CO2 Flux from Volcanic Lakes in the Western Group of the Azores Archipelago (Portugal)

Abstract

Here, we present the first detailed study on diffuse CO 2 degassing in the lakes in the Western Group (Corvo and Flores islands) of the Azores archipelago. This research is of interest in order to determine (1) the overall CO 2 emission from such lakes, as volcanic lakes are often underrepresented in the databases of these water bodies, and (2) the diffuse CO 2 degassing estimates in active volcanic areas such as the Azores. The lake waters on Corvo and Flores islands are mainly of the Na-Cl type, which is likely caused by the lakes' sea salt signatures, arising from nearby seawater spraying; however, a few samples show evidence of slight alkali earth metal and bicarbonate enrichments in the lake waters, suggesting a contribution of water-rock interaction. In this study, diffuse CO 2 flux measurements were taken using the accumulation chamber method, and statistical analyses utilizing the graphical statistical approach (GSA) and sequential Gaussian simulation (sGs) were conducted on the CO 2 flux data, showing that the CO 2 flux values measured in these lakes were relatively low (0.0-18.6 g m −2 d −1 ). The results seem to indicate that there is a single source of CO 2 (a biogenic source), which is also supported by the waters' δ 13 C isotopic signatures. Significant differences in the final CO 2 output values were verified between surveys (e.g., 0.16 t d −1 in R1; 0.32 t d −1 in R2), and these differences are probably associated with the monomictic character of the lakes. CO 2 emissions ranged between 0.18 t d −1 (CE1) and 0.50 t d −1 (CW1) for the Corvo lakes and between 0.03 t d −1 (P1) and 0.32 t d −1 (R2) for the seven lakes studied on Flores Island. The presence of a dense macrophyte mass in a few of the lakes appears to enhance the CO 2 flux in these lakes. measurements should be obtained from the remainder and the fluxes added to the global databases in order to better understand the total volcanic CO 2 output.The study of volcanic lakes permits enhanced knowledge of volcanic systems, as such lakes present a wide range of chemical characteristics, from high total dissolved solid (TDS) brines to meteoric waters; indeed, they have been called "blue windows" into the depths of a volcano [11,12]. Volcanic gases, composed mainly of water vapor and varied amounts of carbon compounds, sulfur, halogens, and several minor constituents, are the main drivers of volcanic water composition [13][14][15][16].Natural CO 2 can have several origins in that it can originate from the mantle, from carbonate rocks existing in the crust, or as the result of biogenic activity [17]. Over the last decade, studies of diffuse degassing CO 2 have been conducted in volcanic lakes all over the world and in other types of water bodies, and such studies have provided important information about their spatial and temporal flux variations and have been shown to be a relevant tool for mapping hidden active faults and/or for active volcano monitoring [10,[18][19][20][21][22][23][24][25]. In addition, these studies have contributed to improvi...

Unraveling the Holocene Eruptive History of Flores Island (Azores) Through the Analysis of Lacustrine Sedimentary Records

“…Despite being a young oceanic volcanic island, Flores did not experience any eruption since the Portuguese settlement in the 15th century and therefore its volcanic hazard is usually considered as low (Chester et al, 2017). Moreover, hydrogeochemical and diffuse CO 2 degassing studies of Flores lakes show that, presently, there is no volcanic/hydrothermal signature (Cruz et al, 2006;Andrade et al, 2019), suggesting that volcanism is either dormant or extinct. However, the geological record of the island shows that a brief, but intense period of monogenetic volcanism occurred 2.9-3 kyr ago (Morisseau and Traineau, 1985;Azevedo and Portugal Ferreira, 2006).…”

The vanishing and the establishment of a new ecosystem on an oceanic island – Anthropogenic impacts with no return ticket

“…The total CO 2 content along the water column ranges from 24.76 to 38.32 mg•L −1 (mean = 30.45 mg•L −1 ). The seasonal effect on the total amount of CO 2 emitted into the atmosphere is present in the studied lake, and during summer is 0.36 t•d −1 , and through winter, the emissions should be higher due to the full mixing (Andrade et al, 2019).…”

“…1). The lake occupies 0.37 km 2 and its total basin area is 3.14 km 2 (Andrade et al, 2019). Lake Funda is a deep monomictic lake (max.…”

“…Stratification starts approximately in May/June and lasts until November. The residence time is 2.7 years (Andrade et al, 2019). Lake Funda has a high bathymetric gradient, and is hydrologically closed with reduced shallow littoral zones (Fig.…”

Long-term hydroclimate variability in the sub-tropical North Atlantic and anthropogenic impacts on lake ecosystems: A case study from Flores Island, the Azores

“…It is located in the interior of Flores Island in the Azores Archipelago and is surrounded by a relatively large (3.14 km 2 ) and steep catchment area that is mostly forested with some agriculture (Andrade et al, 2019). The lake is monomictic as the lake thermally stratifies from the summer to fall (usually May/June to November) and the water column mixes from November/December to May/June.…”