Volcanic lakes of the Azores archipelago (Portugal): Geological setting and geochemical characterization

Abstract

A representative set of 30 lakes from the Azores archipelago (Portugal) has been studied to obtain data on their volcanic setting and water geochemistry. The majority of the studied lakes are located inside volcanic craters and subsidence calderas, while the remaining lakes are located in topographic depressions not associated with volcanic centres, in depressions of tectonic origin and inside lava caves. In general, the Azorean volcanic lakes are small, with surface areas between 8.94 × 10 − 4 km 2 and 4.35 km 2 and volumes between 2.4 × 10 3 and 4.78 × 10 7 m 3; median = 3.99 × 10 4 m 3, presenting maximum depths between 0.8 and 33 m. Mostly of the Na–Cl to Na–HCO 3 type, the lake waters are generally low temperature (11.9–24.6 °C), very dilute (TDS range between 11.0 and 356.2 mg/L; median = 36.8 mg/L) and a clear exception is Furna do Enxofre lake (Graciosa island) that shows a Mg trend. Two types of lake waters can be recognized: 1) one is characterized by Na/Cl ratio close to the seawater value and have evolved, and evolve by marine contribution; and 2) Na–HCO 3 type waters that, in addition to sea-salt input, have been influenced by other processes (e.g., water–rock interaction, hydrothermal seepage into the lake bottom) evident from higher total CO 2 content (365.1 mg/L) and SiO 2 content (74.9 mg/L), as well demonstrated by in the Furna do Enxofre lake. Nevertheless, volcanic contribution cannot be excluded for some lakes as Furna do Enxofre. Vertical profiles made at the deepest lakes of São Miguel reveal that major-ion content varies little with depth, largely independent of the occurrence of thermally driven density stratification in summer for some lakes. However, dissolved CO 2 increases with depth and is kept in the hypolimnion in summer, whereas the HCO 3 − content is stable, suggesting that CO 2 release exceeds acidity neutralization by water–rock interaction. During winter, without stratification, the dissolved CO 2 increase is less sharp for the majority of the lakes, with the exception of Furnas and Furna do Enxofre lakes. The CO 2 enrichment observed for certain lakes (e.g., Furna do Enxofre and Furnas) implies a volcanic input. Moreover, as these lakes themselves are located near hydrothermal surface manifestations, they therefore offer good targets of volcano monitoring in the Azores.