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

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 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 CO2 m 2 s 1 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.