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

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