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Isotopic and chemical composition of Antarctic shelf ice: implications for global changes

Antarctica phase III (1993-1997)

Isotopic and chemical composition of Antarctic shelf ice: implications for global changes

Promotor

Professor Roland Souchez
Université Libre de Bruxelles - C.P. 160/03
Département des Sciences de la Terre et de l'Environnement
Avenue F.D. Roosevelt, 50
B-1050 BRUSSELS
Phone: +32 (0)2 650 22 16
Fax: +32 (0)2 650 22 26
E-mail: glaciol@ulb.ac.be

Topics

In the context of global warming, the question of the stability of Antarctic ice shelves is critical for predicting any sea level rise. Small ice shelves are likely to react more rapidly than large ones to a change in atmospheric and oceanic temperatures.

The behaviour of ice shelves and floating ice tongues is dependent on their boundary conditions. At the ice-ocean interface, these boundary conditions can be appraised by studying:

  • the properties of marine ice forming accretions at the base;
  • the properties of the water column in front and under the ice shelves.

Two case studies were considered in this research-work, both in the Terra Nova Bay area, Victoria Land, of which conclusions are summarized hereunder.
In the first case, marine ice is formed near the grounding line here defined as the limit between grounded ice and floating ice, either if the glacier goes afloat or becomes again grounded.

Two different types of marine ice have been found. Type 1 is bubble-and debris-free ice with properties which can be explained by intrusion of brackish water in open basal fissures. Closing of the fissures by progression of a freezing front from the sides is precluded and filling by frazil ice is favoured. Type 2 is made of thin clear ice and debris layers which are thought to have formed when a subglacial water-filled sediment enters into contact with sea water and is subjected to freezing under a double diffusion process.

It is also stressed that, in a deltaD/delta18O diagram, the alignment of marine ice samples on a mixing line does not, necessarily, imply a mixture of continental water and sea water in varying proportions.

In the second case, extensive frazil ice accumulation occurs under the ice shelf. Two main frazil ice types can be identified which have different crystallographic, isotopic and chemical characteristics indicating contrasted depositional environments.

Results from oceanic water sampling at various time periods clearly show the occurrence of oceanic circulation mode 3 as defined by Jacobs et al. (1992). In this mode, tidal pumping allows seasonally warm waters of the coastal currents to make contact with the base of the ice shelf , to produce melting and to form Ice Shelf Water (ISW) exiting at the front of the ice shelf. The wide occurrence of orbicular frazil ice accreted upstream at the bottom of the ice shelf is the result of circulation mode 1, i.e. Deep Thermohaline Circulation. Banded frazil ice is generated closer to the ice shelf front by adiabatic supercooling or by a double-diffusion process at the limit between ISW-mode 1 and ISW-mode 3, thus partially impeding the net melting loss due to circulation mode 3.

References:

Jacobs S.S., Helmer H.H., Doake C.S.M., Jenkins A. and Frolich R.M. (1992): Melting of ice shelves and the mass balance of Antarctica. J. of Glaciol., 38 (130): 375-387.

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