CO₂ discharge in an active, non-volcanic continental rift area (Czech Republic): Characterisation (δ¹³C, ³He/⁴He) and quantification of diffuse and vent CO₂ emissions

Abstract

In the western Eger Rift (ER) area along the Počatky–Plesná fault zone (PPZ) CO₂ degassing occurs predominantly within two mofette fields Bublák and Hartoušov. We studied 27 wet mofettes belonging to these mofette fields for gas emission rates repeatedly between 2007 and 2009 and selected mofettes for gas composition and isotope ratios (δ¹³C, ³He/⁴He). Detailed ground mapping along the PPZ provided further two separated diffuse degassing structures (DDS) within the mofette fields Hartoušov and Bublák. The DDS Hartoušov was studied in detail by measurements of 682 CO₂ soil gas concentrations, 762 CO₂ soil gas fluxes (max. 10 grid spacing) and partly by analyses of isotope ratios (¹³C/¹²C, ³He/⁴He) of soil gas. At the DDS Hartoušov the total CO₂ soil flux yielded 1.559 t m^− 2 d^− 1 in spring 2009 and the CO₂ emission rate at the wet mofettes of Hartoušov mofette field yielded 0.62 t d^− 1. The total CO₂ discharge of the 27 wet mofettes was 3.75 t d^− 1.

At sites with high CO₂ soil flux, the portion of mantle-derived helium is in the same range as the releasing at wet mofettes; both cover the signature of the subcontinental mantle. Also, the δ¹³C values analysed in the gas releasing from wet mofettes and those analysed in soil gas are nearly the same. Taking in account a biogenic soil CO₂ flux of 25 g m^− 2 d^− 1 as background, the mantle-derived CO₂ flux yielded approximately 1566 t m^− 2 d^− 1. As a result of the CO₂ flux mapping of the DDS Hartoušov, it could be proved that 97.4% of the released soil CO₂ (1.518 t m^− 2 d^− 1) stems from sites with CO₂ fluxes higher than 500 g m^− 2 d^− 1‐pointing to dominantly fault-related CO₂ release.

At the central mofette Bublák (B2), the gas emission rate was determined for the first time in 1993. Measurements repeated between 2007 and 2009 showed a clear increase in the gas emission rate of more than 40%, correlating well with the increase of the ³He/⁴He ratios from 5 Ra to approximately 6 Ra between 1993 and 2008 at this location (Bräuer et al., 2009).

The Bublák mofette field is characterised by the highest CO₂ emission rate along the PPZ, and in combination with the helium isotope signature of the European subcontinental mantle, this area was identified as a deep-reaching fluid injection zone.

Introduction

Natural degassing of CO₂ is well-known predominantly from numerous volcanic-hydrothermal settings, but also from rift areas worldwide (e.g. Kerrick et al., 1995, Chiodini et al., 1996, Chiodini et al., 1998, Chiodini et al., 2008, Chiodini and Frondini, 2001, Evans et al., 2002, Lewicki et al., 2003, Lewicki et al., 2007, Bergfeld et al., 2006, Lan et al., 2007, Doğan et al., 2009, Gal and Gadalia, 2011).

Active fault systems provide migration paths for the degassing of terrestrial gases. Soil gas studies due to fault-related CO₂ release were described, for instance, for the San Andreas fault and for the Calaveras fault in California (Lewicki et al., 2003), for the Chaochou fault in southern Taiwan and for the North Anatolian fault zone in Turkey (Doğan et al., 2009).

The western Eger/Ohře Rift (ER) area (Fig. 1) is characterised as an active, non-volcanic continental rift where CO₂ degassing occurs in the form of mineral springs and mofettes (Kolářová, 1965, Kolářová and Hrkal, 1979, Polyak et al., 1985, Kämpf et al., 2007).

Weinlich et al., 1998, Weinlich et al., 1999 carried out an initial mapping of the gas and isotope composition of the free gas at the wet degassing sites in the ER area, and provided a first estimation of the vent-bound CO₂ emission rate in this rift area. Additional gas- and isotope data were given by Geissler et al. (2005). Three different degassing centres could be distinguished along the ER region: the Cheb Basin (CB), Mariánské Láznĕ (ML) and surroundings, and Karlovy Vary (KV). The gases from all these degassing centres showed a similar δ¹³C signature in CO₂, but a different high level of mantle-derived helium. Further detailed studies indicated that an active hidden magmatic process is ongoing beneath the Cheb Basin (Bräuer et al., 2005, Bräuer et al., 2008, Bräuer et al., 2009).

Based on receiver function studies Geissler et al. (2005) and Heuer et al. (2006) found an up-doming of the Moho beneath the investigation area and detected at ca. 65 km depth in the lithospheric mantle a zone with decreased velocity which was interpreted as astenospheric up-doming and/or may be caused by the occurrence of partial melt. Heuer et al. (2011) assumed a small plume structure with only weak imprint on the 410 km discontinuity beneath this area.

Until a few years ago only two small Quaternary volcanoes were known close to the Cheb Basin. In 2007 the Mýtina maar was discovered (ca. 0.29 Ma old, Mrlina et al., 2007) and there are first indications for the existence of further maar and diatreme structures in or close to the Cheb Basin. The Bublák–Hartoušov mofette fields are arranged only about 20 km north of the Mýtina maar. Due to the indications of ongoing active magmatic processes beneath the area of the mofette fields, a potential future hazard (phreatomagmatic eruption) for the western ER cannot be excluded (Mrlina et al., 2009).

In the mofette fields along the PPZ, the cold dry CO₂ release may be accumulated near the ground because the CO₂ dominated gas is denser than air. This process influences flora and fauna and results in the death of small animals (mostly insects, mice, frogs and birds) which are often found in such carbon sinks. In the Hartoušov DDS (diffuse degassing structure) the effect of CO₂ release was in the focus of multidisciplinary studies due to the consequence of interaction processes on near surface degassing conditions and ecological studies by geophysical, sedimentologic, pedologic, botanic and zoologic tools (Flechsig et al., 2008, Pfanz and Sassmannshausen, 2008, Schulz and Potapov, 2010, Rennert et al., 2011 and Russell et al., 2011).

The aim of the presented detailed study at degassing structures in the Cheb Basin (CB) was to quantify the mantle-derived CO₂ emission rate along the Počatky–Plesná fault zone (PPZ) (Fig. 1). Furthermore, detailed CO₂ soil gas concentration and flux measurements, including isotope analyses (δ¹³C, ³He/⁴He) of soil gas are presented to support the quantification of diffuse mantle-derived CO₂ release.