Adjustments of the Rock-Eval® thermal analysis for Soil Organic and Inorganic Carbon (SOC and SIC) quantifications

Quantifying Soil Organic and Inorganic Carbon (SOC & SIC) separately in carbonate soils involves successive pretreatments and/or measurements to separate the two carbon forms. The Rock-Eval® (RE) thermal analysis has been developed in the 70’s by IFPEN to study oil bearing rocks with a ramped heating pyrolysis and oxidation. The RE has been increasingly used over the past 20 years to quantify and characterize SOC without pretreatments and with a single analysis in any kind of soils. To improve the SOC and SIC quantifications by RE, Disnar & al. (2003) and Sebag & al. (2022a; 2022b) suggested statistical corrections of the TOC and MinC standard parameters, which are SOC and SIC content estimators respectively. However, few applications have focused on SIC quantification and no adjustments of the RE standard analysis cycle have been investigated. This study aims at adjusting the RE standard analysis protocol to quantify SOC and SIC. For this, a panel of carbonate soils with a wide range of SOC and SIC contents was analyzed by RE. The SOC and SIC quantifications by RE were compared to those obtained by elemental analysis (EA, standard method to quantify soil carbon). A too high SIC amount in the RE crucible (> 4 mg) led to an underestimation of the SIC content, even after correcting the MinC parameter. The higher the SIC amount in the RE crucible, the more the SIC content was underestimated. The standard analysis cycle was too short to achieve a complete thermal decomposition of the SIC amount in the crucible leading to an underestimation of the SIC content. Thus, we propose to extend the last oxidation isotherm from 3 min to 5 or 7 min to achieve the complete SIC thermal decomposition. Moreover, the temperature limit used during the pyrolysis to distinguish the CO and CO₂ emitted by organic matter thermal cracking and carbonate thermal decomposition phase is often inappropriate as the thermoresistant part of SOC and the main part of SIC decompose simultaneously after 550°C. Thus, we suggest to stop the pyrolysis phase at the onset of SIC decomposition to drag all SIC signal during the oxidation phase only. This modification of the standard analysis protocol for carbonate soils allows us to avoid some of the statistical corrections suggested by Disnar & al. (2003) and Sebag & al. (2022a; 2022b). Finally, we propose a new RE analysis protocol to simplify the calculations and improve the accuracy of SOC and SIC quantifications in carbonate soils.   

Disnar, J.R., B. Guillet, D. Keravis, C. Di-Giovanni & D. Sebag, 2003. Soil organic matter (SOM) characterization by Rock-Eval pyrolysis: scope and limitations. Organic Geochemistry 10.1016/S0146-6380(02)00239-5

Sebag, D., V. Lamoureux-Var, I. Kowalewski, D. Pillot & H. Ravelojoana, 2022a. Procédé pour la quantification et la caractérisation du carbone dans les sols. IFP Energies Nouvelles Patent No. 3121225. France.

Sebag, David; Lamoureux-Var, Violaine; Kowalewski, Isabelle; Ravelojoana, Herman; Lefrançois, Noémie, 2022b. Improved quantification of SOC and SIC in Rock-Eval® thermal analysis. SOM (8th International Symposium on Soil Organic Matter), Seoul, Korea.