Coralline algal Mg-O bond strength as a marine pCO2 proxy

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 matm 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 CO 2 concentrations.