Oxygen-Isotope Ratios and Glacier Movement

Abstract

The ratio of the common isotopes of oxygen, O¹⁸ to O¹⁶, in snow is influenced by the degree of cooling and the amount of condensation experienced by the air masses involved and by the temperature at which precipitations occur. Analyses of firn from the Saskatchewan Glacier, Canada, and the Greenland Ice Sheet show that the individual strata composing an annual firn layer have distinctly different oxygen-isotope ratios which reflect: altitude of accumulation, seasonal influences, differences among individual storms, and subsequent diagenetic changes. Lower precipitation temperatures usually yield lower O¹⁸/O¹⁶ ratios. Samples of ice taken along a centerline profile on Saskatchewan Glacier from the firn limit to the terminus show an irregular trend toward lower O¹⁸/O¹⁶ ratios downglacier which is the reverse of the normal precipitation pattern. This could be due to the fact that ice at progressively lower positions on the ice tongue originated at successively higher sites in the accumulation area. If so, the isotope ratios nicely confirm deductions made 60 years ago by Reid concerning flow lines in a valley glacier. Isotope ratios in ice samples from a transverse profile 6.75 km above the terminus of Saskatchewan Glacier indicate that the marginal ice comes from higher parts of the firn area than the center ice. This is partly the result of topographic configuration and partly the result of a lower flow velocity and a higher ablation rate in the marginal zones. A greater dispersion of isotope ratios in closely spaced samples from marginal ice may be due to the steep velocity gradient and greater shear displacements near the margin which bring ice masses of distinctly different composition into juxtaposition. Although a high degree of homogenization has been affected in Saskatchewan Glacier ice by diagenetic changes in the firn or by processes associated with deformation and flowage, identification of vestigial sedimentary layering in the glacier tongue may be possible in some instances by means of oxygen-isotope analyses. The generally higher O¹⁸/O¹⁶ ratios in the ice tongue of Saskatchewan Glacier compared to the firn is as yet an unsolved problem perhaps involving diagenetic changes in the firn or alterations associated with flowage. The possibility that they reflect a major climatological change during accumulation of the material composing this glacier seems unlikely. Isotope ratios in ice samples from the Malaspina Glacier, Alaska, confirm deductions concerning the constitution and structure of this large piedmont sheet made on the basis of earlier field studies. The O¹⁸/O¹⁶ ratios show that intensely deformed bands of clean and debris-rich ice probably represent original valley glaciers and medial moraines from far back in the accumulation area. The ratios also confirm that one of the major ice units in the Malaspina consists of material accumulated in sedimentary fashion in the large basin of the upper Seward Glacier. Variations in oxygen-isotope ratios appear to constitute a parameter of considerable potential value for investigation of glaciers.

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