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Determination Of Calcite And Dolomite Content In Soils And Paleosols By Continuous Coulometric Titration

“Determination of Calcite and Dolomite Content in Soils
and Paleosols by Continuous Coulometric Titration” Soil Science Society of America Journal, 1100-1006

Daniel R. Hirmas, Brian F. Platt, Stephen T. Hasiotis

Quantitative determination of calcite/dolomite ratios in soil carbonate fractions is important for understanding the pedogenic history of a soil or paleosol. This is particularly relevant to stable carbonate-carbon and-oxygen isotope analyses where the presence of dolomite in a carbonate fraction can lead to erroneous results and inaccurate interpretations of the paleoenvironment. The goal of this work was to develop and test an accurate method using coulometric titration combined with differential kinetic principles for determining calcite and dolomite fractions in a sample. An automated titrator was connected to a temperature-controlled carbonate reactor where 2 M HCIO4 was introduced to the sample and mixed by bubbling CO2 – free air at a constant rate of 100 mL min-1. Samples were ground to <53 µm to control for differences in particles size and reactive surface area. The concentration of C released from the reaction was monitored and recorded every 6 s for the length of the reaction. Data were fit with both a pseudo-first order kinetic model and Weibull model for comparison. The latter outperformed the pseudo-first order kinetic model and the Weibull parameter, λ,fit to titration data from unknown samples was compared to λ values fit to data from mixtures of known fractions of pure calcite and dolomite to quantitatively obtain the content of the two minerals. Dolomite fractions obtained by this method qualitatively compared well with values obtained X-ray diffraction. Our method allows a precise and accurate measurement for both total carbonate and the ratio of calcite to dolomite in a sample.

Abbreviations:cal, calcite; CT, total dissolved inorganioc carbon; DMSO, dimethyl sulfoxide; dol, dolomite; DTA, differential thermal analysis; ETA, ethanolamine; MS residuals, mean square of the residuals; PTFE, polytetrafluoroethylene; RMSE, root mean square error; SIC, soil inorganic carbon; XRD, X-ray diffraction.

The ability to accurately determine the fraction of calcite and dolomite composing the total carbonate in a sample is important in investigations of soil genesis, determination of mineral weathering and formation rates, reconstruction of paleoenvironments, and identifications of sediment source area (Levine et al., 1989; Long et al., 1997; Capo et al., 2000; Muhs et al., 2001; Bustillo and Alonso-Zarza, 2007). This ability has aided studies in discriminating pedogenic pathways of soil inorganic carbon (SIC) accumulation from those where soil carbonates have been inherited from lithogenic sources or precipitated as a result of groundwater processes and is, thus, important for accurately inventorying true SIC sequestration (Bellanca and Neri, 1993; Wang and Anderson, 2000). In addition, investigations that use stable isotopes of paleosol carbonates – such as paleoclimatic and paleohydrologic reconstructions – depend on an accurate determination of the calcite and dolomite content in a sample.

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