Mass-dependent Corrections and Atmospheric Invasion: Working with the Radiocarbon Content of CO2 in the Soil Gas Environment

Abstract:

Radiocarbon is becoming a more commonly utilized tool for C cycling studies, as it helps constrain biotic ecosystem processes such as C turnover times and sources of production. However, for studies that focus on CO₂, the sampling methods for Δ¹⁴CO₂ (surface chambers and subsurface gas wells) can be affected by abiotic processes, which may bias results as a function of gas transport regime (diffusion and advection). The radiocarbon community currently uses a δ¹³C correction to account for mass-dependent fractionation, but to date this correction has not been validated for the soil gas environment, where atmospheric invasion and gas transport are important. This study used an analytical soil gas transport model across a range of soil diffusivities and production rates, in which we could control Δ¹⁴CO₂ and δ¹³CO₂ signatures of production and atmosphere. This synthetic situation allowed us to assess the bias that results from using the conventional correction method for estimating Δ¹⁴CO₂ of soil production. We found that the conventional correction is not strictly valid in this setting for interpreting the signature of production and does not account for diffusion and atmospheric invasion. The resultant Δ¹⁴CO₂ bias scales with soil diffusivity and production rates. We propose a new two-step correction for Δ¹⁴CO₂ work in the soil environment that accounts for atmospheric invasion and the δ¹³CO₂ correction, and is able to reproduce a true value of Δ¹⁴CO₂ of production. This work not only assists in data interpretation, but also helps clarify a methodological window of opportunity for accurately measuring the Δ¹⁴CO₂ of soil production using subsurface sampling.