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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.

Subglacial hydrology modulates basal motion but remains poorly constrained, particularly for soft-bedded Greenlandic outlet glaciers. Here, we report detailed measurements of the response of subglacial water pressure to the connection and drainage of adjacent water-filled boreholes drilled through kilometre-thick ice on Sermeq Kujalleq (Store Glacier). These measurements provide evidence for gap opening at the ice-sediment interface, Darcian flow through the sediment layer, and the forcing of water pressure in hydraulically-isolated cavities by stress transfer. We observed a small pressure drop followed by a large pressure rise in response to the connection of an adjacent borehole, consistent with the propagation of a flexural wave within the ice and underlying deformable sediment. We interpret the delayed pressure rise as evidence of no pre-existing conduit and the progressive decrease in hydraulic transmissivity as the closure of a narrow (< 1.5 mm) gap opened at the ice-sediment interface, and a reversion to Darcian flow through the sediment layer with a hydraulic conductivity of ≤ 10−6 m s−1. We suggest that gap opening at the ice-sediment interface deserves further attention as it will occur naturally in response to the rapid pressurisation of water at the bed.

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

AbstractObjectiveTo externally validate four commonly used rules in computed tomography (CT) for minor head injury.DesignProspective, multicentre cohort study.SettingThree university and six non-university hospitals in the Netherlands.ParticipantsConsecutive 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 measuresThe primary outcome was any intracranial traumatic finding on CT; the secondary outcome was a potential neurosurgical lesion on CT, which was defined as an intracranial traumatic finding on CT that could lead to a neurosurgical intervention or death. The sensitivity, specificity, and clinical usefulness (defined as net proportional benefit, a weighted sum of true positive classifications) of the four CT decision rules. The rules included the CT in head injury patients (CHIP) rule, New Orleans criteria (NOC), Canadian CT head rule (CCHR), and National Institute for Health and Care Excellence (NICE) guideline for head injury.ResultsFor the primary analysis, only six centres that included patients with and without CT were selected. Of 4557 eligible patients who presented with minor head injury, 3742 (82%) received a CT scan; 384 (8%) had a intracranial traumatic finding on CT, and 74 (2%) had a potential neurosurgical lesion. The sensitivity for any intracranial traumatic finding on CT ranged from 73% (NICE) to 99% (NOC); specificity ranged from 4% (NOC) to 61% (NICE). Sensitivity for a potential neurosurgical lesion ranged between 85% (NICE) and 100% (NOC); specificity from 4% (NOC) to 59% (NICE). Clinical usefulness depended on thresholds for performing CT scanning: the NOC rule was preferable at a low threshold, the NICE rule was preferable at a higher threshold, whereas the CHIP rule was preferable for an intermediate threshold.ConclusionsApplication of the CHIP, NOC, CCHR, or NICE decision rules can lead to a wide variation in CT scanning among patients with minor head injury, resulting in many unnecessary CT scans and some missed intracranial traumatic findings. Until an existing decision rule has been updated, any of the four rules can be used for patients presenting minor head injuries at the emergency department. Use of the CHIP rule is recommended because it leads to a substantial reduction in CT scans while missing few potential neurosurgical lesions.

Various guidelines for minor head injury focus on patients with a Glasgow Coma Scale (GCS) score of 13–15 and loss of consciousness (LOC) or post-traumatic amnesia (PTA), while clinical management for patients without LOC or PTA is often unclear. We aimed to investigate the effect of presence and absence of LOC or PTA on intracranial complications in minor head injury. A prospective multi-center cohort study of all patients with blunt head injury and GCS score of 15 was conducted at six Dutch centers between 2015 and 2017. Five centers used the national guideline and one center used a local guideline—both based on the CT in Head Injury Patients (CHIP) prediction model—to identify patients in need of a computed tomography (CT) scan. We studied the presence of traumatic findings and neurosurgical interventions in patients with and without LOC or PTA. In addition, we assessed the association of LOC and PTA with traumatic findings with logistic regression analysis and the additional predictive value of LOC and PTA compared with other risk factors in the CHIP model. Of 3914 patients, 2249 (58%) experienced neither LOC nor PTA and in 305 (8%) LOC and PTA was unknown. Traumatic findings were present in 153 of 1360 patients (11%) with LOC or PTA and in 67 of 2249 patients (3%) without LOC and PTA. Five patients without LOC and PTA had potential neurosurgical lesions and one patient underwent a neurosurgical intervention. LOC and PTA were strongly associated with traumatic findings on CT, with adjusted odds ratios of 2.9 (95% confidence interval [CI] 2.2–3.8) and 3.5 (95% CI 2.7–4.6), respectively. To conclude, patients who had minor head injury with neither LOC nor PTA are at risk of intracranial complications. Clinical guidelines should include clinical management for patients without LOC and PTA, and they should include LOC and PTA as separate risk factors rather than as diagnostic selection criteria.