Abstract
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.
Highlights
Kinetic fractionation results from discrimination against the heavy isotopes of both carbon and oxygen during the hydration and hydroxylation of CO2 at the site of calcification[8,9,14,17,18]
Building on the results of Dassié et al.[13], we examined the relationship between annual extension rates and annually averaged coral skeletal δ13C in multi-century long Porites coral cores from
The atmospheric 13C Suess effect forcing of CO2 into the surface ocean would have been relatively uniform across our South Pacific study region, the removal of the 13C Suess effect from the coral δ13C
Summary
Kinetic fractionation results from discrimination against the heavy isotopes of both carbon and oxygen during the hydration and hydroxylation of CO2 at the site of calcification[8,9,14,17,18]. Photosynthesis by endosymbiotic zooxanthellae affects coral skeletal δ13C by preferentially consuming 12CO2, resulting in 13C enrichment in the DIC in the internal calcification pool[9,14]. The rate of respiration decreases, the internal carbon pool used by the coral animal to make its skeleton becomes relatively enriched in 13C resulting in higher skeletal δ13C8,11,12,18. Many studies have interpreted 20th century trends towards lower coral skeletal δ13C to be the result of the 13C Suess effect in the surface ocean[9,10,13,31,32,33,34,35,36,37]. Of the 37 corals included in that study, 23 (62%) had a statistically significant trend toward lower δ13C from 1900 to the present leading to the conclusion that this trend was related to the 13C Suess effect.
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