Conference Agenda

The carbonate pump, an integral component of the biological carbon pump, plays a pivotal role in regulating the global carbon cycle by facilitating the production, sinking, and sequestration of particulate inorganic carbon (PIC), while also modulating the export and transfer of particulate organic carbon (POC). However, progress in understanding these complex dynamics remains limited by the scarcity of PIC observations from surface to depth.

Here we present two autonomous optical sensor prototypes designed to measure PIC concentrations in seawater. Both exploit the birefringence of calcium carbonate, the primary constituent of PIC, by detecting near-forward depolarized scattering with either linear or circular polarizers. Laboratory experiments confirmed that both designs were sensitive to variations in the concentration of PIC derived from cultured coccolithophores, a major calcifying plankton group, across an oceanic concentration range spanning more than three orders of magnitude. The linear prototype demonstrated higher sensitivity, while the circular prototype yielded stronger optical signals and improved alignment stability. and the circular prototype offering greater mechanical stability. We also observed differences in mass-specific depolarization between species, reflecting variations in coccolith morphology.

PIC sensor prototypes were deployed on several research cruises, operating in underway flow-through mode at a sampling rate of 1 Hz. Sensor signals correlated very well with PIC concentration obtained from discrete water samples, demonstrating the capability for autonomous, high-resolution sensing of PIC in surface waters. These sensors provide much-needed in situ observations to improve satellite-based PIC retrieval algorithms and to advance our understanding of the biological processes driving the carbonate pump.

Griet Neukermans^1,2, Clémence Goyens¹, Alexandre Castagna¹, Qiming Sun¹, Andrea van Langen Roson^1,2, Nils Haentjes³, Emmanuel Boss³, Thanos Gkritzalis², and Peter Landschützer².

¹ Marine Optics and Remote Sensing Group (MarSens), Ghent University, Ghent, Belgium

² Flanders Marine Institute (VLIZ), Ostend, Belgium

³ School of Marine Sciences, University of Maine, Maine, USA

Integrating measurements of carbonate chemistry, marine reflectance, and bio-optical properties is essential to capture and understand the coupled physical–biogeochemical processes driving CO₂ dynamics and to link in situ observations with satellite remote sensing. This is particularly so in coastal shelf seas, comprising optically-complex waters with strong spatial and temporal variations in biological activity and carbonate chemistry.

We build on the observational capacity of RV Simon Stevin, a Flemish ICOS (Integrated Carbon Observation System) Ocean Station operating in the North Sea, equipped with state-of-the-art sensors for continuous measurement of carbonate system parameters on pumped surface water, including the partial pressure of CO₂ (pCO₂). We expanded the vessel’s underway system with a flow-through Autonomous uNderway near-real-Time HYperspectraL Optical Properties PackagE (ANTHYLOPE), measuring hyperspectral backscattering (Sequoia hyperBB), attenuation, and absorption (Seabird AC-S), single wavelenght backscattering, fluorescence of CDOM and Chlorophyll-a (RBR Tridente), UV fluorometry (Seapoint SUVF), particle size distribution (Sequoia LISST-200X), red light attenuation (LISST-Tau) and Particulate Inorganic Carbon (prototype optical sensor, developed in collaboration with Sequoia Scientific), complemented by a thermosalinograph (Seabird TSG). Lastly, an autonomous hyperspectral radiometry system for the measurement of above-water reflectance (R_rs), (IMO DALEC) was mounted on a pole on the bow of the vessel.

Our integrated monitoring system was first put in operation in May 2024 and has been tested and improved during several measurement campaigns. Here, we present the data processing and quality control pipelines and discuss the challenges associated with the operation of the ANTHYLOPE and DALEC systems. We present preliminary results on the mulitple uses of the integrated dataset. First, we show that the characteristics of the particle assemblage (particle concentration, composition, and size) can be retrieved from inherent optical properties. Next, we show the improved retrieval of biogeochemical variables by leveraging the hyperspectral nature of the signals. We also test and (re-) calibrate commonly used remote sensing algorithms for the retrieval of SPM, POC, and Chlorphyll-a from R_rs. Lastly, we investigate the spatio-temporal dynamics of pCO₂ and examine its physical, chemical, and biological drivers.