[M-IS01] アジア・モンスーンの進化と変動，新生代寒冷化との関連

[MIS01-02] Precession-band variance missing from East Asian monsoon runoff

★Invited Papers

*Steven C Clemens¹、Ann Holbourn²、Yoshimi Kubota³、Kyung Eun Lee⁴、Zhengyu Liu⁵、Guangshan Chen⁶、Arin Nelson¹、Baylor Fox-Kemper¹ (1.Earth, Environmental and Planetary Sciences, Brown University, Providence, RI USA. 、2.Institute of Geosciences, Christian-Albrechts-University, Kiel, Germany、3.Geology and Paleontology, National Museum of Nature and Science, Tsukuba, Japan、4.Ocean Science, Korea Maritime and Ocean University, Busan, South Korea、5.Atmospheric Sciences, The Ohio State University, Columbus, OH, USA、6.Institute of Earth Environment, Chinese Academy of Sciences, Xi’an, China.)

キーワード：East Asian Monsoon, speleothem oxygen isotopes, seawater oxygen isotopes, East China Sea, Paleoclimate, greenhouse gas

The iconic Yangtze River Valley (YRV) composite speleothem δ¹⁸O record [Cheng et al., 10.1038/nature18591] is often cited as representative of the East Asian Monsoon (EAM) climate. The spectrum of this record is unique among EAM proxies in that it is dominated by 23-kyr (precession-band) variance whereas other EAM proxies are dominated by variance at ~100-kyr (eccentricity-band), 41-kyr (obliquity-band), or heterodynes thereof. These spectral differences lead to fundamentally different interpretations of the underlying climate mechanisms driving the EAM. Dominance of precession-band variance in YRV speleothem δ¹⁸O leads to the interpretation that EAM variability responds dominantly and directly to northern hemisphere summer insolation whereas dominance of spectral variance at the 100-kyr, 41-kyr, and heterodyn bands, (with very little 23-kyr variance) leads to the conclusion that EAM variability is dominated by global ice volume and greenhouse gas forcing; fundamentally different interpretations of how this critical component of our climate system functions.

We address this difference by attempting to replicate the YRV speleothem δ¹⁸O record at offshore Site U1429 in the East Chain Sea (ECS), drilled during IODP Expedition 346 (Tada et al., 10.2204/iodp.proc.346.101.2015). We utilize the same proxy system, δ¹⁸O of CaCO₃, but recorded in surface-dwelling planktonic foraminifera (Foram δ¹⁸O). Foram δ¹⁸O replicates the precession-band and millennial-scale structure in YRV speleothem δ¹⁸O to a very high degree. We next quantitatively remove the effects of local surface temperature and global source-water δ¹⁸O to reconstruct local seawater δ¹⁸O, a record that responds primarily to dilution by local precipitation and runoff. Unlike speleothem δ¹⁸O, dominated almost exclusively by precession-band (23-kyr) variance, local seawater δ¹⁸O is dominated by eccentricity (100-kyr), obliquity (41-kyr), and heterodyne variance, with almost no precession-scale variance. The spectral structure of this record closely matches the spectral structure of global ice volume and greenhouse gasses, indicating that the EAM is more sensitive to high latitude ice sheet and greenhouse gas forcing than to direct insolation forcing.