Water-driven fracture propagation beneath supraglacial lakes rapidly transports large volumes of surface meltwater to the base of the Greenland Ice Sheet1. These drainage events drive transient ice-sheet acceleration1, 2, 3 and establish conduits for additional surface-to-bed meltwater transport for the remainder of the melt season1, 4, 5, 6. Although it is well established that cracks must remain water-filled to propagate to the bed7, 8, 9, the precise mechanisms that initiate hydro-fracture events beneath lakes are unknown. Here we show that, for a lake on the western Greenland Ice Sheet, drainage events are preceded by a 6–12 hour period of ice-sheet uplift and/or enhanced basal slip. Our observations from a dense Global Positioning System (GPS) network allow us to determine the distribution of meltwater at the ice-sheet bed before, during, and after three rapid drainages in 2011–2013, each of which generates tensile stresses that promote hydro-fracture beneath the lake. We hypothesize that these precursors are associated with the introduction of meltwater to the bed through neighbouring moulin systems (vertical conduits connecting the surface and base of the ice sheet). Our results imply that as lakes form in less crevassed, interior regions of the ice sheet10, 11, 12, 13, 14, where water at the bed is currently less pervasive5, 14, 15, 16, the creation of new surface-to-bed conduits caused by lake-draining hydro-fractures may be limited.
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Extended data figures and tables
Extended Data Figures
- Extended Data Figure 1: WorldView image taken on 21 July 2011 of an empty North Lake basin after the 2011 rapid drainage event. (383 KB)
Yellow outline shows M1 and M2 location along the hydro-fracture trace (endpoints marked by black arrows). Yellow triangles mark GPS stations within the map area. Image copyright 2015 DigitalGlobe.
- Extended Data Figure 2: Images and DEM of 2011 and 2013 North Lake basin. (290 KB)
a (d), WorldView image chosen to map the 2011 (2013) North Lake pre-drainage shoreline position. b (e), WorldView image of an empty North Lake basin obtained on 21 July 2011 (5 July 2013) used to create the 2011 (2013) North Lake DEM. c (f), The 2-m horizontal resolution DEM (2-m vertical contours in black) for the North Lake region, with the North Lake shoreline (red), M1 (yellow), and hydro-fracture trace (blue) mapped over contours. Images copyright 2015 DigitalGlobe.
- Extended Data Figure 3: North Lake depths in 2011 and 2013. (472 KB)
Two-metre resolution DEMs were created from the first available post-drainage WorldView stereo pair obtained of the region in (a) 2011 and (b) 2013. Shoreline positions from 2011 and 2013 derived from last pre-drainage WorldView or TerraSAR-X images obtained over the region are shown in red. The last pre-drainage WorldView image for 2011 occured 2 days before the drainage event on 17 June 2011; the last pre-drainage SAR image for 2013 occured 1 day before the event on 17 June 2013. Filling the empty basin DEM up to the greatest known pre-drainage shoreline extent generated North Lake depths (1-m vertical contours in black) in relation to the greatest known pre-drainage shoreline extents and were used to calculate minimum 2011 and 2013 North Lake pre-drainage volumes. The trace of the vertical hydro-fracture crack is shown in grey; M1 is outlined in yellow.
- Extended Data Figure 4: The 2012 basal slip and cavity opening at hydro-fracture initiation and maximum hydro-fracture opening. (463 KB)
NIF-calculated (a) extra basal slip accumulated, (b) basal cavity opening, and (c) hydro-fracture crack opening at the time of the 2012 (a–c) hydro-fracture initiation and (d–f) maximum hydro-fracture opening (time points shown in Fig. 2a). Moulin location, last known lake shoreline, GPS stations, and NIF vertical crack surface trace derived from SAR imagery are shown as a yellow circle, blue line, black triangles, and black line, respectively. Vector fields show GPS (NIF) displacement less background velocities in black (green) for (a) the period between the start of the precursor and hydro-fracture initiation, and (d) the period between hydro-fracture initiation and maximum hydro-fracture opening. Error ellipses of 1 sigma are shown for the GPS displacements (blue ellipses). Basal sub-elements are 0.83 km by 0.83 km, resulting in 144 sub-elements over a 10 km × 10 km region.
