STABLE ISOTOPIC VARIABILITY IN TWO GLACIER MELTWATER STREAMS, TAYLOR VALLEY, ANTARCTICA

Stable isotopes of water, δ¹⁸O and δD, were used to characterize variations in glacier meltwater and hyporheic zone exchange in two glacier meltwater streams (Von Guerard Stream and Andersen Creek) of the McMurdo Dry Valleys (MDV), Antarctica. In theory, this should be more straightforward to accomplish in MDV than in a temperate setting because there are no vascular plants on the landscape, water primarily flows in discrete stream channels, glacier melt is the primary water source, and there is no significant contribution from groundwater or overland flow. However, evaporation and hyporheic exchange do occur, and influence the isotopic signature of the stream water. We investigated the importance of the hyporheic zone interaction over the course of an entire stream flow season (48 days). Previous studies of the MDV hyporheic zone were conducted over 0.3–16 days. Stream water collection started with the onset of flow in each stream and continued over the melt season, while hyporheic zone sampling was conducted weekly. Stream water δ¹⁸O and δD were more negative at the beginning of the season and less negative later in the season. D-excess measurements were used as an indicator of stream water mixing because the evaporative signature was evident and distinguishable between the two end-members. Hyporheic zone influence was variable throughout the season with a strong control on stream water chemistry, except at the highest seasonal discharges.

Andersen Creek, with its direct glacier meltwater source and a small hyporheic zone, displayed hyporheic zone-solute interactions. Andersen Creek water chemistry was also affected by a subsurface hypersaline flow, which provided waters of more negative D-excess and higher TDS. This work supports the idea that Von Guerard Stream has a vast, widespread hyporheic zone that changes with time and discharge amounts. These results provide new information on the role of hyporheic zone-stream interactions, and support the shorter-term, more physically based descriptions of hyporheic zone dynamics explained in the past decade.