Mixing of magmatic CO2 into volcano groundwater flow at Aso volcano assessed combining carbon and water stable isotopes

Abstract

To understand deep groundwater flow systems and their interaction with CO 2 emanated from magma at depth in a volcanic edifice, deep groundwater samples were collected from hot spring wells in the Aso volcanic area for hydrogen, oxygen and carbon isotope analyses and measurements of the stable carbon isotope ratios and concentrations of dissolved inorganic carbon (DIC). Relations between the stable carbon isotope ratio (δ 13 C DIC ) and DIC concentrations of the sampled waters show that magma-derived CO 2 mixed into the deep groundwater. Furthermore, groundwaters of deeper areas, except samples from fumarolic areas, show higher δ 13 C DIC values. The waters' stable hydrogen and oxygen isotope ratios (δD and δ 18 O) reflect the meteoric-water origin of that region's deep groundwater. A negative correlation was found between the altitude of the well bottom and the altitude of groundwater recharge as calculated using the equation of the recharge-water line and δD value. This applies especially in the Aso-dani area, where deeper groundwater correlates with higher recharge. Groundwater recharged at high altitude has higher δ 13 C DIC of than groundwater recharged at low altitude, strongly suggesting that magmatic CO 2 is present to a much greater degree in deeper groundwater. These results indicate that magmatic CO 2 mixes into deeper groundwater flowing nearer the magma conduit or chamber.

Earthquake-induced structural deformations enhance long-term solute fluxes from active volcanic systems

“…Further evidence supporting a deep aquifer origin of the post-earthquake springs comes from the water chemistry. Compared with surface waters from the Aso caldera, the new fluid is characterized by lower stable hydrogen and oxygen isotope ratios (δD H2O = −56.1‰ and δ 18 O H2O = −8.66‰, Table 1 ), similar to the composition of confined groundwater at 50–200 m depth from the surface (−58‰ < δD H2O < −54‰ and −9.0‰ < δ 18 O H2O < −8.5‰) 8 , 30 . Moreover, the new spring water and similar highly saline water from springs that pre-dated the earthquake (Kayahara) (Fig.…”

“…Under the center of the volcanic cones, magma ascends from about 16 km depth toward the surface 31 . Contributions from the central magma chamber were suggested for groundwater with deep flow paths using a δ 13 C DIC tracer method 8 ; although this has not been considered for the waters from the northwestern plain specifically, similar waters could contribute to the springs in the region. Alternatively, a recent electrical resistivity survey 31 , 32 suggests the presence of a localized melt finger deeper than 8 km beneath the area seeping deep fluids in the northwestern plain (yellow dotted line in Fig.…”

Fractionation processes affecting the stable carbon isotope signature of thermal waters from hydrothermal/volcanic systems: The examples of Campi Flegrei and Vulcano Island (southern Italy)

“…Bottinga, 1968;Deines et al, 1974;Mook et al, 1974), this widely accepted classification is not consistent with the isotopic signature of a number of thermal waters from volcanic and hydrothermal areas, which show δ 13 C CO2(aq) values ranging from − 18‰ to − 9‰ vs. V-PDB (e.g. D'Alessandro et al, 1997;Caliro et al, 1999;Federico et al, 2002;Taran et al, 2002;Inguaggiato et al, 2005;Grassa et al, 2006;Yamada et al, 2011;Ruzié et al, 2013;Marrero-Diaz et al, 2015;Morikawa et al, 2016). In most cases, these isotopic values, which are intermediate with respect to those related to mantle/magmatic and biogenic CO 2 , were interpreted as due to mixing processes between deep and shallow/marginal end-members (e.g.…”

The effect of regional groundwater on carbon dioxide and methane emissions from a lowland rainforest stream in Costa Rica

“…In addition to biogenic contributions from the surface, dissolved CO 2 and CH 4 in regional groundwater can originate from volcanic degassing, nonvolcanic escape of gases from the upper mantle, intrusive magma chambers, carbonate‐bearing rocks in the crust, hydrocarbon accumulations [ Mörner and Etiope , ], and remineralization of ancient sedimentary organic matter [ Lovley and Anderson , ; Park et al ., ; Liu et al ., ]. Aquifers transporting high concentrations of dissolved C have been identified in numerous places around the world, for example, Portugal [ Cruz and Amaral , ], the Canary Islands [ Marrero et al ., ], the United States [ Evans et al ., ; Kampman et al ., ], Japan [ Yamada et al ., ], Canada, Costa Rica, Italy [ Genereux et al ., , and references therein], the Lesser Antilles [ Rivé et al ., ], and the Slovak Republic [ Kucharič et al ., ]. However, little is known about the fate of that C once it discharges into surface waters and its effects on C balances of the aquatic and surrounding terrestrial ecosystems.…”