[B-PT04] Biomineralization and the Geochemistry of Proxies

[BPT04-07] Coral calcification in temperate region and the role of dissolved inorganic carbon (DIC) and pH in the calcifying fluid

*Kohki Sowa¹, Kentaro Tanaka², Tsuyoshi Watanabe³, Jens Zinke⁴, Keiichi Nomura⁵, Malcolm McCulloch ^6,7 (1.Japan Agency for Marine-Earth Science and Technology, 2.Atmosphere and Ocean Research Institute, The University of Tokyo, 3.Hokkaido University, 4.Freie Universität Berlin , 5.Kushimoto Marine Park Center Ltd., 6.The University of Western Australia, 7.ARC Centre of Excellence for Coral Reef Studies)

Keywords:Temperate, Coral, Calcification, boron isotope, pH, temperature

Coral skeletons, which consist of calcium carbonate, are one of the main agents for maintaining shallow coral reef structures across tropical to temperate regions. Corals precipitate their skeleton from a calcifying fluid (CF) which is located between the coral skeleton and tissue within a micrometer thickness region. Ongoing ocean acidification (OA) in the upper oceans due to absorption of increasing atmospheric CO₂ may affect the chemical composition of coral CF, and hence potentially reduce the coral calcification. Understanding the interaction between the chemical composition of coral CF and the declining pH of seawater in the natural coral-reef settings is crucial. In this context, the temperate regions are of prime interest because the degree of OA in the temperate area is higher than that in the tropical area due to inherently low seawater temperature. However, the carbonate dynamics and interactions between temperate coral CF and changes in ambient seawater is unclear.

Here we report a ~70-year record of changing dissolved inorganic carbon (DIC) and pH of CF in a massive Porites coral growing near its latitudinal habitable limits at Kushimoto, Japan. The annually resolved boron components (δ¹¹B and B/Ca) and Li/Mg, which are the proxies of carbonate chemistry in coral CF and temperature, are reported here. The geochemical data provide constraints on both the carbonate composition of the CF and ambient temperature during calcification. We find that the reconstructed DIC_CF and pH_CF are significantly higher than that of seawater, thereby enhancing the kinetics for precipitation of the corals calcium carbonate skeleton. DIC_CF is negatively correlated to the pH_CF, which is in agreement with the tropical coral. The ΔDIC (DIC_CF-DIC_Seawater) in temperate coral is higher than that in tropical coral at given temperature (24-25^oC) and there is an inverse correlation between seawater temperature and DIC_CF. These findings imply that corals strongly manipulate the chemical composition of their CF in response to changes their ambient environment. To understand the role of the aragonite saturation state (Ω_CF) and temperature for temperate coral calcification, we calculate Ω_CF and calcification rate based on the inorganic chemical reactions of carbonate (Burton and Walter 1987). The comparison between measured and calculated calcification rate indicates that the temperature mainly controls calcification rate, not Ω_CF in the natural setting. These results imply enhanced resilience to OA but the high vulnerability of temperate corals to thermal stress due to recent global warming.


Burton EA, Walter LM (1987) Relative precipitation rates of aragonite and Mg calcite from seawater: Temperature or carbonate ion control? Geology 15(2):111-114