Fluctuation of deep and surface ocean circulations off the northern Japanese Islands in the northwestern Pacific during the last 27 kyr

Monday, 5 September 2005

This presentation is part of: Poster Session I

Fluctuation of deep and surface ocean circulations off the northern Japanese Islands in the northwestern Pacific during the last 27 kyr

Kayo Minoshima¹, Hodaka Kawahata¹, Tomohisa Irino², Ken Ikehara¹, Masao Uchida³, Minoru Yoneda⁴, and Yasuyuki Shibata⁴. (1) National Institute of Advanced Industrial Science and Technology, Higashi 1-1-1, Ibaraki 305-8567, Tsukuba, Japan, (2) Graduate School of Environmental Sience, Hokkaido Univetsity, Sapporo, Japan, (3) Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan, (4) National Institute for Environmental Studies, Tsukuba, Japan

The Oyashio current, which has high nutrient concentration due to strong upwelling, shows high primary production in the northwestern Pacific. Elevated primary production increases export production, which reduces carbon dioxide concentration in the surface ocean. In addition, the surface ocean in this area has many cold and warm eddies. Therefore this area is characterized by the wind-driven Oyashio current and the mixed region. In order to understand the fluctuation of marine environment through the entire section of the water column on a glacial-interglacial time scale, we retrieved the piston core PC-6 from 2,215 m water depth on the continental margin off Sanriku (40N, 143E) during R/V Kairei KR02-15 cruise, 2002. We analyzed ¹⁴C ages of benthic and planktonic foraminifera to evaluate changes in the circulation of the Pacific deep water and total organic carbon (TOC), biogenic opal and C37 alkenone contents in order to reconstruct the changes in past primary production after the last deglaciation. Differences of ¹⁴C ages between benthic and planktonic foraminifera ranged from 1160 to 1460 years (av. 1326 years) without an apparent variation after 15 ka. It suggests that modern state of deep water ventilation was established around 15 ka. It is in contrast to the results obtained off Shimokita (41N, 142E; Ahagon et al., 2003; Ohkushi et al., 2004), where the ventilation ages were smaller during the deglaciation than the present. TOC and C₃₇ alkenone contents were relatively low (av. 1.04 % and av. 1.56 μg/g, respectively) during the glacial (27 - 18 ka) but gradually increased after 15 ka. After 6 ka, TOC contents still increased whereas C₃₇ alkenone contents decreased slightly. Biogenic opal contents were low (av. 0.75 wt. %) from the glacial to deglaciation and increased in the Holocene. Especially the rapid increase was observed after 5 ka (up to 31.2 wt. %). These lines of evidence suggest that modern surface water condition was formed after 5 ka and nutrient supplied by eddies resulted in high primary production. It indicates that eddies in this region play an important role in primary production.

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