Iron is an essential micronutrient regulating marine primary productivity and the global carbon cycle. Despite extensive observational and modeling studies, the environmental controls on iron speciation and complexation remain poorly represented in Earth system models, limiting projections of future marine iron cycling.
Here we develop a novel parameterization describing the environmental dependence of iron complexation based on the NICA–Donnan model. Simulated titration experiments are conducted using observationally constrained seawater chemistry to quantify the stability constants of two representative ligand classes under varying environmental conditions. The resulting parameterizations explicitly relate iron complexation strength to seawater pH, temperature, dissolved organic carbon, and dissolved iron concentrations.
The parameterization is evaluated against GEOTRACES observations and demonstrates robust agreement across diverse oceanic environments, indicating that it reliably captures the variability of iron complexation under present-day conditions. By coupling this parameterization with multiple CMIP6 Earth system models, we investigate future changes in marine iron complexation under ocean warming and acidification scenarios. Our results indicate that future ocean conditions generally enhance ligand complexation strength and increase the capacity of seawater to maintain dissolved iron, suggesting an increase in iron solubility under climate change.
This framework provides a computationally efficient approach for representing environmentally responsive iron complexation in global ocean biogeochemical models and offers new insights into the future evolution of marine iron cycling.

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