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Antarctic ice-shelf hydrology, instability and break-up

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 now undertaken two field seasons on the McMurdo Ice Shelf, Antarctica as part of an NSF-funded field project. 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. The ultimate aim is to use these data to constrain numerical models of ice-shelf stability and breakup.

We instrumented 4 lakes in the 2016/17 summer season. Water-depth data reveal that lakes filled and drained over multiple week timescales, which had a simultaneous effect on vertical ice deflection in the area. Differential GPS data over three months show that vertical deflection varies as a function of distance from the maximum load change (i.e. from the lake centres). Analysis of seismic data indicates contrasting behavior between the 'wet' and 'dry' areas of the ice shelf, with a diurnal signal in the wet area that potentially relates to warming/freezing cycles. Data analysis is ongoing.

Publications

Papers stemming from this project so far:

  • 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.

Figure

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.

Figure

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.

Figure

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.

Figure

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