Borehole-based characterization of deep crevasses at a Greenlandic outlet glacier

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-modelled 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 their 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.

1Centre for Glaciology, Department of Geography and Earth Sciences, Aberystwyth University, Aberystwyth, UK. 2Scott Polar Research Institute, University of Cambridge, Cambridge, UK.

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Figure S1 Caption for Figure S1 Caption for Movie S1 Figure S1. Full depth temperature profiles for BH18c showing measured englacial temperatures (red dots) and modelled temperatures (black dots). The depth axis extends from the ice surface (at 0 m) to the glacier bed at˜949 m.
Movie S1. Video of a surface meltwater channel˜15 cm across that has exploited and revealed a crevasse trace cropping out at the surface of Store Glacier. Note the clear, bubble-free ice bounding the mm-thick central bubble-rich lamina, similar to that imaged by our OPTV log of BH18c (see Figure 3 of main text).

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Borehole logging by optical televiewer reveals the presence of planes, interpreted to be crevasse 9 traces, to a depth of 265 m at Store Glacier, Greenland. 10 • Crevasses are inferred, from excess ice temperature, to penetrate to a depth of ~400 m. 11 • Crevasse traces are non-vertical and offset in azimuth from orthogonal to the local direction of 12 principal extending strain, indicating formation by mixed-mode fracturing, probably ~8 km upglacier. 13 • Crevasse traces show evidence of multiple reactivation phases, indicating that they represent planes 14 of fracture weakness. 15 Abstract Optical televiewer borehole logging within a crevassed region of fast-moving Store Glacier, 16 Greenland, revealed the presence of 35 high-angle planes that cut across the background primary 17 stratification. These planes were composed of a bubble-free layer of refrozen ice, most of which hosted thin 18 laminae of bubble-rich 'last frozen' ice, consistent with the planes being the traces of former open crevasses. 19 Several such last-frozen laminae were observed in four traces, suggesting multiple episodes of crevasse 20 reactivation. The frequency of crevasse traces generally decreased with depth, with the deepest detectable 21 trace being 265 m below the surface. This is consistent with the extent of the warmer-than-modelled 22 englacial ice layer in the area, which extends from the surface to a depth of ~400 m. Crevasse trace 23 orientation was strongly clustered around a dip of 63° and a strike that was offset by 71° from orthogonal to 24 the local direction of principal extending strain. The traces' antecedent crevasses were therefore interpreted 25 to have originated upglacier, probably ~8 km distant involving mixed-mode (I and III) formation. We conclude 26 that deep crevassing is pervasive across Store Glacier, and therefore also at all dynamically similar outlet 27 glaciers. Once healed, their traces represent planes of weakness subject to reactivation during their 28 subsequent advection through the glacier. Given their depth, it is highly likely that such traces -particularly 29 those formed downglacier -survive surface ablation to reach the glacier terminus, where they may represent 30 foci for fracture and iceberg calving. 31  1 Introduction   44   Surface crevassing contributes to bulk ice motion and enables the transfer of water and its thermal energy  45  from the surface of an ice mass to its subsurface. Theoretically, creep closure limits the depth of dry crevasses  46 to some tens of metres (see Mottram & Benn, 2009;Nye, 1955), but this can be increased substantially by 47

Plain language summary
hydrofracturing of water-filled crevasses (van der Veen, 1998; Weertman, 1973). Once initiated, 48 hydrofracture extends the tip and, if enough water is available to increase its depth accordingly, propagation 49 can continue to the glacier bed. This mechanism has been used to explain the rapid transfer of meltwater to 50 the glacier bed during discrete surface lake drainage events (e.g., Chudley   Lyn, 2008). Structural analysis and plotting were carried out using software packages BiFAT (Malone,  113 Hubbard, Merton-Lyn, Worthington, & Zwiggelaar, 2013), WellCAD, and Stereonet v.11 (Cardozo & 114 Allmendinger, 2013). In the analysis presented herein, local declination of -30° (west) was corrected for when 115 mapping the orientation of OPTV-imaged features. All feature azimuths are therefore presented (as three 116 digits) clockwise from grid (NSIDC Polar Stereographic North) north. 117 Borehole temperatures were recorded by sensor strings installed into two boreholes, BH18b and 118 BH18d, both located <10 m from BH18c. Englacial temperature was measured using Fenwal UNI-curve 192 119 thermistors (see  and DS18B20 digital temperature sensors (Table S1; Figure S1). 120 Undisturbed ice temperatures were estimated following established methods ( bed topography resampled to the 1 km fixed grid of our model. Surface air temperature was specified at the 130 same 1 km resolution using output from the RACMO regional climate model (Noël et al., 2018). (examples of which are indicated by red arrows in Figure 2b) that cut across the less distinct, lower-angle 153 banding noted above. Enlargements of the log (Figure 3) reveal that these layers form regular sinusoids, 154 indicating uniform and planar layers in 3D space. Each layer is formed of one or more millimetre-scale 155 laminae that are bright (reflective) and hence inferred to be bubble-rich. In 28 cases, these laminae are 156 enveloped by centimetre-to decimetre-thick layers of ice that appear dark and are hence transparent and 157 devoid of reflective bubbles. Of these 28 layers, 24 host a single central lamina (e.g., Figure 3a and b), one 158 hosts two such laminae, two host three such laminae, and one (a ~0.25 m-thick layer at a depth of ~184 m) 159 hosts nine such laminae (Figure 3c).  following Kamb (1959). Eigen analysis data are given in Table 1

