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In the western Eger Rift (ER) area along the Pocatky–Plesna fault zone (PPZ) CO 2 degassing occurs predominantly within two mofette fields Bublak and Hartousov. 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 (δ 13 C, 3 He/ 4 He). Detailed ground mapping along the PPZ provided further two separated diffuse degassing structures (DDS) within the mofette fields Hartousov and Bublak. The DDS Hartousov was studied in detail by measurements of 682 CO 2 soil gas concentrations, 762 CO 2 soil gas fluxes (max. 10 grid spacing) and partly by analyses of isotope ratios ( 13 C/ 12 C, 3 He/ 4 He) of soil gas. At the DDS Hartousov the total CO 2 soil flux yielded 1.559 t m − 2 d − 1 in spring 2009 and the CO 2 emission rate at the wet mofettes of Hartousov mofette field yielded 0.62 t d − 1 . The total CO 2 discharge of the 27 wet mofettes was 3.75 t d − 1 . At sites with high CO 2 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 δ 13 C 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 CO 2 flux of 25 g m − 2 d − 1 as background, the mantle-derived CO 2 flux yielded approximately 1566 t m − 2 d − 1 . As a result of the CO 2 flux mapping of the DDS Hartousov, it could be proved that 97.4% of the released soil CO 2 (1.518 t m − 2 d − 1 ) stems from sites with CO 2 fluxes higher than 500 g m − 2 d − 1 ‐pointing to dominantly fault-related CO 2 release. At the central mofette Bublak (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 3 He/ 4 He ratios from 5 Ra to approximately 6 Ra between 1993 and 2008 at this location (Brauer et al., 2009). The Bublak mofette field is characterised by the highest CO 2 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. |
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