Effects of Soil Organic Matter on the Results of Isotope Analysis of ¹³С/¹²С and ¹⁸О/¹⁶О in Calcite

The isotopic composition of pedogenic carbonates offers valuable insights into the paleoenvironments in which they formed. However, the analysis of these carbonates is complicated by the simultaneous presence of soil organic matter (SOM) alongside inorganic carbon (IC), the removal of which is a labor-intensive process. Previous research into the influence of organic matter on the isotope analysis of carbonates has yielded contradictory results, and some authors attributed the discrepancies to the properties and quantity of the organic matter. Some researchers suggested that sample preparation methods—such as thermal treatment, plasma ashing, the use of chemical reagents to remove organic matter, and the digestion of samples with phosphoric acid—affect the resulting δ¹³C and δ¹⁸O values more than the organic matter itself. There is no consensus about the necessity of removing SOM before the isotope analysis of carbonates. To assess the impact of SOM on the results of the isotope analysis of carbonates from peat soils, we conducted a series of experiments aimed to select the optimal sample preparation procedure for analysis by continuous-flow isotope ratio mass spectrometry. The study involved two horizons of Sapric Drainic Histosol the histic horizon (TE₂), with 41.67% soil organic carbon (SOC) content and a lower degree of decomposition, and the organogenic horizon (TT), with 4.45% SOC content and a higher degree of decomposition. For the δ¹³C analysis, the best results, comparable to those obtained by removing organic matter with H₂O₂, were achieved using 105% H₃PO₄ to decompose air-dried samples from the TT horizon, with an IC/SOC ratio up to 1 : 10. However, applying this approach to samples with an IC/SOC ratio of 1 : 20, or to soil from the TE₂ horizon with a higher SOC content and lower degree of decomposition, led to a significant depletion of ¹³C in the resulting CO₂. The use of 98% H₃PO₄, thermal treatment of samples at 90°C, and substantial reductions in equilibration time also caused deviations in the δ¹³C values towards a lighter isotopic composition. Deviations from the true values of δ¹⁸O were observed in all experiments. The best results were obtained when samples were digested with 105% H₃PO₄ without thermal treatment or the use of chemical reagents, regardless of the initial SOC content or the IC/SOC ratio.