Carbon isotopic ratio of dissolved inorganic carbon in the spring water around Asama volcano

Estimation of the contribution rate of volcanic CO₂

Abstract

In order to determine the source and formation process of dissolved inorganic carbon (DIC) in spring water and to evaluate quantitatively the contribution of volcanic gas to water chemistry of springs distributed on and around Asama volcano, the carbon isotopic ratio of DIC (δ¹³C_DIC) with major dissolved solids has been measured. The measurements of carbon isotopic ratios of volcanic and soil CO₂, which are the source materials of DIC, were also carried out in Jigokudani fumarole and in the forest soil of several points of volcano flank, respectively.
The spring waters in Asama volcano have been classified into nine groups (A∼I) based on the physicochemical characteristics, such as water temperature, electrical conductivity and chemical compositions. As δ¹³C_DIC increase with increasing DIC content, the origin of DIC in spring water from Asama volcano was can be assessed by mixing process between isotopically light soil CO₂ (organic origin) and ¹³C-enriched volcanic CO₂ (deep origin with mantle component), except for the springs of group B. On the basis of two components mixing, the contribution rate of volcanic CO₂ to DIC in spring water was computed by using the carbon isotopic ratio of CO₂ equilibrated with DIC (δ¹³C_CO2) as an indicator. Consequently, the contribution rates of volcanic CO₂ were ranged from 40 to 60% in the groups C, F and H located on the flank of the mountain. In particular, the strong contribution of more than 90% was confirmed in the group I located on the higher part of the mountain, that is near the crater. These groups were correspondent with those in which influence of volcanic gases was assumed from the geochemical characteristics of spring water. By contrast, influence of volcanic CO₂ was almost not found in other groups A, D, E and G.
The spring waters of group B which are not plotted on the two components mixing line and located at the terminal of Onioshidashi lava flow have highest δ¹³C_DIC in spite of low DIC content. These ¹³C-enriched spring waters are probably derived from dissolved CO₂ degassing of thegroundwater affected by volcanic CO₂ during the discharge process. Since the groundwater moves in the clinker, which is fractured zone developed in lower and upper part of the lava flow and is extremely porous, as not laminar flow but turbulent flow, the CO₂ degassing would be effectively caused.