Reconstruction of Kuroshio Variability Based on High-Resolution Carbon and Oxygen Isotope Analyses Recorded in Coral from Okinoerabu Island

Abstract

The Kuroshio Current flows northward along the western side of the Ryukyu Islands, and the Ryukyu Current flows northward along the eastern side. The amount of continuous and long-term data on these currents is limited. To understand current variability in this region, it is necessary to utilize a variety of proxy data. In this study, we conducted high-resolution analyses of D¹⁴C, Sr/Ca ratios, d¹⁸O, and d¹³C in coral skeletons from Okinoerabu Island to investigate the long-term variability of the Kuroshio-Ryukyu Current system. The D¹⁴C time series from Okinoerabu revealed a sharp and short (less than one year) spike originating from nuclear weapons testing. The delayed arrival of this spike compared to that at Kikai Island, located farther north, suggests that the signal may have been transported via the Kuroshio Countercurrent. Additionally, a notable decrease in D¹⁴C between 1961 and 1963 was observed, which may reflect the influence of localized vertical mixing or basin-scale variability.("D" refers to uppercase delta, while "d" refers to lowercase delta.)