[B-BG21_1AM1] Material cycles in tropical- subtropical coastal ecosystems

[BBG21-01] Ocean Acidification and its effect on calcification since the late 19th century revealed by d11B of Ogasawara coral

*Kaoru KUBOTA¹, Yusuke YOKOYAMA¹, Tsuyoshi ISHIKAWA², Atsushi SUZUKI³ (1.Atmosphere and Ocean Research Institute, University of Tokyo, 2.Japan Agency for Marine-Earth Science and Technology, 3.National Institute of Advanced Industrial Science and Technology)

Keywords:boron, Ogasawara, coral, calcification, Ocean Acidification

Boron isotopes (d¹¹B) of coral skeleton are known as a pH meter in the seawater. As pH of seawater is closely related to partial pressure of CO₂ (pCO₂) in the atmosphere, it is expected that d¹¹B becomes pCO₂ indicator in the geological past too. However, d¹¹B-pH is under scrutinized since coral calcification itself probably affects the relationship. Although many studies have focused on d¹¹B measurements for cultured corals under pH-controlled aquarium, those for living corals outdoors have rarely measured, which are limited to, for example, Great Barrier Reef and Guam. Here we show 125 years-records (AD1873-1998) of d¹¹B and boron concentration (B/Ca ratio) for long-lived massive coral (Porites sp.) that was sampled at Chichi-jima, Ogasawara Islands, North West Pacific. They clearly reveal Ocean Acidification after the industrial revolution. We will discuss a relationship between ocean acidification and coral calcification from a slope of pH decline that is obtained from observational data. We will also discuss how B/Ca of calcium carbonate skeleton that is produced by marine calcifiers is reliable proxy for seawater pH, which is being paid a great attention mainly due to relative easiness to measure compared to isotopes.