Geologic carbon sources may confound ecosystem carbon balance estimates: Evidence from a semiarid steppe in the southeast of Spain

At a semiarid steppe site located in the SE of Spain, relatively large CO<inf>2</inf> 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 CO<inf>2</inf> was of geologic origin. This investigation included a survey of soil CO<inf>2</inf> efflux, together with carbon isotope analyses of the CO<inf>2</inf> in the soil atmosphere, soil CO<inf>2</inf> 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 CO<inf>2</inf> efflux rates were significantly higher (i.e., up to 6.3 and 1.4 mmol CO<inf>2</inf> m^-2 s^-1) than measurements in a comparable site that was some distance from fault sites (means of 1.0 and 0.43 mmol CO<inf>2</inf> m^-2 s^-1 in March and June, respectively). The CO<inf>2</inf> 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 (δ¹³C from -10.2% to -16.6%), consistent with the isotopic composition of the soil CO<inf>2</inf> efflux estimated by Keeling plots (i.e., -16.6%). ¹³C-enriched CO<inf>2</inf> 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 CO<inf>2</inf> sources of geologic origin when assessing the net ecosystem carbon balance of sites that may possibly be affected by circulation of such CO<inf>2</inf>-rich fluids. © 2012. American Geophysical Union. All Rights Reserved.