[M-IS03] Evolution and variability of the Asian Monsoon and Indo-Pacific climate during the Cenozoic Era

[MIS03-08] Constraining the recording depth of TEX₈₆ at the edge of the Indo-Pacific Warm Pool using multispecies Mg/Ca records spanning the last glacial cycle

*Akshat Gopalakrishnan¹、Liang-Jian Shiau^2,3、Min-Te Chen²、Kuo-Fang Huang⁴、Ai-Lin Chen^1,4、Masanobu Yamamoto⁵、Sze Ling Ho¹ (1.Institute of Oceanography, National Taiwan University, Taipei, Taiwan、2.Institute of Earth Sciences, National Taiwan Ocean University, Keelung, Taiwan、3.Exploration and Development Research Institute, CPC Corporation Taiwan, Miaoli, Taiwan、4.Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan、5.Faculty of Environmental Earth Science, Hokkaido University, Sapporo, Japan)

キーワード：IPWP, LGM, Multiproxy, Mg/Ca, Foraminifera, TEX86

The Indo-Pacific Warm Pool (IPWP) holds the warmest surface ocean waters on earth and is a major source of heat and moisture to the global climate. Thus, past changes in the upper ocean temperature here are of high interest to the palaeoceanographic community. In this regard, one of the highly debated issues is the diverging temperature reconstructions during the Pliocene based on TEX₈₆ and Mg/Ca of foraminifera. This discrepancy has been attributed to secular changes in seawater Mg/Ca or TEX₈₆ recording a combined surface and subsurface signal. To further shed light on the Mg/Ca-TEX₈₆ discrepancy in the IPWP and depth origin of TEX₈₆ signal, we reconstruct surface and subsurface temperatures at the edge of the IPWP using U^K’₃₇, TEX₈₆, and multi-species foraminiferal Mg/Ca. We focus on the past 25 kyr, which allows us to rule out the effect of long term variations in seawater composition as a driver of the proxy discrepancy. Our multiproxy surface and subsurface temperatures dataset shows cooler LGM and warmer Holocene. U^K’₃₇ and surface-dwelling G. ruber temperatures agree well, exhibiting ~2°C warming in the surface ocean since the LGM. G. ruber displays peak warming during Early Holocene followed by cooling towards Late Holocene, whereas U^K’₃₇ approaches the saturation limit and thus does not exhibit the same trend. The Holocene cooling trend for G. ruber at our location appears to be linked to the austral winter insolation, which differs from the trends observed at other sites in the warm pool. When interpreted as sea surface temperature (SST), commonly applied SST calibrations yield TEX₈₆ temperatures that were considerably higher than modern SST and temperatures derived from U^K’₃₇ and G. ruber Mg/Ca at our site. Relatively high GDGT 2/3 ratios (7-12) throughout the record suggests that the TEX₈₆ likely registers subsurface temperature signal at our site. Indeed, when interpreted as subsurface temperature using three different calibrations, TEX₈₆ record shows a good agreement with the Mg/Ca records of thermocline dwelling P. obliquiloculata , N. dutertrei and G. tumida . These results imply that TEX₈₆ likely reflects temperature at the thermocline close to the habitat depth of these thermocline dwelling foraminifera in the range of ~ 100 - 200 m water depth. The subsurface warming trend across the last glacial cycle recorded by foraminifera and GDGTs at our site is consistent with the observations at other sites in the IPWP region, indicating a coherent regional glacial-interglacial climate shift. Our study highlights the potential of utilizing a multiproxy approach to constrain proxy interpretation and improve the robustness of the paleoclimate reconstruction.