Abstract
Abstract. This study investigates the combined hydrogen deuterium and triple oxygen isotope hydrology of the Salar del Huasco, an endorheic salt flat with shallow lakes at its centre that is located on the Altiplano Plateau, N Chile. This lacustrine system is hydrologically dynamic and complex because it receives inflow from multiple surface and groundwater sources. It undergoes seasonal flooding, followed by rapid shrinking of the water body at the prevailing arid climate with very high evaporation rates. At any given point in time, ponds, lakes, and recharge sources capture a large range of evaporation degrees. Samples taken between 2017 and 2019 show a range of δ18O between −13.3 ‰ and 14.5 ‰, d-excess between 7 ‰ and −100 ‰, and 17O-excess between 19 and −108 per meg. A pan evaporation experiment conducted on-site was used to derive the turbulence coefficient of the Craig–Gordon isotope evaporation model for the local wind regime. This, along with sampling of atmospheric vapour at the salar (-21.0±3.3 ‰ for δ18O, 34±6 ‰ for d-excess and 23±9 per meg for 17O-excess), enabled the accurate reproduction of measured ponds and lake isotope data by the Craig–Gordon model. In contrast to classic δ2H–δ18O studies, the 17O-excess data not only allow one to distinguish two different types of evaporation – evaporation with and without recharge – but also to identify mixing processes between evaporated lake water and fresh flood water. Multiple generations of infiltration events can also be inferred from the triple oxygen isotope composition of inflow water, indicating mixing of sources with different evaporation histories. These processes cannot be resolved using classic δ2H–δ18O data alone. Adding triple oxygen isotope measurements to isotope hydrology studies may therefore significantly improve the accuracy of a lake's hydrological balance – i.e. the evaporation-to-inflow ratio (E / I) – estimated by water isotope data and application of the Craig–Gordon isotope evaporation model.
Highlights
The majority of water in the hydrologic cycle on Earth represents a dynamic isotopic equilibrium between evaporation from the ocean, precipitation, and continental runoff, defining a linear relationship between δ17O and δ18O or δ2H and δ18O – i.e. the global meteoric water line (GMWL; Clark and Fritz, 1997; Luz and Barkan, 2010)
This study investigates the combined hydrogen deuterium and triple oxygen isotope hydrology of the Salar del Huasco, an endorheic salt flat with shallow lakes at its centre that is located on the Altiplano Plateau, N Chile
Springs sampled in September 2017 comprise average values of −12.5 ± 0.6 ‰ for δ18O, 2 ± 3 ‰ for d-excess, and 11 ± 7 per meg for 17O-excess, which are in good agreement with published isotopic data of springs and wells in the Salar del Huasco basin (−12.56±1.36 ‰ in δ18O and 3.2±5.7 ‰ in dexcess; data from Fritz et al, 1981; Uribe et al, 2015; Jayne et al, 2016)
Summary
The majority of water in the hydrologic cycle on Earth represents a dynamic isotopic equilibrium between evaporation from the ocean, precipitation, and continental runoff, defining a linear relationship between δ17O and δ18O or δ2H and δ18O – i.e. the global meteoric water line (GMWL; Clark and Fritz, 1997; Luz and Barkan, 2010). A number of continental water reservoirs, e.g. lakes in semi-arid or arid environments, may deviate from that state as a result of evaporation – which imparts an enhanced kinetic isotope effect on these waters – or because of mixing with preevaporated water. Such deviations are described by the secondary parameters 17O-excess [= δ 17O − 0.528 · δ 18O with δ = 1000 · (δ/1000 + 1)] and d-excess [= δ2H − 8 · δ18O], which assume values of 33 per meg and 10 ‰, respectively, for waters plotting on the GMWL. The model variables of the C–G equation – relative humidity, temperature, the isotopic composition of atmospheric vapour, continuous groundwater recharge, wind
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