Water isotope signatures of lakes, streams and ground ice from the Ojos del Salado, Dry Andes – interannual fluctuations and moisture sources

Targeted sampling campaigns were carried out at the high elevation permafrost zone of the Dry Andes for a systematic multiannual isotopy hydrological investigation to reveal the potential role of thawing permafrost in the regional water cycle. Ephemeral lakes and streams were sampled between 2018 and 2024. Elevation of the sampling spots ranged from 5,240 to 5,900 m asl. Ground ice samples representing the topmost 20–30 cm of the frozen section were excavated in the 2024 field season. The stable hydrogen and oxygen isotope composition (δ²H and δ¹⁸O) of the samples was determined by laser spectroscopy (LGR LWIA-24i) and tritium (³H) activities were analyzed using the ³He-ingrowth method at the Institute for Nuclear Research, Debrecen, Hungary.

Stable hydrogen and oxygen isotope composition of fresh snow samples collected in late February, 2018 were -69.5‰ for δ²H and -10.33‰ for δ¹⁸O, the ³H activity was 5.36±0.10 TU. Stable isotope composition of surface waters collected around the Ojos del Salado between 2018 and 2024 ranged from -114.3 to -31.4‰ for δ²H, and from -15.32 to -2.0‰ for δ¹⁸O. Tritium activities of the samples ranged from 4.7 to 10.8 TU. Ground ice samples showed a narrow range around -40.8‰ for δ²H and -4.2‰ for δ¹⁸O, with tritium activity concentrations 6 and 6.6 TU.

Neither the snow sample, nor the surface water samples yielded as negative isotopic values as expected. Snow and stream water samples exhibit a relatively more depleted composition in heavy isotopes and fit well to the Northern Chile meteoric water line (MWL), while lake water samples usually show relatively more enriched composition and are scattered below the regional MWL. Water stable isotopic characteristics suggest that freezing of liquid water, presumably with high freezing rate and relatively thick boundary layer characterized the formation of ground ice samples. The ³H data of the ground ice samples argue for the recent (< 8yrs) origin of these subsurface ice accumulation.

Acknowledgement: The research was supported by the National Research, Development and Innovation Office (NRDI) Fund project K-147424.