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Using foraminiferal Mg/Ca ratios to detect an ocean-warming trend in the twentieth century from coastal shelf sediments in the Bungo Channel, southwest Japan

Center for Marine Environmental Studies, Ehime University, Ehime 790-8577, Japan, mkuwae{at}sci.ehime-u.ac.jp

Yuichi Hayami

Ariake Sea Research Project, Saga University, Saga 840-8502, Japan

Hirotaka Oda

Center for Chronological Research, Nagoya University, Nagoya 464-8602, Japan

Azumi Yamashita

The Ehime Prefectural Fisheries Experimental Station, Ehime 798-0104, Japan

Atsuko Amano

Center for Marine Environmental Studies, Ehime University, Ehime 790-8577, Japan

Atsushi Kaneda

Center for Marine Environmental Studies, Ehime University, Ehime 790-8577, Japan

Minoru Ikehara

Center for Advanced Marine Core Research, Kochi University, Kochi 783-8502, Japan

Yoshio Inouchi

Faculty of Human Sciences, Waseda University, 1-7-14 Nishi Waseda 404, Shinjukuku, Tokyo 169-0051, Japan

Koji Omori

Center for Marine Environmental Studies, Ehime University, Ehime 790-8577, Japan

Hidetaka Takeoka

Center for Marine Environmental Studies, Ehime University, Ehime 790-8577, Japan

Hodaka Kawahata

Ocean Research Institute, University of Tokyo, Tokyo 164-8639, Japan

To assess abnormal warming of temperature in southwest Japan's coastal seas during the twentieth century, we developed a 200 yr interannually resolved record of planktonic and benthic foraminiferal Mg/Ca-based temperature using neritic coastal sediment cores. The winter to late spring bottom temperature (50 m) record, based on benthic foraminiferal (Nonion japonicum) Mg/Ca ratios in the Bungo Channel (BC), showed consistent variation with observed temperatures on a five-year average basis. The BC bottom temperature record showed a significant increasing trend of 1.5°C /100 yr during the twentieth century, which was never apparent in nineteenth century. That result suggests that our Mg/Ca-based thermometry approach using coastal benthic foraminifera can detect abnormally rising temperatures in neritic coastal seas in southwest Japan. The abnormal warming of winter to late spring bottom temperature in the BC contrasts with the lack of an increasing trend in the Globigerinoides ruber-based summer to autumn temperature for the upper 20 m in the slope region of southwest Japan and the lack of an increasing trend in the steric sea level in the region. These results indicate a warming trend of the neritic coastal ocean in southwest Japan, especially in winter to late spring. The timing of the onset of the prolonged abnormal high-temperature stage, which started in the early twentieth century, suggests a link of the neritic coastal ocean in Japan with human-induced global warming.

Key Words: Global ocean warming • foraminiferal Mg/Ca • neritic coastal sediments • Bungo Channel • southwest Japan.

The Holocene, Vol. 19, No. 2, 285-294 (2009)
DOI: 10.1177/0959683608100573


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