Buoyant flexure and basal crevassing in dynamic mass loss at Helheim Glacier

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1 SUPPLEMENTARY INFORMATION DOI: /NGEO2204 Buoyant flexure and basal crevassing in dynamic mass loss at Helheim Glacier Timothy D. James*, Tavi Murray, Nick Selmes, Kilian Scharrer and Martin O Leary Glaciology Group, Department of Geography, Swansea University, Singleton Park, Swansea, SA2 8PP, United Kingdom Supplementary Methods Helheim Glacier is a major outlet of the Greenland Ice Sheet draining an area of ~52,000 km 2 (ref. 1). Its recent behaviour has been under much scrutiny due to reports of acceleration 2, 3, 4, retreat and thinning 4 found to occur quasi-synchronously with other marine-terminating glaciers in the southeast 1, 4. As Helheim is the closest major outlet glacier to southeast Greenland s main settlement, Tasiilaq, it has been a primary target of data collection efforts over the last decade. DEM Error Assessment The quality of topographic data of a dynamic surface like the calving margin of Helheim Glacier is difficult to quantify. For the terrestrial imagery, the photogrammetric block adjustment uses measured points and camera calibration information to predict the location and attitude of the cameras whose positions were surveyed with differential global positioning system data (dgps) providing an indication of the quality of the image block adjustment. The root mean square error (RMSE) of the predicted camera positions (Table S1) were <2m in XY and sub-metre in Z indicating a high relative accuracy between DEMs. Comparison to dgps camera positions give the absolute accuracy of the DEMs. Typically, NATURE GEOSCIENCE 1

2 error due to the image correlation stage of DEM generation is evaluated by comparing the data to a ground truth data set, which is of course not available here. Therefore, we conservatively estimate the error of our DEMs at ±5 m in the vertical and ±5 m horizontal at the calving front but degrading quickly with distance from the cameras. We base these estimates on the block adjustment results and the ability of our DEMs to resolve the daily flow of the glacier which is expected to be ~20 m day -1. Bed Topography Glacier behaviour is largely driven by bed topography especially at the marine terminus 5. Radar Depth Sounder lines have been flown at Helheim by the Center for the Remote Sensing of Ice Sheets (CReSIS) since 2001 and two gridded DEM products have been produced from these data. The CReSIS composite bed product ( and the Bamber bed elevation data set for Greenland 6, 7 provide gridded bed data sets for the Helheim catchment. However, the glacier bed beneath a heavily crevassed surface is notoriously difficult to measure from radar backscatter and signal loss from the rough surface. Due to the consequent sparse data in the vicinity of the calving front, we use only actual data points from CReSIS flightline product rather than the gridded bed products. Bed DEM root mean square error is quoted for the full data set as ~46 m based on radar line crossover analysis. However, in vicinity of the terminus, error can be up to 200 m (Figure S8). The most recent radar flightlines, 2011 and 2013, use the newer Multichannel Coherent Radar Depth Sounder (MCoRDS) developed for improved performance over crevassed ice where weak and ambiguous bed echoes are problematic. However, the still large uncertainty is complicated by the fact that where the glacier bed is not in contact with the fjord bed (i.e. 2

3 at the calving front), the radar images the ice base, not the fjord bed. We interpret the upturned end of the 2013 line to be the bottom of the rotating calving section shown in the surface profile. While we expect the data from the newer instrument to be more accurate than the older data, the 2001 line (used in ref. 8) was flown when the glacier was further advanced, thicker and slower, which may provide better conditions for imaging the bed. 3

4 Supplementary Results Figure S1 Example of a DEM generated from stereo terrestrial photography. (a) DEM presented in shaded-relief and coloured by elevation. (b) Corresponding oblique terrestrial photograph with a similar viewing angle to (a). Movie S2 Time-lapse video of calving event at Helheim Glacier. This video captures a major calving event at Helheim on 12 July 2010 in 10 second time-lapse imagery between 18:40 and 20:10 UTC. The main event was followed by a smaller event (at 00:56) on the south side of the fjord in which sudden ice fracturing prior to the event can be seen. 4

