Past ocean acidification recorded in the geological record facilitates the understanding of rates and influences of contemporary pCO(2) enrichment. Most pH reconstructions are made using boron, however there is some uncertainty associated with vital effects and isotopic fractionation. Here we present a new structural proxy for carbonate chemistry; Mg-O bond strength in coralline algae. Coralline algae were incubated in control (380 mu atm pCO(2)), moderate (750 matm pCO(2)), and high (1000 matm pCO(2)) acidification conditions for 24 months. Raman spectroscopy was used to determine skeletal Mg-O bond strength. There was a positive linear relationship between pCO(2) concentration and bond strength mediated by positional disorder in the calcite lattice when accounting for seasonal temperature. The structural preservation of the carbonate chemistry system in coralline algal high-Mg calcite represents an alternative approach to reconstructing marine carbonate chemistry. Significantly, it also provides an important mechanism for reconstructing historic atmospheric CO2 concentrations.