ASSESSMENT OF TRIPLE OXYGEN ISOTOPE FRACTIONATION IN PERENNIALLY ICE-COVERED LAKE FRYXELL, MCMURDO DRY VALLEYS, ANTARCTICA

Antarctica’s McMurdo Dry Valleys is the continent’s largest ice-free region with various lake systems that can help elucidate the region’s glacial and hydrologic history. Located in Taylor Valley, Lake Fryxell is a closed-basin perennially ice-covered lake fed by seasonal meltwater from neighboring glaciers. As a result of its ice cover, the water column is both physically and chemically stratified. In these lake systems, high-precision triple oxygen isotope analyses (Δ’¹⁷O) present a novel application to the study of their geochemistry as they would allow us to evaluate equilibrium or post-depositional processes throughout the lakes’ lifespan. To assess this method, we utilize carbonate phases from a 33-cm-long sediment core from 11.2 m depth in the lake basin collected in 2012. Secondary electron microscopy of core sections shows acicular aragonite crystals and biogenic material consistent with previous observations (Lawrence and Hendy, 1985); carbonate abundance in calcareous mud sections varies along the core and can be either enriched or as low as ~20% abundance. The core was sub-sampled at ~1 cm intervals where carbonate was present and subsequently analyzed through CO₂ released via phosphoric acid digestion on an Aerodyne laser mass spectrometer at the University of New Mexico’s Center for Stable Isotopes. Carbonates from analyzed samples contain Δ’¹⁷O signatures ranging from -0.21 to 0.01‰ (VSMOW) and δ¹⁸O signatures ranging from -30.81 to -26.48‰ (VPDB). Whereas recent sediment at the top 5 cm has relatively consistent Δ’¹⁷O values ranging from -0.05 to -0.07‰ (VSMOW), there are two zones of extremely light values ranging from -0.21 to -0.14‰ at 10-15 cm and 33 cm of the core, which may be explained by evaporation processes. Further exploration of Δ’¹⁷O signatures throughout the Lake Fryxell basin can aid in providing a framework for triple oxygen isotope systematics in Taylor Valley as it relates to assessing changes in water balance.