5 Figure S3 Image feature tracking prior to the 14 July :00-18:00 UTC calving event. This event involved the whole front except a small section on the south (left) side of the fjord. Larger error is visible due to the poor lighting conditions, however, the movement of the front is still clearly visible. Note also that the front on the north (right) side is sufficiently lifted to obscure the depression. 5

6 Figure S4 Image feature tracking prior to the 19 July :00-00:00 UTC calving event. This event involved the southern (left) third of the glacier and a small section on the north (right) shore. 6

7 Figure S5 Image feature tracking prior to the 12 Aug :00-23:00 UTC calving event. This event involved the full glacier width and was unique in that the southern section (red line) produced an overturning iceberg whereas the northern section (blue line) produced two large tabular icebergs. While the frontal uplift was only seen ahead of the overturning calving event, a surface depression formed across the entire calving width. On the south side 7

8 where the iceberg overturned, the width of the calving section was much narrower in the direction of ice flow than the tabular calving section ice on the north side. Figure S6 Interactive annotated time-lapse of the evolution of the Helheim calving front. View using fit-to-screen and the arrow keys to navigate forward and backward through time-series. 8

9 Figure S7 ASTER satellite image showing rotated front section. An ASTER image from 18 July 2004 shows the surface of Helheim three days prior to a large calving event 8. Contours highlight the surface depression and lifted front caused by the rotation of the front section. In places the calving front is >30 m higher than the elevation of the depression and the calving face itself can be seen clearly from the satellite platform indicating that significant rotation has already occurred. A large rift (700 m x 140 m) has formed where the ice failed at the most depressed point of the surface. The red dot shows the point of lowest elevation; ~90 m above geoid, ~40 m above sea-level. No water was visible in the crevasses at the calving front in these images. 9

10 Figure S8 Bed data at the Helheim calving margin. Blue circles represent bed elevation measurements from the most recent CReSIS bed product, the Helheim Composite (HHC, see Supplemental Methods, Bed Topography). Blue squares with same depth colouration are 2001 depths presented in ref. 8. Discrepancies in bed depth between flightlines are shown at crossover points as pink circles. Grey dots represent points that occur in the HHC product but where only surface measurements are available. White line shows location of Figure 2 profiles. Background is an 08 July 2010 ASTER scene. 10

11 Table S1 Photogrammetric block adjustment results. Root mean square errors (RMSE in meters) are provided for the predicted camera positions relative to their mean and relative camera positions measured by dgps. RMSE X RMSE Y RMSE Z Camera 1 Relative Camera 2 Relative Camera 1 DGPS Camera 2 DGPS Supplementary References 1. Murray T., Scharrer K., James T. D., Dye S. R., Hanna E., Booth A. D., et al. Ocean regulation of glacier dynamics in south-east Greenland and implications for ice-sheet mass changes. J. Geophys. Res.-Earth 115, doi: /2009JF (2010). 2. Luckman A., Murray T., de Lange R.& Hanna E. Rapid and synchronous icedynamic changes in East Greenland. Geophys. Res. Lett. 33, L03503, doi: /02005GL (2006). 3. Rignot E.& Kanagaratnam P. Changes in the velocity structure of the Greenland ice sheet. Science 311, (2006). 4. Howat I. M., Joughin I., Fahnestock M., Smith B. E.& Scambos T. A. Synchronous retreat and acceleration of southeast Greenland outlet glaciers : ice dynamics and coupling to climate. J. Glaciol. 54, (2008). 5. Vieli A.& Nick F. M. Understanding and modelling rapid dynamic changes of tidewater outlet glaciers: issues and implications. Surv. Geophys. 32, (2011). 11

12 6. Allen C. IceBridge MCoRDS L3 Gridded Ice Thickness, Surface, and Bottom. Version 2. Helheim_2008_2012_Composite., Bamber J. L., Griggs J. A., Hurkmans R. T. W. L., Dowdeswell J. A., Gogineni S. P., Howat I., et al. A new bed elevation data set for Greenland. The Cryosphere 7, (2013). 8. Joughin I., Howat I., Alley R. B., Ekstrom G., Fahnestock M., Moon T., et al. Icefront variation and tidewater behavior on Helheim and Kangerdlugssuaq Glaciers, Greenland. J. Geophys. Res. 113, F01004 (2008). 12