Abstract
At a semiarid steppe site located in the SE of Spain, relatively large CO2 emissions
were measured that could not be attributed to the ecosystem activity alone. Since the study
site was located in a tectonically active area, it was hypothesized that a part of the measured
CO2 was of geologic origin. This investigation included a survey of soil CO2 efflux,
together with carbon isotope analyses of the CO2 in the soil atmosphere, soil CO2 efflux
(i.e., Keeling plots), groundwater and local thermal springs. These measurements
confirmed the hypothesis of degassing from geologic sources. In areas with local faults
and ancient volcanic structures, soil CO2 efflux rates were significantly higher (i.e., up to
6.3 and 1.4 mmol CO2 m2 s1) than measurements in a comparable site that was some
distance from fault sites (means of 1.0 and 0.43 mmol CO2 m2 s1 in March and June,
respectively). The CO2 concentration in the soil atmosphere at the eddy covariance site
reached 0.14% v/v at 0.70 m soil depth with a 13C-enriched isotopic composition
(d13C from 10.2‰ to 16.6‰), consistent with the isotopic composition of the soil
CO2 efflux estimated by Keeling plots (i.e., 16.6‰). 13C-enriched CO2 also occurred in
local aquifers, and there was evidence of degassing from deep crust and mantle at regional
scale by the helium isotopic ratio in spring waters located about 30 km (R/Ra: 0.12)
and 200 km (R/Ra: 0.95) NW of the eddy covariance site. This study highlights
the importance of considering CO2 sources of geologic origin when assessing the
net ecosystem carbon balance of sites that may possibly be affected by circulation of
such CO2-rich fluids.
