Antarctic ice-shelf hydrology, instability and break-up

Antarctic ice-shelf instability and break-up, as exhibited by the Larsen B ice shelf in 2002, remains one of the most difficult glaciological processes to observe directly. It is vital to do so, however, because ice-shelf breakup has the potential to influence the buttressing controls on inland ice discharge, and thus to affect sea level. Several mechanisms enabling Larsen B style breakup have been proposed, including the ability of surface lakes to introduce ice-shelf fractures when they fill and drain.

We have undertaken two field seasons on the George VI Ice Shelf, Antarctic Peninsular in Nov. 2019 and Nov. 2021 and have a third season planned for Nov. 2022 as part of a joint US NSF - UK NERC funded project, with field logistics provided by the British Antarctic Survey. Specific aims of the project are to monitor the filling and draining of surface lakes, and the effect of these processes on ice-shelf flexure and possible fracture. The ultimate aim is to use these data to constrain numerical models of ice-shelf stability and breakup. The work builds on a similar successful project involving SPRI and US collaborators on the McMurdo Ice Shelf.

On the George VI Ice Shelf, we instrumented 3 transects, each radiating out from a lake. There was an unprecedented volume of water ponding on the ice shelf in the 2019/20 summer which we have quantified from satellite imagery and which we see in time lapse photography (the first of its kind). We are currently analysing GPS data to identify the effects of this on ice shelf flexure. We are also analysing weather station data, and shallow subsurface firn/ice temperature data, which we are using to validate climate model output. Together with shallow ice cores extracted during the 2021/22 summer, the data suggest the ice shelf is largely impermeable as a result of so much meltwater simply refreezing over the last several years. The work complements that being undertaken by European Space Agency,Climate Change Initiative Fellow, Becky Dell and PhD student Karla Boxall.

Publications

Papers stemming from this project so far:

• Dell, R.L., Banwell, A.F., Willis, I.C., Arnold, N.S., Halberstadt, A.R.W., Chudley, T.R. and Pritchard, H.D., 2022. Supervised classification of slush and ponded water on Antarctic ice shelves using Landsat 8 imagery – CORRIGENDUM. Journal of Glaciology, v. 68, p.415-416. doi:10.1017/jog.2022
• Dell, R.L., Banwell, A.F., Willis, I.C., Arnold, N.S., Halberstadt, A.R.W., Chudley, T.R. and Pritchard, H.D., 2021. Supervised classification of slush and ponded water on Antarctic ice shelves using Landsat 8 imagery. Journal of Glaciology, v. 68, p.401-414. doi:10.1017/jog.2021.114.
• Banwell, A.F., Datta, R.T., Dell, R.L., Moussavi, M., Brucker, L., Picard, G., Shuman, C.A. and Stevens, L.A., 2021. The 32-year record-high surface melt in 2019/2020 on the northern George VI Ice Shelf, Antarctic Peninsula. The Cryosphere, v. 15, p.909-925. doi:10.5194/tc-15-909-2021.
• Dell, R., Arnold, N., Willis, I., Banwell, A., Williamson, A., Pritchard, H. and Orr, A., 2020. Lateral meltwater transfer across an Antarctic ice shelf. The Cryosphere, v. 14, p.2313-2330. doi:10.5194/tc-14-2313-2020.
• Dunmire, D., Lenaerts, J.T.M., Banwell, A.F., Wever, N., Shragge, J., Lhermitte, S., Drews, R., Pattyn, F., Hansen, J.S.S., Willis, I.C., Miller, J. and Keenan, E., 2020. Observations of Buried Lake Drainage on the Antarctic Ice Sheet. Geophysical Research Letters, v. 47, p.e2020gl087970-. doi:10.1029/2020gl087970.
• MacAyeal, D.R., Willis, I.C., Banwell, A.F., MacDonald, G.J. and Goodsell, B., 2020. Diurnal lake-level cycles on ice shelves driven by meltwater input and ocean tidal tilt. Journal of Glaciology, v. 66, p.231-247. doi:10.1017/jog.2019.98.
• Banwell, A., Willis, I., Macdonald, G., Goodsell, B. and MacAyeal, D., 2019. Direct Measurements of Ice-Shelf Flexure caused by Surface Meltwater Ponding and Drainage. Nature Communications, doi:10.1038/s41467-019-08522-5.
• Macdonald, G., Banwell, A., Willis, I., Mayer, D., Goodsell, B. and MacAyeal, D., 2019. Formation of pedestalled, relict lakes on the McMurdo Ice Shelf, Antarctica. Journal of Glaciology.
• MacAyeal, D., Banwell, A.F., Okal, E., Lin, J., Willis, I., Goodsell, B. and Macdonald, G., 2018. Diurnal Seismicity Cycle Linked to Subsurface Melting on an Ice Shelf. Annals of Glaciology, doi:10.1017/aog.2018.29.
• Banwell, A.F., 2017. Glaciology: Ice-shelf stability questioned. Nature, v. 544, p.306-307. doi:10.1038/544306a.
• Banwell, A.F., Willis, I.C., Goodsell, B. Macdonald, G.J., Mayer, D., Powell, A. and MacAyeal, D.R. 2017. Calving and Rifting on McMurdo Ice Shelf, Antarctica. Annals of Glaciology. doi.org/10.1017/aog.2017.12
• Banwell, A.F. and MacAyeal, D.R., 2015. Ice-shelf fracture due to viscoelastic flexure stress induced by fill/drain cycles of supraglacial lakes. Antarctic Science, v. 27, p.587-597. doi:10.1017/S0954102015000292.
• MacAyeal, D.R., Sergienko, O.V. and Banwell, A.F., 2015. A model of viscoelastic ice-shelf flexure. Journal of Glaciology, v. 61, p.635-645. doi:10.3189/2015JoG14J169.
• Banwell, A.F., Caballero, M., Arnold, N.S., Glasser, N.F., Cathles, L.M. and MacAyeal, D.R., 2014. Supraglacial lakes on the Larsen B ice shelf, Antarctica, and at Paakitsoq, West Greenland: A comparative study. Annals of Glaciology, v. 55, p.1-8. doi:10.3189/2014AoG66A049.
• Banwell, A.F., MacAyeal, D.R. and Sergienko, O.V., 2013. Breakup of the Larsen B Ice Shelf triggered by chain reaction drainage of supraglacial lakes. Geophysical Research Letters, v. 40, p.5872-5876. doi:10.1002/2013GL057694.

