Coral carbon isotope sensitivity to growth rate and water depth with paleo-sea level implications

Although reef coral skeletal carbon isotopes (δ(13)C) are routinely measured, interpretation remains controversial. Here we show results of a consistent inverse relationship between coral δ(13)C and skeletal extension rate over the last several centuries in Porites corals at Fiji, Tonga, Rarotonga and American Samoa in the southwest Pacific. Beginning in the 1950s, this relationship breaks down as the atmospheric (13)C Suess effect shifts skeletal δ(13)C > 1.0‰ lower. We also compiled coral δ(13)C from a global array of sites and find that mean coral δ(13)C decreases by −1.4‰ for every 5 m increase in water depth (R = 0.68, p < 0.01). This highlights the fundamental sensitivity of coral δ(13)C to endosymbiotic photosynthesis. Collectively, these results suggest that photosynthetic rate largely determines mean coral δ(13)C while changes in extension rate and metabolic effects over time modulate skeletal δ(13)C around this mean value. The newly quantified coral δ(13)C-water depth relationship may be an effective tool for improving the precision of paleo-sea level reconstruction using corals.