How hydroclimate variability drives triple oxygen isotope dynamics in permanent and temporal lakes in Southern Spain

Triple oxygen isotopes of lacustrine gypsum and carbonate deposits are used to quantitatively assess past hydroclimate conditions. An accurate interpretation of these paleo-archives requires a fundamental understanding of processes driving the variability of ¹⁷O-excess in hydrologically different lake systems and their dynamics. Here, we present results of an ongoing monitoring study of triple oxygen and hydrogen isotopes and lake hydrology of two adjacent lakes in southern Spain, which differ in morphology, hydrogeological functioning, and water chemistry. Laguna Grande is a permanent, ∼8 m deep water body that receives groundwater discharge from the aquifer. In contrast, Laguna Chica is a temporal lake with a maximum depth of 1.5 m, which is only fed by precipitation and basin discharge and desiccates during exceptionally long periods of drought. The region is characterized by semi-arid climate and strong seasonality of precipitation. The dataset comprises three hydroperiods between 2020 and 2023. This period has been extremely dry in the South of Spain. We compare non-steady-state isotope and hydrological mass balance model results to monthly observations of lake levels and lake water isotope data to assess the mechanisms that control lake hydrology from daily to annual scale.

Laguna Grande showed significantly less isotope variability (δ¹⁸O ranged from 5.7 to 9.6‰, ¹⁷O-excess from -34 to -86 per meg, d-excess from -28 to -43‰) than Laguna Chica (δ¹⁸O ranged from -1.5 to 20.8‰, ¹⁷O-excess from -7 to -153 per meg, d-excess from -2 to -89‰). In general, ¹⁷O‑excess and d‑excess decreased with increasing δ¹⁸O, indicating the impact of evaporation. The highest δ¹⁸O and lowest ¹⁷O-excess and d-excess values occurred at the end of summer before the start of the next rainy season. Annual average δ¹⁸O of Laguna Grande increased by ∼0.7‰ per year, while ¹⁷O-excess (∼10 per meg per year) and d-excess (∼2 ‰ per year) decreased slightly. This indicates that evaporation exceeded water inflows, which is consistent with the 2 m water level drop observed over the study period. Laguna Chica showed high interannual isotope variability. In particular, the timing of desiccation determines its maximum evaporative isotope enrichment. The highest δ¹⁸O and lowest ¹⁷O-excess and d-excess were observed in October 2022, just before complete desiccation. The lake was refilled in the subsequent rainy season but dried up in May 2023 preventing it from reaching the high level of evaporation observed in the preceding hydroperiods. Changes in the length of the hydroperiod and the timing of desiccation need to be considered when interpreting isotope data of paleo-lake water obtained from lake sediment archives. The non-steady-state isotope-hydrological mass balance model agrees reasonably well with observations, showing that lake’s isotope variability can be predicted even in highly dynamic systems. However, uncertainty in the lake volume-to-surface area ratio at low water level stages challenge accurate prediction of the lake isotope composition.