[M-IS12] 古気候・古海洋変動

[MIS12-09] Increased tropical South Pacific western boundary current transport by coral skeletal-bound nitrogen isotope record

*Wen-Hui Chen¹、Haojia Abby Ren¹、John C. H. Chiang^2,3、You-Lin Wang^3,4、Ren-Yi Cai-Li¹、Yi-Chi Chen¹、Chuan-Chou Shen^1,5,6、Frederick W. Taylor⁷、Thomas M. DeCarlo⁸、Chau-Ron Wu^3,4、Horng-Sheng Mii⁴、Xingchen Tony Wang⁹ (1.Department of Geosciences, National Taiwan University, Taipei, Taiwan、2.Department of Geography, University of California, Berkeley, CA, USA、3.Research Center for Environmental Changes, Academia Sinica, Taipei, Taiwan、4.Department of Earth Sciences, National Taiwan Normal University, Taipei, Taiwan、5.High-Precision Mass Spectrometry and Environment Change Laboratory (HISPEC), Department of Geosciences, National Taiwan University, Taipei, Taiwan、6.Research Center for Future Earth, National Taiwan University, Taipei, Taiwan、7.Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, Austin, USA、8.College of Natural and Computational Science, Hawai’i Pacific University, Honolulu, HI, USA、9.Department of Earth and Environmental Sciences, Boston College, Chestnut Hill, Massachusetts, USA)

キーワード：coral skeletal-bound nitrogen isotope, western boundary current, decadal climate variability, Solomon Sea

Instrumental and paleoclimate records show decadal climate variability in the tropical Pacific. Climate models suggest that this decadal variability arises from the wind-driven meridional overturning circulation between the tropical and subtropical oceans, which regulates decadal-scale temperature fluctuations in the Pacific Ocean and globally. However, it is still challenging to demonstrate observationally, owing to the insufficient instrumental record of oceanic processes. The Solomon Sea in the Southwest tropical Pacific is a major connection between the subtropics and the equator. The equatorward Low Latitude Western Boundary Current represents a key component of the meridional circulation cell in the Pacific and is a major source of water mass for the Equatorial Undercurrent, however, long-term observations of its transport remain unclear. The nitrogen isotopes in the Solomon Sea thermocline are particularly sensitive to changes in the water masses and circulation. Upwelling in the Eastern Equatorial Pacific brings nitrate to the surface. The westward and off the equator elevated ¹⁵N/¹⁴N surface water transport due to partial assimilation of the nitrate. On the other hand, the subtropical waters entering the Solomon Sea are characterized by lower ¹⁵N/¹⁴N due to a high N₂ fixation rate in the subtropics. As a result, variabilities in the ¹⁵N/¹⁴N of the thermocline nitrate in the Solomon Sea are primarily influenced by altering the subtropical vs. equatorial water. Here we present a skeletal-bound nitrogen isotopic record from 1919 to 2012 of a Porites coral core drilled from the western Solomon Island. From a long-term perspective, the ¹⁵N/¹⁴N ratio declines as the global surface temperature rises. The record suggests that the South Pacific western boundary current has strengthened in the past century, and which may contribute to the reported strengthening of the equatorial undercurrent. In addition, the ¹⁵N/¹⁴N record shows strong decadal variability, indicative of weaker equatorial Pacific upwelling and stronger western boundary transport when the eastern equatorial Pacific is in the warm stage of the Pacific Decadal Oscillation.