Oxygen-isotope ratios in the Blue Glacier, Olympic Mountains, Washington, U.S.A.

Abstract

The mean per mil deviation from a standard (average ocean water) in the O^(18)/O^(16) ratio of 291 specimens of ice, firn, snow, and rain from the Blue Glacier is −12.4; extremes are −8.6 and −19.2. This is consistent with the moist temperate climatological environment. The O^(18)/O^(16) ratio of snow decreases with declining temperature of precipitation, and it also decreases with increasing altitude at 0.5/100 meters. Analyses of the three principal types of ice, coarse-bubbly, coarse-clear, and fine, composing lower Blue Glacier, show that ratios for coarse-clear ice are generally lower and for fine ice they are mostly higher than the ratios for coarse-bubbly ice. This indicates that the fine ice represents masses of firn and snow recently incorporated into the glacier by filling of crevasses or by infolding in areas of severe deformation. Coarse-clear ice masses may represent fragments of coarse-bubbly ice within a breccia formed in the icefall. Because of unfavorable orientation, these fragments could have undergone exceptional recrystallization with reduction in air bubbles and, possibly, a relative decrease in O^(18). A longitudinal septum in the lower Blue Glacier is characterized by higher than normal O^(18)/O^(16) ratios. These values are consistent with an origin for this feature involving incorporation of much surficial snow and firn near the base of the icefall. Samples from longitudinal profiles on the ice tongue suggest that ice close to the snout comes from high parts of the accumulation area. Analyses from the light and dark bands of ogives are compatible with the concept that the dark bands represent greatly modified insets of firn-ice breccia filling icefall crevasses. The range in ratios of materials is much greater in the accumulation area than in the ice tongue. This is attributed to homogenization, much of which takes place during the conversion of snow to glacier ice. This is supported by comparative analyses of snow layers when first deposited and months later, after alteration. Refreezing of rain and meltwater percolating into underlying cold snow is an important mechanism as shown by analyses of ice layers and lenses in the firn formed in this manner.