Isotopic composition of ice cores and meltwater from upper fremont glacier and Galena Creek rock glacier, Wyoming

Meltwater runoff from glaciers can result from various sources,including recent precipitation and melted glacial ice. Determining the origin of the meltwater from glaciers through isotopic analysis can provide information about such things as the character and distribution of ablation on glaciers. A 9.4 m ice core and meltwater were collected in 1995 and 1996 at the glacigenic Galena Creek rock glacier in Wyoming's Absaroka Mountains. Measurements of chlorine-36 (Cl-36), tritium (H-3). sulphur-35 (S-35), and delta oxygen-18 (delta(18)O) were compared to similar measurements from an ice core taken from the Upper Fremont Glacier in the Wind River Range of Wyoming collected in 1991-95. Meltwater samples from three sites on the rock glacier yielded Cl-36 concentrations that ranged from 2.1+/-1.0x10(6) to 5.8+/-0.3x10(6) atoms/l. The ice-core Cl-36 concentrations from Galena Creek ranged from 3.4+/-0.3x10(5) to 1.0+/-0.1x10(6) atoms/l. Analysis of an ice core from the Upper Fremont Glacier yielded Cl-36 concentrations of 12+/-0.2x10(6) and 5.2+/-0.2x10(6) atoms/l for pre-1940 ice and between 2x10(6) and 3x10(6) atoms/l for post-1980 ice. Purdue's PRIME Lab analyzed the ice from the Upper Fremont Glacier. The highest concentration of Cl-36 in the ice was 77+/-2x10(6) atoms/l and was deposited during the peak of atmospheric nuclear weapons testing in the late 1950s. This is an order of magnitude greater than the largest measured concentration from both the Upper Fremont Glacier ice core that was not affected by weapons testing fallout and the ice core collected from the Galena Creek rock glacier. Tritium concentrations from the rock glacier ranged from 9.2+/-0.6 to 13.2+/-0.8 tritium units (TU) in the meltwater to -1.3+/-1.3 TU in the ice core. Concentrations of H-3 in the Upper Fremont Glacier ice core ranged from 0 TU in the ice older than 50 years to 6-12 TU in the ice deposited in the last 10 years. The maximum H-3 concentration in ice from the Upper Fremont Glacier deposited in the early 1960s during peak weapons testing fallout for this isotope was 360 TU. One meltwater sample from the rock glacier was analyzed for S-35 with a measured concentration of 5.4+/-1.0 millibecquerel per liter (mBeq/l). Modern precipitation in the Rocky Mountains contains 35S from 10 to 40 mBeq/L. The delta(18)O results in meltwater from the Galena Creek rock glacier (-17.40+/-0.1 to -17.98+/-0.1 per mil) are similar to results for modem precipitation in the Rocky Mountains. Comparison of these isotopic concentrations from the two glaciers suggest that the meltwater at the Galena Creek site is composed mostly of melted snow and rain that percolates through the rock debris that covers the glacier. Additionally, this water from the rock debris is much younger (less than two years) than the reported age of about 2000 years for the subsurface ice at the mid-glacier coring site. Thus the meltwater from the Galena Creek rock glacier is composed primarily of melted surface snow and rain water rather than melted glacier ice, supporting previous estimates of slow ablation rates beneath the surface debris of the rock glacier.