Multidecadal-to-interannual modulations of Late Holocene climate variations reconstructed from oxygen isotope ratios of a 237-year-long lived fossil coral in Kikai Island, southwestern Japan

Abstract

For this study, to elucidate the East Asian monsoon variations on seasonal, interannual, and decadal timescales during the Late Holocene, we generated a 237-year-long time series of bimonthly resolved oxygen isotope ratios (δ18O) from a 3.5–3.2 ka Porites coral in Kikai Island, southwestern Japan. The coral data clearly show seasonal-to-interdecadal variations reflecting seawater temperature and salinity variations for 237 years, which can be a robust benchmark with respect to subtropical Pacific climate variability during the Late Holocene. Along with modern and fossil coral records published earlier from Kikai Island and Okinawa Island, the time-weighted δ18O averages for overlapped 14C ages demonstrate that the annual and summer values had slightly increased by <0.2‰ for 6–3 ka and that the winter values had increased by ∼0.2‰ for 6–5 ka and decreased by ∼0.2‰ for 5–3 ka. The result indicates that the mean climate state around the island had been almost stable on a centennial-to-millennium timescale throughout the transition from the Middle to Late Holocene, but with several short-term cooling excursions of ∼1 °C at 4–3 ka. On interdecadal-to-interannual timescales, amplitudes of the fossil coral δ18O variations for winter and summer were similar or larger at 3.5–3.2 ka relative to today, revealing significant periodicities at 36–38 years, 20–21 years, 6–9 years, and 3–4 years by spectral analysis at 95% confidence intervals. Furthermore, winter and summer coral δ18O time series of the respective periodic components indicate that interannual variations of SST (and/or seawater δ18O) around the island have changed on interdecadal and decadal timescales, which have been modulated on a multidecadal timescale of approximately 60–90 years/cycle. These results imply that the relationships of the East Asian winter and summer monsoon to the El Niño/Southern Oscillation and Pacific Decadal Oscillation dominant in the North Pacific might have been modulated by or associated with the Atlantic Multidecadal Oscillation dominant in the North Atlantic during the Late Holocene.