Triple Isotope Water Measurements of Lake Untersee Ice using Off-Axis ICOS

Abstract:

Lake Untersee (71.348°S, 13.458°E) is the largest surface freshwater lake in the interior of the Gruber Mountains of central Queen Maud Land in East Antarctica. The lake is permanently covered with ice, is partly bounded by glacier ice and has a mean annual air temperature of -10°C. In contrast to other Antarctic lakes the dominating physical process controlling ice-cover dynamics is low summer temperatures and high wind speeds resulting in sublimation rather than melting as the main mass-loss process. The ice-cover of the lake is composed of lake-water ice formed during freeze-up and rafted glacial ice derived from the Anuchin Glacier. The mix of these two fractions impacts the energy balance of the lake, which directly affects ice-cover thickness. Ice-cover is important if one is to understand the physical, chemical, and biological linkages within these unique, physically driven ecosystems.

 We have analyzed δ²H, δ¹⁸O, and δ¹⁷O from samples of lake and glacier ice collected at Lake Untersee in Dec 2014. Using these data we seek to answer two specific questions: Are we able to determine the origin and history of the lake ice, discriminating between rafted glacial ice and lake water? Can isotopic gradients in the surface ice indicate the ablation (sublimation) rate of the surface ice?

The triple isotope water analyzer developed by Los Gatos Research (LGR 912-0032) uses LGR’s patented Off-Axis ICOS (Integrated Cavity Output Spectroscopy) technology and incorporates proprietary internal thermal control for high sensitivity and optimal instrument stability. This analyzer measures δ²H, δ¹⁸O, and δ¹⁷O from water, as well as the calculated d-excess and ¹⁷O-excess. The laboratory precision in high performance mode for both δ¹⁷O and δ¹⁸O is 0.03 ‰, and for δ²H is 0.2 ‰.

Methodology and isotope data from Lake Untersee samples are presented.

Figure: Ice samples were collected across Lake Untersee from both glacial and lake ice regions for this study.