[M-IS18] Paleoclimatology and paleoceanography

[MIS18-P01] Long-term variation of the Western Pacific Warm Pool since the 18th century reconstructed from modern coral geochemistry

*Akane Hosoda¹, Ryuji Asami², Shusaku Sugimoto³, Hideko Takayanagi², Ryuichi Shinjo^4,5, Toru Nakamori², Yasufumi Iryu² (1.Faculty of Science, Tohoku University, 2.Institute of Geology and Paleontology, Graduate School of Science, Tohoku University, 3.Department of Geophysics, Graduate School of Science, Tohoku University, 4.Research Institute for Humanity and Nature, 5.Department of Physics and Earth Science, Faculty of Science, University of Ryukyus )

Keywords:coral, Sr/Ca, temperature, salinity, Western Pacific Warm Pool

Knowledge of climate and ocean variability in the past is crucial for understanding current and future climate change. However, instrumental climate records are limited before the 1950s. Hermatypic corals are an excellent recorder for past ocean environments. The coral skeleton with annual density bands is composed of aragonite and contains several minor/trace elements that can be used to reconstruct past oceanographic conditions. The oxygen isotope composition of corals varies with seawater temperature and oxygen isotope composition. The latter is related to sea surface salinity (SSS). Whereas coral Sr/Ca ratios vary with seawater temperature. Therefore, we can reconstruct independent time series of seawater temperature and oxygen isotope composition by analyzing these components. The Western Pacific Warm Pool is an extensive water mass characterized by high sea surface temperature (SST) and low SSS. It significantly influences ocean-atmosphere interactions, such as the heat transportation to middle and high latitudes and El Niño-Southern Oscillation. In recent years, the warming and freshening of the warm pool have been shown by statistical analyses of oceanographic data, and its area has been expanding since the late 20th century. However, little is known about the long-term trend and variability of the warm pool since the Industrial Revolution.
Here we analyzed Sr/Ca ratios of Porites corals collected at Guam and Saipan Islands, located around the northern edge of the warm pool. In addition, by coupling oxygen isotope records of the corals, we generated annually-resolved time series of seawater temperature and oxygen isotope composition for the last three centuries. Results show that the SST in Guam and Saipan has increased since the late 19th century, and their warming rates for the late 20th century are higher than those for the early 20th century. The oxygen isotope composition of seawater in Guam shows a long-term freshening trend for the last century. Compared with previously published coral records, our data indicated that the surface area of the warm pool has been expanding toward north and south for the last century.