[M-IS14] Paleoclimatology and paleoceanography

[MIS14-P18] East Asian monsoon fluctuations with high temporal resolution recorded in coral skeletons in Kikaijima Island, Japan

Ayuka Kuribara¹, Tsuyoshi Watanabe^3,2, *Atsuko Yamazaki^4,5 (1.Graduate School of Science, Hokkaido University, 2.Research Institute for Humanity and Nature, 3.Faculty of Science, Hokkaido University, 4.Graduate School of Environmental Studies, Nagoya University, 5.KIKAI Institute for Coral Reef Sciences)

Keywords:Coral skeleton, Oxygen isotope, East Asian Monsoon

The climate of the East China Sea coastal region is strongly influenced by the East Asian Monsoon (EAM). In this study, we use geochemical ploxies of coral skeletons to reconstruct East Asian monsoon variability in high temporal resolution and clarify how the intensity of the EAM changes in response to recent global climate change. Kikaijima Island, located in the center of the Ryukyu Archipelago and at the eastern edge against the East China Sea, experiences a distinct change in wind direction between summer and winter due to the EAM, resulting in the baiu and autumn rains. This precipitation pattern follows the variability of the East Asian Summer Monsoon (EASM). This study uses oxygen isotope ratios of reef coral skeletons to capture changes in the EASM and the East Asian Winter Monsoon (EAWM). Coral oxygen isotope ratios (δ¹⁸O_coral) in reef coral skeletons are affected by both seawater temperature and seawater oxygen isotope ratios (δ¹⁸O_sw). On Kikaijima Island, high water temperature and wet environment in summer and low water temperature and dry environment in winter are expected to be recorded by the intensity of EAM. We used a modern Porites coral cores (1927-2015) and fossil Porites coral core (1798-1931) drilled at the Shiomichi Bay and modern Porites coral cores (169 annual rings, including growth arrest surfaces) collected in the Tekutsuku area, facing the open ocean off. The core samples were formed into slabs, and soft X-ray images were taken to observe the annual bands. Micro-powder samples were taken along the maximum growth axis of the annual bands. at intervals of 1.2 mm (about 12 points/year) and at intervals of 0.2 mm (about 50 points/year) for the Tekutsuku core. δ¹⁸O_coral were measured with stable isotope mass spectrometer connected to a carbonate pretreatment device (Kiel4-253plus from Thermo fisher scientific).
The δ¹⁸O_coral of all coral skeletal samples showed a clear annual cycle. Coral skeletal samples from Shiomichi and Tekutsuku showed δ¹⁸O_coral variation over 217 years (1798-2015) and 43 years (1980-2023). Using observed water temperature (SST) at each site and the relationship between surface salinity (SSS) and δ¹⁸O_sw in the East China Sea (Oba, 1988), we found that 41% of the seasonal variation in Shiomichi δ¹⁸O_coral depends on SSS, and 59% can be explained by SST. Similarly, 17% of the seasonal variation in δ¹⁸O_coral at Tekutsuku depends on SSS, and SST can explain 83% of the variation. Thus, it is likely that precipitation and/or terrestrial water effects have a significant influence on the δ¹⁸O_coral in the Shiomichi, inner bay environment. Winter δ¹⁸O_coral in the open ocean environment of Tekutsuku was significantly correlated with SST along the coast of Japan and in the northwestern Pacific. Correlation maps of winter δ¹⁸O_coral and North Pacific SSTs separated by time windows showed that winter SSTs at Tekutsuku during 1980-1989 were positively correlated with SSTs in the Indian Ocean to the western Pacific around Japan, and winter SSTs during 2005-2013 were negatively correlated with the central to eastern Pacific. This suggests that the intensity of the winter monsoon in Kikaijima varies with the meandering of westerly winds and the configuration of atmospheric pressure.