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The composition and origin of basal ice from cold and polythermal glaciers

S. Mager1, S. Fitzsimons1, R. Frew2

1 Department of Geography, University of Otago, Dunedin, New Zealand
2 Chemistry Department, University of Otago, Dunedin, Zealand

 

 Abstract

The study of ice composition sheds light on the various processes operating at the ice-bedrock interface and on glacier dynamics. By analyzing the ice composition inferences can be made about ice formation and its origin. This paper compares the isotopic and solute composition of basal ice from the Rhone and Taylor glaciers which are located in the Taylor Valley, South Victoria Land, Antarctica. Rhone Glacier is a small dry-based glacier with a basal ice temperature of -17ºC.  Its basal zone is characterised by 3.5 m thick basal sequences of stratified and amber ice that rests below 18 m of clean englacial ice. Taylor Glacier is a large thick outlet glacier that flows from Taylor Dome, an independent dome within the east Antarctic ice sheet.  At the terminus Taylor Glacier has a basal temperature of 18ºC but 8 km upstream it is believed to be at pressure melting point (Robinson, 1984). The Taylor basal zone consists of over 2.5 m of debris-bearing ice that is overlain and underlain by clean englacial ice. Tunnels excavated in both glaciers provided access to the basal ice and facilitated the removal of frozen samples. The basal ice facies were mapped detailing sedimentary structures, ice facies, and physical characteristics, and blocks of ice were extracted for laboratory analysis.

In the laboratory these blocks of ice were sub-sampled and the stable isotope signatures (d18O and dD) and solutes (Sodium, Calcium, Magnesium, Potassium, and Chloride). This analysis revealed a strong contrast between the different ice facies. The ice facies strongly contrast in physical appearance, as there is a clear distinction between englacial ice, amber ice, solid debris, and stratified ice facies. The englacial ice is characterised by very low solute concentrations and isotopic values plot on the local meteoric water line. The amber ice has a distinct yellow-greenish discoloration, low debris concentration and relatively high solute concentrations. The stratified ice faces contains layers of debris and clean ice and is characterised by high debris concentration and very high solute concentrations. The isotopic values of debris-bearing ice from Rhone Glacier show a linear relationship that is very similar to the local meteoric water line. In contrast isotopic values of debris-bearing ice from Taylor Glacier show a linear relationship with a slope that is considerably lower than that of the local meteoric water line. The differences in the physical and chemical signatures of the ice facies reflect different origins of the basal ice. The high solute concentrations and low slope of the isotopic values from Taylor Glacier show that meltwater plays a crucial role in the formation of the basal ice.  Differences in the basal ice stratigraphy between the Rhone and Taylor glaciers reflect intrinsic differences in their thermal regimes.

Robinson, P.H. (1984) Ice dynamics and thermal regime of Taylor Glacier, South Victoria Land, Antarctica. Journal of Glaciology 30(105): 153-160.

 

Reference

Mager, S., Fitzsimons, S., Frew, R. 2003. Comparison of the composition and origin of basal ice in cold and polythermal glaciers. Abstract presented at Seventh International Symposium on Antarctic Glaciology (ISAG-7) in Milano, Italy in August, 2003.

© 2009 Department of Geography, University of Otago, Dunedin, New Zealand