- Extended Data Figure 5: The 2013 basal slip and cavity opening at hydro-fracture initiation and maximum hydro-fracture opening. (496 KB)
NIF-calculated (a) extra basal slip accumulated, (b) basal cavity opening, and (c) hydro-fracture crack opening at the time of the 2013 (a–c) hydro-fracture initiation and (d–f) maximum hydro-fracture opening (time points shown in Fig. 2a). Moulin location, last known lake shoreline, GPS stations, and NIF vertical crack surface trace derived from SAR imagery are shown as a yellow circle, blue line, black triangles, and black line, respectively. Vector fields show GPS (NIF) displacement less background velocities in black (green) for (a) the period between the start of the precursor and hydro-fracture initiation, and (d) the period between hydro-fracture initiation and maximum hydro-fracture opening. Error ellipses of 1 sigma are shown for the GPS displacements (blue ellipses). Basal sub-elements are 0.83 km by 0.83 km, resulting in 144 sub-elements over a 10 km by 10 km region.
- Extended Data Figure 6: The 2011 station time series. (381 KB)
a–c, Flowline, crack-normal, and uplift GPS displacements (in metres) (grey stars), respectively, for stations used in the 2011 NIF. NIF station fits from the three displacement sources (Extended Data Fig. 7) shown in red, and NIF station fits including Li(t) (random benchmark wobble term) are shown in black. Stations are ordered roughly north to south on the y axis, offset by 0.5 m.
- Extended Data Figure 7: The 2011 NL08 and NL04 Station flowline, crack-normal, and uplift displacements computed from NIF displacement sources. (295 KB)
Flowline, crack-normal, and uplift GPS displacements less background velocity field (grey stars) are plotted the for (a–c) NL04 and (d–f) NL08 over the 2 days before and 1 day after the 2011 rapid North Lake drainage. These stations are two examples chosen from the full array because they capture displacement on both the northern (NL04) and southern (NL08) side of the lake, are located at roughly the same longitude as M1, and are within 2 km of the lake. NIF-calculated surface ice displacements at NL04 and NL08 stations from the three displacement sources are plotted for the (red) hydro-fracture crack opening, (green) basal cavity opening, and (blue) extra basal slip. The sum of all three NIF displacement sources is shown in black.
- Extended Data Figure 8: MLE of NIF hyperparameters. (235 KB)
MLE of the vertical hydro-fracture plane temporal smoothing parameter, α, for (a) 2011, (b) 2012, and (c) 2013 NIF. The MLE corresponds with the minimum value on the −2 × likelihood plots19. Minimum likelihood estimates are outlined in red circles, with the value used in each year’s inversion outlined indicated with a black diamond (Methods).
- Extended Data Figure 9: Stress changes across North Lake basin. (199 KB)
Stress changes during (a) supraglacial lake formation, (b) rapid drainage precursor, and (c) hydro-fracture opening.
Extended Data Tables
- Video 1: Hydro-fracture crack opening, basal cavity opening, and additional basal slip during the 2011 North Lake rapid drainage. (5.06 MB, Download)
- Video spanning the two days prior and one day following the 2011 drainage event, depicting (d) opening along the hydro-fracture crack as a percentage of the maximum opening (Extended Data Table 1) of the hydro-fracture crack, (e) basal cavity opening, and (f) additional basal slip above background velocity values for all NIF subelements. GPS stations are shown with black triangles; station NL08 is shown as a white triangle. The yellow box outlines the subelement of maximum opening, uplift, or slip within the hydro-fracture crack, basal cavity opening, and basal slip planes, respectively, once opening or slip has exceeded a threshold value of 20% of maximum opening for the hydro-fracture crack and 0.2 m basal cavity opening or additional basal slip for the basal planes. Station fit between GPS station NL08 and the NIF is shown in the left column, with NL08 (a) crack-normal, (b) uplift, and (c) along flowline displacements less background velocities in grey, and the NIF inversion at the location of NL08 in black. The blue diamond follows the NIF inversion at the location of NL08 along the time of the NIF panels. “Start of Precursor,” “Hydro-fracture Initiation,” and “Maximum Hydro-fracture Opening” times are marked as vertical grey lines are at the same time points delineated in Figure 2 and Extended Data Table 1.
- Video 2: Hydro-fracture crack opening, basal cavity opening, and additional basal slip during the 2012 North Lake rapid drainage. (4.65 MB, Download)
- This video shows the same as Video 1 but for the 2012 North Lake rapid drainage.
- Video 3: Hydro-fracture crack opening, basal cavity opening, and additional basal slip during the 2013 North Lake rapid drainage. (7.96 MB, Download)
- This video shows the same as Video 1 but for the 2013 North Lake rapid drainage.