Using a new and rapid NanoSIMS-based method, we quantified the sensitivity of skeletal Sr/Ca in coral to the aragonite saturation state of seawater (Ω_(SW)). Skeletal Sr/Ca is a common proxy for temperature while Ω_(SW) is a parameter that varied in the past ocean and is predicted to change with continued ocean acidification. Five adult branches of the surface coral Stylophora pistillata were grown at different Ω_(SW) from 2.7 to 4.9 (pH of 7.9–8.5) but at a constant temperature of 25 °C. Despite a large range of growth parameters and a twofold range in calcification rates, the average skeletal Sr/Ca of coral exposed to each condition are within 1.2% of each other (2σ std. dev. of the 5 means). Furthermore, the average skeletal Sr/Ca measured in this study agrees with the results of two previous coral culture experiments conducted at the same temperature but where Ω_(SW) was not controlled. These results suggest that aragonite saturation has little or no influence on Sr/Ca paleothermometry over the range of Ω_(SW) investigated. Combined with existing data for low Ω_(SW) conditions, our results were used to elucidate the mechanisms controlling calcifying fluid acid–base chemistry during coral biomineralization. Assuming that coral drive precipitation through alkalinity pumping, our data suggest that this pumping occurs until the calcifying fluid reaches a target pH. Below a threshold Ω_(SW) bounded by 1 < Ω_(SW) < 2.4, however, coral do not pump enough alkalinity to reach the target pH and instead pump a maximal but finite amount of alkalinity. In this low Ω_(SW) regime, calcifying fluid pH is expected to decrease with Ω_(SW). The interplay between these two alkalinity pumping regimes and external seawater composition explain the full range of observed Sr/Ca sensitivity to Ω_(SW) and suggest that surface coral may become increasingly sensitive to ocean acidification below a threshold Ω_(SW) bounded by 1 < Ω_(SW) < 2.4.