Alison Banwell and Laura Stevens installing time-lapse camera. Recently installed GPS unit stands to the right.

Schematic view of stress regime, flexure, and fracture patterns associated with loaded (filled) and unloaded (drained) supraglacial lakes on ice shelves. (a) As water fills an idealized circular depression, the accumulated mass creates a depression that induces an upward deflected forebulge. Downward propagating, ring-type fractures form in the forebulge at the ice shelf surface. At the ice shelf base, tension with upward radial propagating fractures form at the antipode of the lake. The neutral plane, where flexure stresses are zero and across which flexure stresses vary linearly to maximum amplitude at the surface and base of the ice shelf, is identified. (b) When a lake drains, hydrostatic rebound causes tensile stress to be induced in an inverted forebulge (surface moat). It is here that upward propagating ring-type fractures are likely to form. The drained lake is also missing some original ice shelf mass due to enhanced lake-bottom ablation. At the ice shelf surface, tension with downward radial propagating fractures form at the antipode of the lake. (c) Fractures introduced by repeated filling and draining of lakes over a number of years can potentially yield a mixed-mode fracture pattern, consisting of ring-type fractures surrounding the lake, and radial-type fractures below the lake depression. From Banwell et al, 2013.

Chain reaction drainage of supraglacial lakes. (a) Observed lakes are represented by circular disks of equal area and constant depth (5 m). The lake found to trigger the drainage of most neighboring lakes is labeled "starter lake." Colored surrounding lakes indicate those that are induced to drain either directly by the starter lake's effect on flexure stresses (stage = 1) or indirectly by lakes which are drained at an earlier stage (stage = 2, ..., 10). The color of the lake indicates its stage according to the color bar. When the fracture criterion of 70 kPa is evaluated at each lake's center, a total of 227 lakes are triggered to drain by the starter lake (either directly or indirectly). The radii of colored lakes are drawn at twice the scale to promote visibility. The radii of gray-shaded lakes, which are not drained as a result of the chain reaction, are drawn at true scale. (b) As in (a) but with the fracture criterion reduced to 35 kPa. In this case, a total of 626 lakes are triggered to drain by the starter lake either directly (stage = 1) or indirectly (stage = 2, ..., 14). From Banwell et al, 2013.

Air photo of a lake on the McMurdo Ice Shelf, Antarctica. Is this a real example of a surface depression (moat) formed by drainage of a former lake shown in the cartoon figure above? Photo: Andris Apse.

Installing a GPS antenna on McMurdo Ice Shelf to detect ice shelf flexure associated with lake filling / draining. Photo: Ian Willis.