Baseline Assessment of Carbonate System Parameters and Trace Metals in the North Sea: Implications for Monitoring Ocean Alkalinity Enhancement

Ocean alkalinity enhancement (OAE) is a recent focus in marine carbon dioxide removal (mCDR). The North Sea, a well-studied shelf sea, provides an ideal setting to investigate both the potential benefits and risks of OAE. During the summers of 2024 and 2025, we collected water samples from costal and offshore regions of the North Sea to measure two key parameters of the carbonate system: total alkalinity (TA) and dissolved inorganic carbon (DIC). We also quantified dissolved trace metal concentrations of nickel, vanadium, manganese, cobalt, cadmium and rare earth elements, as OAE interventions can introduce these in significant amounts.  

This study aims to establish a baseline for these parameters in the North Sea and to investigate the diverse sources and sinks of TA, DIC, and trace metals. We also explore tracer-based approaches to enable monitoring of chemical impacts on the coastal environment associated with artificial OAE.

Preliminary depth profile data for TA and DIC indicate similar behavior in the southern North Sea in both years, with concentrations remaining relatively constant throughout the water column. At depth, the variation for TA is around 10 µmol/kg difference between the surface and the bottom. This difference is only slightly higher for the DIC measurements, averaging 20 µmol/kg. In the southern North Sea, the average water depth is about 40 m, and the variability between measuring stations is lower for TA than for DIC. For TA, the range of the measured values is between 2200 µmol/kg and 2400 µmol/kg, while for DIC, values between 1800 µmol/kg and 2300 µmol/kg were measured. In contrast, profiles from the Norwegian Trench with sampling depths up to 513 m show that DIC concentrations are lowest at the surface, averaging 2000 µmol/kg, and increase to an average of 2200 µmol/kg at a depth of 100 m, then remain stable to the seafloor, reflecting the production of organic matter at the surface and subsequent remineralization at depth. Data from both years suggest that TA is less variable than DIC, as it is less influenced by biological processes. This stability highlights TA’s potential as a robust monitoring parameter in the context of OAE. Furthermore, depth-profile data from summer 2025 indicate that most of the trace metals analyzed exhibit higher concentrations near the surface. Rare earth elements have low conentreations, dysprosium for example has a concentration of 1.55 ng/L at the surface and decrease to 0.98 ng/L at the seafloor. Nickel, gadolinium, and dysprosium in particular have higher concentrations in coastal area of the North Sea with low salinity, which is due to river inputs and anthropogenic influences. Near the Baltic Sea, concentrations reach a maximum of 650 ng/l for nickel, 4.2 ng/l for gadolinium, and 2.8 ng/l for dysprosium. These observations underscore the importance of understanding spatial variability in both carbonate system parameters and trace metals when evaluating OAE impacts.

The poster will present spatial patterns of TA, DIC, and trace metal concentrations across the North Sea, discuss potential tracer approaches for OAE monitoring, and highlight implications for future mCDR strategies.