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Accordingly, eigen analysis of the poles to these planes (green dots on Figure 5) indicates a strongly clustered 186 fabric with a dominant principal eigenvalue of 0.83 at a vector of 287° (Table 1). Considering the strength of 187 this mode, the second and third eigenvalues (of 0.1 and 0.07 respectively) have little physical meaning. 188 Table 1

Englacial ice temperature 192
The borehole sensor strings recorded 32 discrete ice temperatures between the near-surface and the ice-193 bed interface ( Figure S1). These temperatures decrease from ~-5 °C at a depth of ~25 m (the uppermost 194 thermistor location in the borehole) to ~-22 °C at 600 -700 m depth, to rise again sharply to temperate at 195 the bed at a depth of ~949 m. Over most of the borehole length, the modelled temperatures correspond 196 closely with the measured temperatures ( Figure S1). However, modelled temperatures diverge from the 197 observed record in the depth ranges 0 -~400 m and 770 -850 m. The former of these is relevant to the 198 present study, and temperature data over the depth range 0 -500 m are plotted in Figure 4b. Here, measured 199 temperatures are notably higher than modelled temperatures, a difference ('residual temperature') that 200 decreases with depth from ~10 °C near the surface to zero at ~400 m (Figure 4c Sweden (Fountain et al., 2005). Such ice is typical of refrozen ice from which gas has been expelled during 213 the freezing process (e.g., Pohjola, 1994), as are their bubble-rich central laminae. Here, gas is rejected at the 214 advancing ice front, increasing its concentration in the remaining reservoir of unfrozen water, eventually 215 reaching saturation. At that point, bubbles form and are incorporated as a lamina into the last-frozen ice 216 layer (Hubbard, 1991). The central position of the bubble-rich last-frozen lamina we observe in most of the 217 OPTV image of BH18c (Figures 3a and b) is consistent with water freezing inwards at a similar rate from both 218 edges of a crevasse. Similar features, also interpreted as crevasse traces, have been identified cropping out 219 at the glacier surface (e.g., Figure 6). 220

