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 1.559tm−2d−1 in spring 2009 and the CO2 emission rate at the wet mofettes of Hartoušov mofette field yielded 0.62td−1. The total CO2 discharge of the 27 wet mofettes was 3.75t 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 25gm−2d−1 as background, the mantle-derived CO2 flux yielded approximately 1566tm−2d−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 (1.518tm−2d−1) stems from sites with CO2 fluxes higher than 500gm−2d−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 5Ra to approximately 6Ra 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.