Coral Carbon Isotope Sensitivity to Growth Rate and Water Depth with Palaeo-Sea Level Implications

Although reef coral skeletal carbon isotopes (δ13C) are routinely measured, interpretation
remains controversial. Here we show results of a consistent inverse relationship between
coral δ13C 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 13C Suess effect shifts skeletal δ13C > 1.0‰
lower. We also compiled coral δ13C from a global array of sites and find that mean coral δ13C
decreases by −1.4‰ for every 5m increase in water depth (R = 0.68, p < 0.01). This highlights
the fundamental sensitivity of coral δ13C to endosymbiotic photosynthesis. Collectively,
these results suggest that photosynthetic rate largely determines mean coral δ13C while
changes in extension rate and metabolic effects over time modulate skeletal δ13C around this
mean value. The newly quantified coral δ13C-water depth relationship may be an effective
tool for improving the precision of paleo-sea level reconstruction using corals.