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Where exposed at the glacier surface, such clear ice crevasse traces commonly erode preferentially, providing 225 channels for supraglacial drainage (Hambrey & Müller, 1978) (also see Figure S2). We interpret multiple last-226 frozen laminae within a crevasse trace, such as that imaged at a depth of 184 m (Figure 3c), in terms of the 227 effects of multiple separate refreezing events. Although multiple central laminae have not, to our knowledge, 228 been reported elsewhere, reactivation of crevasse traces as thrust faults has been proposed (e.g., Goodsell,229 Hambrey, Glasser, Nienow, & Mair, 2005). At Store Glacier, we envisage trace reactivation to involve opening 230 along its bubble-rich last-frozen lamina and splitting that layer into two thinner bubble-rich laminae . The  231 open crevasse then refills with meltwater that eventually refreezes, forming a new (third) bubble-rich last-232 frozen lamina. After that, any subsequent reactivation can open any of the three pre-existing last-frozen 233 laminae and the process is repeated, each time adding a pair of new laminae and therefore generally resulting 234 in an odd total number of such laminae. This is consistent with our observations, and in particular with the 235 feature at 184 m depth which has nine such laminae (Figure 3c), indicating that the initial crevasse trace was 236 reactivated four times. In contrast, we imaged one high-angle layer with an even number of central laminae 237 (two). In this case, the crevasse could have reactivated along a new plane rather than along the pre-existing 238 central laminae. If this were the case then, of the 28 filled crevasse traces imaged in the uppermost 325 m of 239 BH18c, 24 showed no clear evidence of reactivation, three showed evidence of a single reactivation phase, 240 and one showed evidence of four reactivation phases. Of these seven inferred reactivation phases, six 241 occurred along a pre-existing bubble-rich last-frozen lamina. The seven bright laminae that were not visually 242 bounded by transparent (refrozen) ice likely formed either as unopened fractures or as dry crevasses, the 243 traces of which consequently did not host a refrozen ice layer. 244 Geometrical analysis of the crevasse traces intersected in BH18c (Figures 1b and 5)  opening under local strain conditions, with mixed mode fracture explaining the slight offset. However, the 255 englacial traces imaged by our OPTV log are highly unlikely to have formed locally by any combination of 256 fracture mode. This interpretation is supported by our inference of multiple trace reactivation since each 257 such event requires three phases: (i) crevasse opening, (ii) water filling, and (iii) water freezing. It is unlikely 258 that all three phases could occur multiple times within a single year and -although no independent evidence 259 is currently available to evaluate this -variability in the requisite fracturing and temperatures are greatest, 260 and therefore most likely, at the annual timescale. The traces we image -one with four reactivation phases 261 -are therefore likely to be at least four years old, and probably older. With a local ice velocity of ~700 m a -1 , 262 that places their origin at least ~3 km upglacier. Figure 1b shows the presence of two areas of opening 263 crevasses up-flow of the borehole location, one ~4 km distant and the other ~8 km distant. Since extending 264 strain rates are small between the first of these sites and the borehole (and crevasse reactivation is therefore 265 unlikely along this path) this location is unlikely to serve as the origin of the traces intercepted by the 266 borehole. In contrast, crevasses formed at the more distant site both have farther to travel (allowing more 267 time for reactivation) and pass through the lower site of locally high extending strain rates. Indeed, plotting 268 the trajectory of crevasse azimuth at the upper site to the borehole (illustrated by the paths of A to A' and B 269 to B' on Figure 1b) rotates this original azimuth to within 7° of the strike of the englacial traces imaged at the 270 borehole. We therefore consider that the most likely location for the formation of the crevasses whose traces 271 we image by OPTV in BH18c was ~8 km upglacier (although, without further evidence, formation farther 272 upglacier again cannot be discounted). 273 image. However, none of these crevasse traces appears to displace the pre-existing background stratification 281 (Figure 3). While it is possible that vertical displacement of lower-than-detectable magnitude did occur, we 282 estimate -based on the nature of the OPTV images and the irregularity of the stratification -that any such 283 displacement would have been less than a few millimetres in all cases. We therefore consider initial crevasse 284 formation involving a significant Mode II component as unlikely and instead favour Mode I combined with 285 Mode III fracture at, and upglacier of, our study site. 286 We also note that the traces we image dip at a mean angle of 63° and show no systematic change in 287 dip with depth. While the dip of open crevasses is difficult to measure directly for more than a few meters 288 below the surface, shallowly dipping crevasse traces have been reported from valley glaciers. For example, 289 Roberson and Hubbard (2010) identified transverse fractures (their 'S2') in OPTV borehole logs from midre 290 Lovénbreen, Svalbard, which dipped as shallowly as 60°. Similarly, Hambrey and Müller (1978) mapped 291 several sets of crevasse traces exposed at the surface of White Glacier, Canada. This mapping showed that 292 crevasse traces were typically near vertical ~3 km upglacier from the terminus, but that they dipped 293 progressively as they advected downglacier until they became almost horizontal at the terminus. It is not 294 therefore uncommon for crevasse traces to dip as they advect through an ice mass. That the primary 295 stratification has remained close to horizontal in our OPTV images at Store Glacier (Figures 2 and 3) suggests 296 that the crevasse traces have dipped without rotating the pre-existing stratification; thus the crevasses either 297 formed initially at a mean dip of 63°, or formed vertically and subsequently became less steep under simple 298 shear. If the latter, then the mean trace dip of 63° at the borehole site also supports our interpretation that 299 the antecedent crevasses formed several kilometres upglacier. Initially vertical crevasse formation, followed 300 by strain-induced shallowing during passage along the glacier, is also consistent with BH18c not intersecting 301 any local crevasses, despite them being only some metres distant at the glacier surface. 