Applicability and pitfalls of calcimetry in low-calcium carbonate sediments

In Earth and Environmental Sciences, pressure calcimetry is probably the most efficient and fast method to determine CaCO3 content in rocks, sediments and soils. However, measurements of low-CaCO3 samples can be less reliable by calcimetry, depending on the instrument. This problematic is particularly relevant for sediments and soils with low content of CaCO3, very common in past and present lakes, soils and hydromorphic systems (e.g., peatbogs, mires, wetlands), where carbonate analysis contribute to understand water table changes, groundwater oscillation, ecology and wind-related processes, among others. In this context, this work presents a simple protocol to obtain accurate CaCO3 determinations through pressure calcimetry in low-carbonate samples (<4%), even though their carbonate content falls below the limit of detection of the instruments. Calibration curves were first established with a CaCO3 standard to calculate the critical value, detection limit and quantification limit, using two different pressure calcimeters. By considering these thresholds, a set of four natural samples with low-CaCO3 content -also analyzed by Loss-on-Ignition and micromorphology- were measured by pressure calcimetry. Since results with 1 g of sample were lower than the detection limit, gradually higher sediment masses were added until obtaining results above the quantification limit. The amount of CaCO3 per gram was thus inferred. Both calcimeters showed comparable results and high consistency with micromorphological observations. Carbonate content calculated by Loss-on-Ignition showed slightly lower values, likely due to the loss of structural water and dehydroxylation of some minerals exposed to high temperature, affecting calculations of both organic and inorganic carbon.