[U-02] Advanced understanding of Quaternary and Anthropocene hydroclimate changes in East Asia

[U02-P03] High-resolution Δ¹⁴C, δ¹⁸O and δ¹³C Variability Recorded in Coral from Okinoerabu Island

*Shunsuke Imamiya^1,2, Ryuji Asami³, Takahiro Aze¹, Yosuke Miyairi¹, Shoko Hirabayashi¹, Yusuke Yokoyama^1,2,4 (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.Institute of Geology and Paleontology, Graduate School of Science, Tohoku University, 4.Graduate Program on Environmental Sciences, Graduate School of Arts and Sciences, The University of Tokyo)

Keywords:Kuroshio Current, Ryukyu Current, Δ14C spikes, Oxygen isotope ratio, Corals

The Kuroshio Current is a powerful warm current that flows northward in the northwest Pacific Ocean. The Ryukyu Current branches off from the Kuroshio near Taiwan and flows northward along the eastern side of the Ryukyu Islands. These currents play a crucial role in heat transport from low to mid-latitude regions and are considered to have a significant impact on climate variability in East Asia. Observations of the Kuroshio region began in the 1950s, and the physical characteristics of the currents have been studied. However, observation data are discontinuous, and there is a lack of information on climate variability over timescales ranging from several years to decades. Therefore, to understand long-term atmospheric and oceanic variability, it is necessary to reconstruct ocean current characteristics based on continuous and long-term data.

One of the proxies used for reconstructing ocean circulation is the radiocarbon isotope (¹⁴C). Radiocarbon is produced in the upper atmosphere and is introduced into the ocean surface through air-sea CO₂ exchange. Since a ¹⁴C concentration gradient exists between surface and deep waters, ¹⁴C serves as a key tracer for understanding the spatial distribution of water masses. Corals living in the ocean incorporate dissolved inorganic carbon containing ¹⁴C into their calcium carbonate skeletons during growth. Therefore, the ¹⁴C concentration (Δ¹⁴C) in coral skeletons can be used as a long-term indicator of water mass distribution. In particular, radioactive fallout generated by atmospheric nuclear tests conducted in the central Pacific in the 1950s created a distinctive peak in oceanic Δ¹⁴C, known as the Δ¹⁴C spikes. Δ¹⁴C spikes vary in terms of arrival timing, number of peaks, height, and shape between the upstream and downstream regions of the Kuroshio, making it a characteristic indicator of water mass movement and mixing. Δ¹⁴C spikes have been reported in corals from Guam (Andrews et al., 2016), Ishigaki Island (Hirabayashi et al., 2017), and Kikai Island (Zeng et al., 2024). While three peaks were observed in corals from Guam and Ishigaki Island, only one peak was detected in corals from Kikai Island, which is located further downstream. The reduction in the number of peaks is thought to be due to dilution effects during oceanic transport. However, the influence of the Kuroshio Current, the Ryukyu Current, and the Kuroshio Countercurrent (a flow that branches off from the Kuroshio and moves southward or westward) on coral Δ¹⁴C data in this region remains unclear.

Here, we measured Δ¹⁴C of a coral from Okinoerabu Island (27.3°N), located between Ishigaki (24.3°N) and Kikai (28.3°N) Islands, to reconstruct oceanic transport in the Kuroshio region with higher precision. Additionally, other proxies, such as oxygen isotope ratios (δ¹⁸O) and carbon isotope ratios (δ¹³C), are often used to obtain oceanographic information. δ¹⁸O, when used in conjunction with sea surface temperature data, allows for the reconstruction of salinity variations. δ¹³C is influenced by the photosynthetic activity of symbiotic zooxanthellae within corals and is therefore considered to reflect changes in solar radiation to some extent. This study aims to reconstruct ocean circulation from a broader perspective by incorporating δ¹⁸O and δ¹³C along with Δ¹⁴C. In the presentation, we will discuss these measurement results by comparing them with coral data from other regions reported in previous studies.

References

Andrews et al. (2016). Journal of Geophysical Research: Oceans, 121(8), 6351-6366.

Hirabayashi et al. (2017). Geochemistry, Geophysics, Geosystems, 18(4), 1608-1617.

Zeng et al.(2024). Global Biogeochemical Cycles, 38(4), e2023GB007927.