Abstract
There are hundreds of volcanic lakes around the world that represent an important hazard due to the potential occurrence of phreatomagmatic or limnic eruptions. Variations in geochemical and geophysical parameters could help to identify potential risks for these events. Cuicocha and Quilota volcanic lakes, located at the North Andean Volcanic Zone of Ecuador, are geologically young, with gas emissions manifested mainly as CO2via bubbling gases. Both lakes present a limited monitoring record. Therefore, volcanic monitoring is a priority task due to the potential hazard they represent by the possibility of water stratification and CO2accumulation. During 2012-2018 period, geochemical investigation based mainly on diffuse CO2surveys and analyzing the chemical and isotopic composition of bubbling gases has been carried out at Cuicocha and Quilotoa lakes. Additionally, vertical profiles of water columns were conducted in both lakes to investigate the possibility of water stratification and CO2accumulation in the lakes. A bathymetric study was also carried out in Quilotoa in 2017, giving further information about the degasification processes and the morphology of the lake bottom. The computed diffuse CO2output for Cuicocha volcanic lake (3.95 km2) showed a range from 53 to 652 t d−1for the period 2006–2018, with a maximum value in 2012, coinciding with a maximum of the3He/4He ratio measured at the bubbling gases and an increase in the seismic activity with an episode of long-period seismicity recorded in 2011–2012. For Quilotoa volcanic lake (3.50 km2) diffuse CO2output was estimated between 141 and 536 t d−1for the period 2014–2018. The chemical and isotopic data show that Cuicocha has a chemical composition typical of worldwide superficial shallow waters and aquifers, while Quilotoa shows a chemical composition typical of crater lakes in active volcanic systems. The distribution of the dissolved gas composition along the vertical profiles shows the existence of different water masses in both lakes, with an increase in the concentration of dissolved gases with depth. The carbon isotopic signature indicates an endogenous origin of the CO2, with a greater contribution in the stratification zone in both lakes. This study shows methods applicable to other volcanic lakes of the world to monitor their activity and potential risks.
Highlights
Volcanic lakes consist of accumulations of water inside a volcanic depression
This work presents the results of a complete geochemical study of the Ecuadorian volcanic lakes Cuicocha and Quilotoa during the period 2006–2018 (diffuse CO2 emission data of 2006 from Padrón et al (2008))
Vertical profiles of water temperature, pH, and EC showed that these lakes are made up of different water masses
Summary
Volcanic lakes consist of accumulations of water inside a volcanic depression. The study of the physical and chemical characteristics of these water masses constitutes a powerful tool to evaluate the activity level of volcanic systems (Mazot and Taran, 2009; Rouwet et al, 2014, 2015; Andrade et al, 2016, 2019, 2021; Hernández et al, 2017). After the two well-known limnic eruptions ocurred in Camerron at Lake Monoun in 1984 and Lake Nyos in 1986, special importance has been paid to the development of CO2 emission surveys in different volcanic lakes (Padrón et al, 2008; Mazot and Taran, 2009; Hernández et al, 2011, 2017; Pérez et al, 2011; Mazot et al, 2014; Melián et al, 2017; Sierra et al, 2021) These periodic CO2 emission surveys in volcanic lakes are an important task for the surveillance of these systems. Bathymetry studies have been employed to identify CO2 degassing vents (Aguilera et al, 2000; Goepel et al, 2015; Hernández et al, 2017; Melián et al, 2017), to evaluate ideal drilling sites into geothermal reservoirs (Brehme et al, 2019, 2021), and to evaluate hazards due to accumulation of CO2 in volcanic lakes (Anzidei et al, 2008)
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