[U-05] Advanced understanding of Quaternary and Anthropocene hydroclimate changes in East Asia

[U05-P05] Skeletal Ba/Ca in a long living Porites coral in Kikai Island, Ryukyu, Japan reveals changes in carbonate chemistry due to recent ocean acidification

*KAWAKUBO YUTA¹, Yusuke Yokoyama^1,2,3,4,5, Shoko Hirabayashi¹, Atsushi Suzuki⁶, Yosuke Miyairi¹ (1.Atmosphere and Ocean Research Institute, The University of Tokyo, 2.Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 3.Graduate Program on Environmental Sciences, Graduate School of Arts and Sciences, The University of Tokyo, 4.Biogeochemistry Program, Japan Agency for Marine-Earth Science and Technology, 5.Research School of Physics, The Australian National University, 6.Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST))

Keywords:Coral, LA-SF-ICPMS, Ocean Acidification

Reef building coral skeletons are widely used for the reconstruction of past Sea Surface Temperature (SST) as well as carbonate chemistry using the various coral proxies such as Sr/Ca, Mg/Ca, U/Ca and B/Ca. Among those proxies, coral Ba/Ca is known as an indicator for ocean upwelling or river runoff. Most samples used in previous studies have been collected from coastal areas where high Ba input is observed mainly due to large riverine input. In this setting, the Ba/Ca signal is dominantly controlled by the frequency and magnitude of flooding of nearby rivers. In studies using laboratory cultured corals, tracking statistics of long-term treatments in ambient environmental backgrounds are often difficult due to relatively short growth time and limited data sets. Here, we examine elemental ratios (Sr/Ca, Mg/Ca, U/Ca, B/Ca, Ba/Ca, B/Mg, Ba/Mg) of Porites coral from Kikai Island, where there is negligible river runoff and upwelling, to evaluate each Me/Ca as an environmental proxy in a natural setting using laser ablation ICPMS technique. The principal component analysis shows that Sr/Ca, Mg/Ca, U/Ca, B/Mg and Ba/Mg predominantly reflect seasonal SST variations. It is also demonstrated that Ba/Ca and B/Ca experienced a gradual increase from the 1960s, which cannot be explained by SST. Instead, correlation between Ba/Ca and B/Ca as well as δ¹¹B suggest that Ba/Ca is reflecting the carbonate chemistry in calcifying fluid, indicating a decline in pH from 1960s. This new insight suggests that the coral Ba/Ca has the potential to detect changes in carbonate chemistry induced by ocean acidification.