CARBONATE QUANTIFICATION BY DIFFUSE REFLECTANCE INFRARED FOURIER TRANSFORM (DRIFT) SPECTROSCOPY

Carbonates are the most abundant form of carbon in the crust and their mineralogy and abundance provide information about surficial processes. The most abundant carbonate minerals are calcite (CaCO₃) and dolomite (MgCa(CO₃)₂). A method to identify and quantify these carbonates in soil and sediment samples was established using Diffuse Reflectance Infrared Fourier Transform spectroscopy (DRIFTS). DRIFTS provides a rapid, efficient, and non-destructive way to quantitatively analyze carbonates in small samples. A survey of known and commonly used FTIR carbonate bands at 1796 cm^-1, 1430 cm^-1, 876 cm^-1, and 712 cm^-1 found them unusable due to interferences from other mineral phases in samples. As such, the absorbance band at ~2500 cm^-1, which showed no signs of interference, was used. Separate calibrations were required for the two carbonate minerals. Calibration curves were created using known mixtures of carbonate (calcite or dolomite) and carbonate-free, lake bed sediment. The quantification methods were tested on stream bank samples from the Sangamon River in Illinois, and sediment samples from the adjoining Lake Decatur. DRIFTS revealed that the stream bank carbonate was dolomite and the lake sediment carbonate was calcite. DRIFTS results were compared to estimates made through more traditional acidification and headspace analyses using a Thermo Scientific GasBench-Delta V IRMS. The correlation between the two techniques was good with DRIFT estimates being higher for the majority of analyzed samples. The good correlation between the DRIFTS and acidification-headspace estimates indicate DRIFTS to be a reliable method to quantify calcite and dolomite in sediments and soils.