Low-Salinity Isolation Event in the Japan Sea in Response to Eustatic Sea-level Drop during LGM: Reconstruction Based on Salinity-Balance Model.

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  • Matsui Hiroyuki
    Department of Geoscience, Graduate School of Science, University of Tokyo
  • Tada Ryuji
    Department of Geoscience, Graduate School of Science, University of Tokyo
  • Oba Tadamichi
    Graduate School of Environmental Earth Science, Hokkaido University

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Other Title
  • 最終氷期の海水準変動に対する日本海の応答　　塩分収支モデルによる陸橋成立の可能性の検証
  • サイシュウ ヒョウキ ノ カイスイジュン ヘンドウ ニ タイスル ニホンカイ
  • Reconstruction Based on Salinity-Balance Model
  • 塩分収支モデルによる陸橋成立の可能性の検証

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Abstract

The Japan Sea is a semi-enclosed marginal sea connected with other seas by four shallow straits. Tsushima Strait (including Korea Strait) and Tsugaru Strait are the deepest, with a sill depth of approximately 130m, which is close in magnitude to the eustatic sea level drop during the Last Glacial Maximum (-105 to -130m). The sill depths of the other two straits are much shallower (55m and 15m), indicating that they were closed completely during the LGM. For this reason, it has been controversial whether the Japan Sea was completely isolated or not during the LGM. Oba (1980) discovered a large negative excursion in δ18O of planktonic foraminifera in the Japan Sea during the LGM, which is as large as 2.5 permil. This negative δ18O excursion was considered by Oba (1984) to be an expression of a low-salinity event and a result of reduced seawater flux from the East China Sea due to the isolation of the Japan Sea. However, a quantitative explanation has never been attempted previously.<br>In this study, we adopt a simple salinity balance model to the Japan Sea in order to reconstruct the negative salinity excursion observed there during the LGM. As a first step, δ18O records of planktonic foraminifera are converted to reflect the salinity of the surface water. Second, sea-water/fresh-water influx ratios are calculated based on the salinity-balance model. Assuming a reasonable range of fresh-water influx to the Japan Sea and a linear relationship between sill depth and inflow rate, we could predict the sill depth changes in the Tsushima Strait during the LGM. The sill depth decreased from 50m at 33ka to 10m at 19ka, stayed at 10m from 19-15ka, then increased from 10 to 40m at 13ka.

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