Multidecadal decoupling between coral calcifying fluid and seawater saturation states

Ocean acidification poses a threat to coral skeleton formation via reductions in the saturation state of aragonite (Ω_Ar) in seawater. Given that corals precipitate their skeletons from a calcifying fluid supplied by seawater, reductions in seawater Ω_Ar should, in theory, confound calcification. Here, we reconstruct up to 200 years of coral calcifying fluid Ω_Ar, using Raman spectroscopy techniques, at approximately monthly resolution in two Porites sp. skeletal cores from the Coral Sea region to investigate (i) the regulation of coral calcifying fluid Ω_Ar and (ii) the skeletal calcification response to industrial-era ocean acidification. Our results reveal a significant increase in calcifying fluid Ω_Ar, suggesting that some corals may adjust to the pace of acidification in the wild more effectively than suggested by short-term laboratory studies.