Furnas Lake is located inside the caldera complex in Furnas Volcano, one of the three Quaternary central volcanoes that dominates the geology of São Miguel Island (Azores, Portugal). This lake has a maximum depth of about 11 meters and the water is slightly acidic, with a circumneutral pH (~6.7), being fed by both meteoric and hydrothermal water. The lake presents a monomictic character, meaning it fully mixes from top to bottom once a year, from autumn to spring. The physicochemical parameters and major-ion content are typically consistent throughout the water column, though there may be some seasonal variations. However, during summer or warm periods the lake stratification prevails, preventing mixing along the water column, with oxygen consumption leading to anoxic conditions in the hypolimnion and a CO2 enrichment is observed. The CO2 flux is strongly influenced by lake stratification, with higher emission to the atmosphere during periods of monomictic behavior.
Samples were collected in a vertical profile in two locations in Furnas lake, one in the deeper section of the water body (~10.8 m) and one in the area where CO2 degassing is higher (~1 m depth), in April and August of 2022. Although the August survey data is still being analyzed, the April campaign showed a stratification in temperature and in certain elements, such as Al, Fe, Mn, Cu, Zn, Sr, and Ba. The strontium isotopic ratios in waters from Furnas Lake were determined, in order to gain more information on the processes governing water geochemistry and to investigate if the water column stratification in summer affects the 87Sr/86Sr ratios. The 87Sr/86Sr ratio values of the lake were higher (0.705882 – 0.706181) than the mineral waters from Furnas (0.705235 – 0.705432), suggesting a mixture of rainwater and hydrothermal water. It is also possible to notice a small difference in the isotopic ratio in the middle depth (0.705882) sample collected in the shallowest area of the lake, it depicts a lower isotopic ratio in comparison to the bottom (0.706181) and the surface (0.706153). However, further analysis of the summer campaign samples are required to evaluate if stratification affects the strontium isotopic ratios.