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

Although reef coral skeletal carbon isotopes (δC) are routinely measured, interpretation remains controversial. Here we show results of a consistent inverse relationship between coral δC 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 C Suess effect shifts skeletal δC > 1.0‰ lower. We also compiled coral δC from a global array of sites and find that mean coral δC 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 δC to endosymbiotic photosynthesis. Collectively, these results suggest that photosynthetic rate largely determines mean coral δC while changes in extension rate and metabolic effects over time modulate skeletal δC around this mean value. The newly quantified coral δC-water depth relationship may be an effective tool for improving the precision of paleo-sea level reconstruction using corals.