Precise determination of dolomite content in marine sediments

Dolomite (CaMg[CO₃]₂) is a common rock-forming mineral. Nevertheless, its mechanisms of formation and the factors that cause dolomite concentration variations within the sedimentary records constitute long-standing geochemical questions. In addition, the flux of Mg²⁺ leaving the ocean by the formation of dolomite is a controversial question, with some studies arguing that dolomite formation is a negligible Mg²⁺ sink in the modern ocean, while others show that it constitutes more than 50% of the total Mg²⁺ removal rate. An important factor that impedes the resolution of the dolomite Mg²⁺ flux is the lack of analytical methods with adequate precision and detection limit to directly measure minute quantities of authigenic dolomite in marine sediments. Here, we present a new analytical method for direct, precise measurement of dolomite content in marine sediments. The method is based on sequential leaching of carbonate minerals in acid and tracks the CO₂ emitted by the dissolution. Based on the measurement of gravimetric standards of calcite and dolomite, the method's detection limit and precision were determined as better than 0.2 and ± 0.2 dry wt% of dolomite, respectively. The method out-performed dolomite quantification made by x-ray diffraction and by inductive coupled plasma mass-spectrometry, which provided precision of ± 2 and ± 1 dry wt%, respectively. Measurements of the dolomite content in modern sediments from the seafloor below the oligotrophic Eastern Mediterranean and the eutrophic Mississippi plume, and in clayey-silty alluvial soil from south-eastern Israel, demonstrated that the aforementioned precisions are also valid for natural samples.