can be caused by hydrofracture, producing new hydrological connections that continue to concentrate the supply of surface meltwater to the bed of the ice sheet throughout the melt season. Our findings therefore indicate that the quantity and resultant dynamic influence of rapid lake drainages are likely being underestimated.This research was funded by the European Research Council as part of the RESPONDER project under the European Union’s Horizon 2020 research and innovation programme (grant 683043). TRC was supported by a Natural Environment Research Council Doctoral Training Partnership Studentship (grant NE/L002507/1)

RESPONDER borehole response test datasets

Datasets from the RESPONDER project as published in the paper by Doyle et al. entitled "Water flow through sediments and at the ice-sediment interface beneath Sermeq Kujallleq (Store Glacier), Greenland". Please cite this paper if using this data