Triple oxygen isotope systematics of evaporation and mixing processes in a dynamic desert lake system

• Main Authors: C. Voigt, D. Herwartz, C. Dorador, M. Staubwasser
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-03-01
• Series: Hydrology and Earth System Sciences
• Online Access: https://hess.copernicus.org/articles/25/1211/2021/hess-25-1211-2021.pdf

<p>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 <span class="inline-formula"><i>δ</i>¹⁸</span>O between <span class="inline-formula">−13.3</span> ‰ and 14.5 ‰, d-excess between 7 ‰ and <span class="inline-formula">−100</span> ‰, and <span class="inline-formula">¹⁷</span>O-excess between 19 and <span class="inline-formula">−108</span> 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 (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M6" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">21.0</mn><mo>±</mo><mn mathvariant="normal">3.3</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="58pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="53283b99f13c535b38a04b1a3307454f"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="hess-25-1211-2021-ie00001.svg" width="58pt" height="10pt" src="hess-25-1211-2021-ie00001.png"/></svg:svg></span></span> ‰ for <span class="inline-formula"><i>δ</i>¹⁸</span>O, <span class="inline-formula">34±6</span> ‰ for d-excess and <span class="inline-formula">23±9</span> per meg for <span class="inline-formula">¹⁷</span>O-excess), enabled the accurate reproduction of measured ponds and lake isotope data by the Craig–Gordon model. In contrast to classic <span class="inline-formula"><i>δ</i>²</span>H–<span class="inline-formula"><i>δ</i>¹⁸</span>O studies, the <span class="inline-formula">¹⁷</span>O-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 <span class="inline-formula"><i>δ</i>²</span>H–<span class="inline-formula"><i>δ</i>¹⁸</span>O 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 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M16" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="c6f00d13d95b9183e3e2526db4298e27"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="hess-25-1211-2021-ie00002.svg" width="8pt" height="14pt" src="hess-25-1211-2021-ie00002.png"/></svg:svg></span></span> I) – estimated by water isotope data and application of the Craig–Gordon isotope evaporation model.</p>