Technical note: Further adjustments to the Rock-Eval® thermal analysis for soil organic and inorganic carbon quantification to avoid post-hoc corrections

Abstract. Accurate quantifications of soil organic (SOC) and inorganic (SIC) carbon are essential for a better understanding of the global carbon cycle. The procedures usually used to quantify SOC and SIC (e.g., elemental analysis after pretreatment) rely on various approximations and can lead to analytical errors. Ramped thermal analyses are increasingly investigated to quantify SOC and SIC by heating a single aliquot and continuously measuring the carbonaceous compounds emitted. The Rock-Eval® thermal analysis (RE) has been standardized to estimate organic and inorganic C contents of oil-bearing rocks through two parameters named TOC and MINC, respectively. Moreover, its pyrolysis phase before the oxidation provides the basis for calculating indices to characterize soil organic matter (SOM). However, statistical post-hoc corrections of TOC and MINC are needed to adjust their estimations of SOC and SIC contents because SOC and SIC decomposition signals overlap at the end of the pyrolysis. A new cycle with a final pyrolysis temperature of 520 °C (PYRO520) instead of 650 °C is investigated to avoid SIC decomposition while preserving OM characterization during pyrolysis. The results are compared to the quantifications obtained with the standard analysis cycle (PYRO650) and by elemental analysis after pretreatments. The PYRO520 cycle corrects the misallocation of the end-of-pyrolysis signals between the TOC and MINC parameters and thus accurately and repeatably estimated SOC and SIC contents measured by EA after pretreatments without needing post-hoc corrections. Moreover, the values and interpretations of the indices characterizing SOM are not drastically modified by the pyrolysis modification.