Coral carbon isotope sensitivity to growth rate and water depth with paleo-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 5 m 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.