A cross-polarized light scattering sensor for measuring the concentration of particulate inorganic carbon in seawater: laboratory validation

Observations of particulate carbon from the surface to depth are essential for understanding the marine carbon cycle. Compared to traditional, costly, and laborious biogeochemical sampling, low-power optical sensors can enable high-frequency measurements and substantially expand the spatial and temporal coverage of biogeochemical observations. Here, we present two autonomous optical sensor prototypes designed to measure Particulate Inorganic Carbon (PIC) concentrations in seawater. Both exploit the birefringence of calcium carbonate (CaCO 3 ), the primary constituent of PIC, by detecting the cross-polarized near-forward light scattering coefficient, Pol X (m −1 ), using either linear polarizers (LP) or circular polarizers (CP). Laboratory experiments confirmed that both designs were sensitive to variations in PIC concentration from cultured coccolithophores, a major calcifying plankton group, over an oceanic concentration range spanning more than three orders of magnitude (0.1-400 μg C L −1 ). A positive relationship was observed between PIC concentration and the optical signal, with significant responses detected down to 0.1 μg C L −1 for all three species investigated. Differences in the PIC-specific cross-polarized near-forward light scattering coefficient Pol m X , (m 2 g −1 ) were observed among the three species, ranging from 19 to 555 m 2 g −1 using CP and from 12 to 271 m 2 g −1 using LP, reflecting variations in coccolith morphology. These results reveal species-specific differences in the optical properties of coccolithophores and help establish the conceptual framework and performance requirements for future in situ autonomous measurements of marine particulate inorganic carbon.