Abstract
The gases dissolved in the waters of volcanic lakes can present a serious hazard if the physical-chemical conditions change due to variations in the supply of magmatic gases. The monitoring of gases such as CO2 and He help us understand the degassing process and their connection with magmatic/hydrothermal system. One of the most acidic volcanic lakes on the planet is the Yugama, on Kusatsu Shirane volcano (Japan). We report the results of an interdisciplinary study carried out in August 2013 at Yugama consisting of the first estimation of rate of diffuse CO2 emission, the chemical and isotopic analysis of water and dissolved gases in samples from vertical lake profiles, and an echo-sounding survey. The lake water has an average temperature of 24-25°C, pH 1.01, concentrations of SO42- between 1,227 and 1,654 mgL−1 and Cl− between 1,506 and 2,562 mgL−1, with gas bubbling at several locations and floating sulfur globules with sulfide inclusions. A total of 66 CO2 efflux measurements were taken at the lake surface by means of the floating accumulation chamber method to estimate the diffuse CO2 output from the studied area. CO2 efflux values ranged from 82 up to 25,800 g m−2 d−1. Estimation of the diffuse CO2 emission at Yaguma Crater Lake was 30 ± 12 t d−1. Normalized CO2 emission rate (assuming an area of 0.066 km2) was 454 t km−2 d−1, a value within the range of acid volcanic lakes. Vertical profiles of major ions and dissolved gases showed variations with increases in ion content and dissolved CO2 and He with depth. Acoustic imaging shows the presence of intense bubbling and provides important information on the bathymetry of the lake. The 50–200 kHz echograms exhibit frequent vertical plumes of rising gas bubbles. Within the crater-lake, three circular submarine vents have been identified showing flares due to a significant activity of sublacustrine emissions. This work shows the first data of diffuse CO2 degassing, dissolved gases in water and echosounding (ES) from Yugama Crater Lake. Periodic hydrogeochemical and hydroacoustic surveys at Yugama Crater Lakemay thus help to document changes in the state of activity of this high-risk volcanic area.
Highlights
Volcanic lakes may have their origin in processes that disturb the watershed: explosion craters, collapse calderas as well as streams and rivers that were cut off by volcanic products like lava flows, lahars, ash flows (Varekamp, 2015)
In August 2013, a field campaign was carried out at YCL consisting of the measurement of the diffuse CO2 efflux through the lake surface (Figure 1C), an echo-sounding (ES) survey, and water sampling at different depths along three vertical profiles: Crater Wall (CW), Yugama Center (YC) (Yugama center) and 83-C (1983 crater; Figure 2A)
The Na, K, Ca, Mg and SO4 contents of two samples (Y3 and Y10) show a different trend, with a different linear correlation versus Cl− and different slopes. This could be explained in terms of increased hydrothermal activity at the bottom of YCL at the Crater Wall and 83-C vertical profile sampling points, since both samples are close to the bottom of YCH
Summary
Volcanic lakes may have their origin in processes that disturb the watershed: explosion craters, collapse calderas as well as streams and rivers that were cut off by volcanic products like lava flows, lahars, ash flows (Varekamp, 2015). Acidic lakes (pH < 2–3) like Yugama, at Kusatsu-Shirane volcano, Japan, release CO2 freely into the atmosphere, which makes the lake chemically transparent to CO2, in contrast to HCl and SO2-H2S, which are the cause of acidity. This means that CO2 represents a transitory phase during its ascent through a lake of pH < 2. HighpH lakes (pH 6–9) are able to store (or even precipitate) CO2 depending on the P-T conditions along the vertical profile of a lake In this case CO2 is not a transient gas and has a long residence time in the lake, especially in the deeper water layers due to hydrostatic loading (Rouwet, 2021)
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