delta C-13 records of diploptene in the Japan Sea sediments over the past 25 kyr

摘要

Stable carbon isotopic compositions were determined for diploptene in the sediment core KH-79-3, L-3 (core length: 235 cm) taken from the Oki Ridge in the southern part of the Japan Sea. Two major groups of diploptene with different delta(13)C values were recognized. The first group appears in the 0-130 cm section (present to 10 kyr B.P.) of the core, the delta(13)C values being -25.0 +/- 0.7 parts per thousand on average. Diploptene in this section should have originated from cyanobacteria. The second group which is strongly depleted in C-13 (-53.1 +/- 5.3 parts per thousand on average), is observed in the 145-204 cm section (12-21 kyr B.P.). Diploptene in this section might have originated from methanotrophic bacteria. This indicates that an anoxic bottom water condition was developed during 12-21 kyr B.P. (the last glacial period), which was perhaps caused by the development of seawater stratification in the Japan Sea.