[B-PT04] Biotic History

[BPT04-P05] Carbonate saturation state in seawater and its effect on coral polyp calcification

*Shunichi Kinoshita¹, Atsushi Suzuki², Iguchi Akira², Mariko Iijima², Yoshikazu Ohno³, Ko Yasumoto³, Hodaka Kawahata⁴ (1.Department of Geology and Paleontology, National Museum of Nature and Science, 2.National Institute of Advanced Industrial Science and Technology (AIST), 3.Kitasato University School of Marine Biosciences, 4.The Faculty of Science and Engineering, Waseda University)

Keywords:Cretaceous Sea Water, ocean acidification, culturing experiment

Many reports suggest that the skeletal growth rate of corals and other calcifying organisms is influenced by the saturation state of seawater with respect to carbonate. During the Cretaceous, the carbon dioxide partial pressure of the atmosphere in the surface seawater is thought to have been greater than 1000 µatm, and the predicted pH was as low as 7.6. On the other hand, total alkalinity is thought to have been about twice as high as it is now, which would have maintained high carbonate saturation state in seawater and allowed marine organisms to calcify. To confirm this hypothesis, we prepared seawater that reproduced the seawater composition of the Cretaceous period with high alkalinity and low pH, and conducted a rearing experiment on early-stage polyps of the reef-building coral Acropora(Experimental condition 1). In addition, an experimental condition with normal alkalinity and low pH to simulate future ocean acidification conditions (Experimental condition 2), an experimental condition with Cretaceous seawater equilibrated under the current level of the partial pressure of atmospheric CO₂ (Experimental condition 3), and control (modern surface seawater) were also established. The carbonate saturation state of the Cretaceous seawater was set at the same level as that of the control area, and after one month of polyp rearing, there were significant differences in the amount of calcification and microfocus X-ray CT observations among experimental conditions. The Cretaceous seawater showed a calcification rate similar to that of the control. The experimental condition 3 showed the largest amount of calcification among the four experimental conditions, while the condition 2 showed the smallest. These preliminary results confirm that ongoing ocean acidification has an inhibitory effect on corals, and that the carbonic acid saturation state of seawater is an important parameter for calcification, with implications for ocean alkalinization that is being considered to mitigate global warming.