Lateral meltwater transfer across an Antarctic ice shelf

Abstract

Abstract. Surface meltwater on ice shelves can exist as slush, it can pond in lakes or crevasses, or it can flow in surface streams and rivers. The collapse of the Larsen B Ice Shelf in 2002 has been attributed to the sudden drainage of ∼3000 surface lakes and has highlighted the potential for surface water to cause ice-shelf instability. Surface meltwater systems have been identified across numerous Antarctic ice shelves, although the extent to which these systems impact ice-shelf instability is poorly constrained. To better understand the role of surface meltwater systems on ice shelves, it is important to track their seasonal development, monitoring the fluctuations in surface water volume and the transfer of water across ice-shelf surfaces. Here, we use Landsat 8 and Sentinel-2 imagery to track surface meltwater across the Nivlisen Ice Shelf in the 2016–2017 melt season. We develop the Fully Automated Supraglacial-Water Tracking algorithm for Ice Shelves (FASTISh) and use it to identify and track the development of 1598 water bodies, which we classify as either circular or linear. The total volume of surface meltwater peaks on 26 January 2017 at 5.5×107 m3. At this time, 63 % of the total volume is held within two linear surface meltwater systems, which are up to 27 km long, are orientated along the ice shelf's north–south axis, and follow the surface slope. Over the course of the melt season, they appear to migrate away from the grounding line, while growing in size and enveloping smaller water bodies. This suggests there is large-scale lateral water transfer through the surface meltwater system and the firn pack towards the ice-shelf front during the summer.

Distribution and seasonal evolution of supraglacial lakes on Shackleton Ice Shelf, East Antarctica

“…This was particularly important in our study given the large area of nunataks (Bunger Hills) adjacent to the ice shelf. After rock masking was performed, we applied an NDWI threshold value of 0.25, following previous studies (Banwell et al, 2019;Dell et al, 2020;Doyle et al, 2013;Williamson et al, 2017;Yang and Smith, 2013), meaning pixels with NDWI ice > 0.25 were assumed to be water-covered. This threshold was found to be the most appropriate for detecting lakes, apart from in 11 scenes, where the threshold value was manually tuned in 0.01 increments to improve SGL identification by including pixels which were visibly water-covered in true-colour composites of the area (Table S1).…”

“…Some manual editing of SGL extents was necessary where floating ice in the centre of SGLs had resulted in pixels being misclassified as water. The transition from melting ice to saturated firn, slush, shallow ponds and deeper SGLs makes it challenging to adopt a clear binary definition of "lake" versus "non-lake" (Dell et al, 2020). To account for this uncertainty we used a minimum size threshold of 2 pixels, in order to remove very small SGLs likely comprised solely of mixed pixels (i.e.…”

“…1) are indicative of persistent snow surface scouring by easterly katabatic winds. These winds descend from the Polar Plateau, generated by large surface slopes (> 20 • ), and reach 19 m s −1 in summer (Doran et al, 1996). Surface meltwater production is enhanced as the winds warm adiabatically as they descend to the grounding zone, which lowers humidity and raises near-surface air temperatures (Doran et al, 1996;Lenaerts et al, 2017).…”

32-year record-high surface melt in 2019/2020 on north George VI Ice Shelf, Antarctic Peninsula

“…, and more recently, Antarctica (Dell et al, 2020). This algorithm calculates lake water depth using the rate that sunlight passing through a water column is attenuated with depth, lake-bottom albedo, and optically deep water reflectance (Philpot, 1989).…”

“…Refreezing of this meltwater releases latent heat into the firn, causing additional melting, firn saturation, and firn air content depletion; eventually facilitating meltwater to pond on the ice-shelf surface (Kuipers Munneke et al, 2014;Holland et al, 2011). Extensive surface ponding (Arthur et al 2020;Dell et al, 2020;Kingslake et al, 2017) may threaten ice-shelf stability due to stress variations associated with meltwater movement, ponding and drainage (Scambos et al, 2000(Scambos et al, , 2003MacAyeal et al, 2003;Banwell and MacAyeal, 2015;Banwell et al 2019). These processes may initiate meltwater-induced vertical fracturing ('hydrofracturing') (Van der Veen, 2007;Dunmire et al, 2020;Lai et al 2020), especially if the ice shelf is already damaged with a high density of crevasses (Lhermitte et al, 2020).…”