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

Linsley, Braddock; Dunbar, Robert B.; Dassié, Emilie P.; Tangri, Neil; Wu, Henry C.; Brenner, Logan; Wellington, Gerard

Reef building corals face an uncertain future due to the accelerating input of anthropogenic CO2 into the surface ocean, ocean acidification and from rising ocean temperatures. Coral skeletal carbon isotope ratios (13C/12) is one potential tool to understand both changes in coral growth processes over time and how corals are responding to the influx of anthropogenic CO2, but interpretation of coral 13C/12C data remains controversial. Here we show for the first time using multi-century long coral cores from a broad region of the South Pacific, that skeletal extension rate and skeletal 13C/12C had a strong inverse correlation that was stable over many centuries. However, our coral results also demonstrate the cessation of this relationship in the mid 20th century as a response to the influx of anthropogenic CO2. This fundamental conundrum suggests some change in the coral calcification process and could be related to coral reef decline in the 20th century.

Our results also highlight the fundamental sensitivity of mean coral 13C/12C to endosymbiotic photosynthetic rate. We quantified the water depth effect on mean coral 13C/12C using different coral genera from the Indo-Pacific and Atlantic Oceans. This novel compilation reveals a large amplitude and significant trend towards lower skeletal 13C/12C as depth increases and light levels decrease in the upper 20m of the water column (1.4‰ decrease per 5m of depth). We discuss how this relationship could be used to improve the accuracy of sea level reconstructions using fossil corals and result in a better understanding of the relationship between past climatic forcing and sea level response.

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