302 It takes ~40 years for ice to move from the borehole location to the front of Store Glacier, during 303 which time ~100 m of surface ice would be lost to ablation (assuming a surface ablation rate of 2.5 m a -1 ). 304 Thus, crevasse traces deeper than this at the borehole site, as well as those formed deep enough farther 305 down-glacier, would survive to the glacier's terminus, some to depths of 100s of metres. Given several of the 306 crevasse traces we report herein show evidence of reactivation -implying they are weaker than adjacent 307 host ice -and the continued formation of similar crevasses downglacier, nearer to the terminus, it is possible 308 that crevasse traces reaching the terminus present sub-vertical planes of weakness there, offering locations 309 of preferential fracture. If this is the case, then models of fracture mechanics may need to account for the 310 presence of such deep and geometrically recurring planes of weakness. 311 The frequency of intersected crevasse traces shows a general decrease with depth (Figure 4a), 312 consistent with their antecedent crevasses forming at the surface and terminating at different depths. 313 Assuming this depth distribution represents that of the original crevasses, then only seven (or 20%) of the 35 314 intersected by the borehole were shallower than 40 m depth and 12 (34%) were deeper than 100 m. Although 315 specific to our study site, the relationship between the crevasses present (%) per 30 m depth range (C(30)%) 316 and depth (D, m), as illustrated in Figure 4a, can be approximated (R = 0.69) as a logarithmic function: 317 (30) % = 40 − 6.2 ln( ) Eq. 1 This fit both suggests the presence of crevasses below the lowermost crevasse trace imaged by our OPTV log 318 (at a depth of 265 m) and is consistent with the difference between measured and modelled englacial 319 temperatures at our field site (Figure 4c), suggesting that crevasses warmed englacial ice to a depth of at 320 least ~400 m. Fitting a logarithmic curve to this residual temperature (Te, °C) against depth, as illustrated in 321 Figure 4c, yields a close match (R = 0.95) described by: 322 The similarity of the relationships between crevasse frequency and depth (Figure 4a; Eq. 1) and residual 323 temperature and depth (Figure 4c; Eq. 2) lends support to our interpretation that crevasses propagate to at 324 least 400 m below the surface of Store Glacier, warming ice to that depth by the presence and refreezing of 325 crevasse-filling meltwater. 326 5 Summary and conclusions 327 Our OPTV log of a borehole drilled in a crevassed area of fast-moving Store Glacier, Greenland, has 328 successfully imaged deep crevasses in the GrIS for the first time. Combining analysis of this log with thermo-329 mechanical modelling has revealed the following. 330 • Thirty-five traces of surface crevasses were imaged directly to a maximum depth of 265 m. It is possible 331 that the borehole intersected crevasses below this, but that they could not be distinguished from the 332 host ice. Although trace separation increased with depth, approximately one third of all traces were 333 intersected below a depth of 100 m. 334 • The borehole intersected traces of crevasses that were highly unlikely to have formed locally. 335 Comparison of trace orientation with the surface strain rate field of Store Glacier indicates their 336 antecedent crevasses most likely formed ~8 km upglacier. 337 • Despite drilling in an active crevasse field, and between two open crevasses spaced ~10 m apart, the 338 borehole intersected no local crevasses. This indicates that the active local crevasses were shallow 339 and/or did not deviate sufficiently from vertical to intersect the uppermost 325 m of the borehole. 340 • Crevasse fields analysed are not aligned perpendicular to the orientation of principal extending strain 341 rate, indicating mixed-mode formation, likely Modes I and III since the traces did not appear to displace 342 primary stratification vertically. 343 • Of the 35 traces imaged, 28 showed evidence of having been filled with meltwater that subsequently 344 refroze. This meltwater and its refreezing released heat, assumed to be responsible for warming the 345 ice to a depth of ~400 m, suggesting crevasses extended to this depth -although not imaged below 346 265 m by our OPTV log. 347 • Refreezing of crevasse-fill water creates a distinctive ice layer formed of a planar bubble-free ice layer 348 some centimetres to decimetres thick that envelopes a planar mm-thick central layer of bubble-rich 349 last-frozen ice. 350 • We hypothesize that, once formed, crevasse traces represent a plane of weakness that may be 351 reactivated during advection through the glacier. Visual analysis of the traces imaged by borehole 352 OPTV suggests that reopening occurs preferentially along the bubble-rich last-frozen layer(s), which 353 then refill and refreeze, creating additional new last-frozen layers. The time required for crevasse 354 opening, filling and refreezing is not known but is likely to be some years, consistent with the ~8 km 355 distance separating the borehole from the proposed location of initial crevasse formation. 356 • The crevasse traces imaged by our OPTV log, supplemented by new ones formed downglacier, extend 357 deep enough to survive ablation and reach the glacier terminus. Here, it is possible that these relatively 358 weak traces precondition the precise location of ice fracture, a process that may need to be addressed