The carbonate paleoredox proxy, I / Ca, has shown its potential to quantify the redox change in the past ocean, which is of broad importance for understanding climate change and evolution. Here, we tuned and optimized the marine iodine cycling embedded in an Earth system model, “cGENIE”, against modern ocean observations and then tested its ability to estimate I / Ca in the Cretaceous ocean. Our study implies cGENIE’s potential to quantify redox change in the past using the I / Ca proxy.