How seasonal hydroclimate variability drives the triple oxygen and hydrogen isotope composition of small lake systems in semiarid environments

Abstract. This research investigates the influence of seasonal hydroclimate variability on the triple oxygen and hydrogen isotope composition of small, shallow lake systems that show substantial intra- and interannual fluctuations in the water level. The study was conducted at Laguna Honda, a semi-permanent lake located in the semiarid Mediterranean environment of southern Spain. Over one year, lake water level was monitored continuously and water samples from the northern and southern margin were taken monthly for major ion concentration and triple oxygen and hydrogen isotope analyses. Over the study period, the lake water level dropped from 1.4 m to 0.6 m, while salinity increased from 23 g L^-1 to 130 g L^-1 and δ¹⁸O, δ²H and ¹⁷O-excess of lake water varied from -2 ‰ to 15 ‰, -26 ‰ to 51 ‰ and -9 per meg to -87 per meg, respectively. Hydrological mass balance calculations indicate that precipitation, basin discharge and evaporation control lake water level changes in Laguna Honda, and major inflow from other sources, such as groundwater, is absent. The lake water's isotope composition is mainly driven by changes in relative humidity (34–73 %), while precipitation and basin discharge can cause transitional mixing effects that however remain small in magnitude (< 10 %). In the ¹⁷O-excess vs. δʹ¹⁸O space, the lake water forms a loop evolving from low δ¹⁸O and high ¹⁷O-excess in winter to higher δ¹⁸O and lower ¹⁷O-excess in summer along a convex curvature, and back to low δ¹⁸O and high ¹⁷O-excess with the beginning of the subsequent rainy season along a concave curvature. The triple oxygen isotope system allows to identify non-steady state conditions, which is challenging using δ²H and δ¹⁸O alone, due to the linearity of trends in this isotope system. The large seasonal variability of triple oxygen isotopes should be considered when interpreting isotope data obtained from paleo-archives from lake sediments in semiarid and arid environments.