Distribution and isotopic signature of deep gases in submerged soils in an island of the Lower Delta of the Paraná River, Argentina

Subsoil CH4 and CO2 concentrations, δ13C-CH4 and δ13C-CO2 signatures, total organic carbon (TOC) and δ13C-TOC, together with C/N ratio of organic matter, were evaluated throughout a soil profile up to the atmosphere to understand the dynamics of CH4 and CO2 in the waterlogged environment of an island of the Lower Delta of the Paraná River, Argentina. The analysis of the vertical profile showed that a significant fraction of CH4 exists as gas trapped within the sediment column, compared to CH4 dissolved in soil solution. CH4 concentration measurements in sub-saturated soils showed that free CH4 is 1 order of magnitude smaller than CH4 recovered from soil cores by ultrasonic degassing. The highest concentrations of CH4 occurred at the 90–120-cm layer. At this depth, δ13C-CH4 values resulting from methanogenesis were around − 71‰, which is well within the range of CH4 produced from CO2 reduction, and δ13C values of the associated CO2 were enriched (~ − 7‰). Isotope mass balance models used to calculate the fraction of oxidized CH4 indicated that around 30% of the CH4 produced was oxidized prior to atmospheric release. In contrast to methanogenesis, during oxidation processes δ13C-CH4 shifts to more positive values. The mineralogical, textural, isotopic, and geochemical characterization of subsoil sediments with abundant organic matter, like Paraná Delta, demonstrated that CH4 storage capacity of the soil, production, consumption, and transport are the main factors in regulating the actual flux rates of CH4 to the atmosphere.