A rapid birefringence method for measuring suspended CaCO3 concentrations in seawater

The extreme birefringence of calcium carbonate (CaCO3) relative to other major components of marine particulate matter provides a basis for making optical in situ measurements of particulate inorganic carbon (PIC) in seawater. This concept was tested with a benchtop spectrophotometer equipped with a 1- and 10-cm path length sample cell and modified with linear polarizers to measure the birefringence of suspended particles. Sample suspensions containing 3–100% CaCO3 (by weight) were prepared from calcareous marine sediment material and varying amounts of non-birefringent diatomaceous earth. The samples ranged in total suspended material from 0.003 to 249 mg l−1 and PIC from 0.03 to 1820 μmol CaCO3 l−1. A positive relationship was observed between birefringence and PIC, with response falling off as the total particle concentration and the relative abundance of non-CaCO3 particles in the sample increased. Sensitivity increased linearly with optical path length, and absolute detection limits of 0.2–0.4 and 0.04–0.08 μmol CaCO3 l−1, respectively, were determined for path lengths of 1- and 10-cm based on the intrinsic signal noise of the modified spectrophotometer. Conventional (i.e., non-polarized) transmittance measurements were used to correct the birefringence signal for the sensitivity loss due to interference from scattering and absorption. Without further modification, this spectrophotometer-based method can be used (with a 10-cm cell) to quantify PIC in most surface ocean waters—including those influenced by coccolithophore blooms. The spectrophotometer results define performance requirements and design parameters for an in situ instrument capable of operating over the oceanic range of PIC.