Organic matter removal for the analysis of carbon and oxygen isotope compositions of siderite

Abstract

The measurement of stable carbon (C) and oxygen (O) isotope compositions in siderite from sediments and soils can be useful to constrain carbonate genesis processes and/or to reconstruct paleoclimates. In order to evaluate our ability to determine C and O isotope compositions of siderite in modern sediments and soils containing immature organic matter, we prepared and analyzed synthetic samples made of variable proportions of pure siderite and yeast (selected for representing an immature organic matter because of its potential high reactivity to H3PO4 digestion). Replicate analyses of CO2 produced by phosphoric acid (H3PO4) digestion of our pure siderite standard at 130°C provided δ13Csid and δ18Osid values of −12.3±0.1‰ and −16.2±0.3‰ (2σ) relative to PDB. Analysis of the synthetic sample mixtures shows δ13C and δ18O values undistinguishable from those of pure siderite within uncertainties, for proportion of yeast in the mixture equal to or lower than 10wt.% (corresponding to ~5wt.% of total organic carbon). In contrast, samples with proportions of yeast higher than 10wt.% were progressively shifted to more negative values (down to −21.8‰ for C and −19.6‰ for O) with increasing proportion of yeast, as a result of an increasing contribution of CO2 produced by the reaction of yeast with H3PO4 at 130°C. Although in the case of natural samples this organic matter CO2 contribution probably strongly depends on both organic matter source organisms and diagenetic history, our data indicate that the removal of organic matter prior to siderite analysis may be often required. We thus tested three different methods for organic matter removal on our synthetic samples, using either oxidation in solution with NaOCl or H2O2, or oxidation in low-temperature oxygen-plasma ashing system. For both methods based on oxidation in aqueous solution, we show that the determination of δ13C and δ18O is improved for concentration of yeast lower than 75wt.% in the “siderite–yeast” mixture but is still shifted to lower values by 4.1 and 0.7‰ for C and O respectively, for a yeast proportion higher than 75wt.%. Moreover, these two methods induce partial siderite dissolution, preventing determination of siderite content in the samples. The best method of yeast removal for coupled C and O isotopes analysis is the oxidation by low-temperature oxygen-plasma ashing, which strongly improves the accuracy of the δ13C and δ18O measurements and perfectly preserves siderite. In conclusion, any study of siderite isotope composition in organic-rich samples should include an evaluation of the need for organic matter removal using low-temperature oxygen-plasma ashing, the only method shown here to be efficient while preserving the siderite.