δ¹³C records of diploptene in the Japan Sea sediments over the past 25 kyr

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 δ¹³C values were recognized. The first group appears in the 0-130 cm section (present to 10 kyr B.P.) of the core, the δ¹³C values being -25.0 ± 0.7‰ on average. Diploptene in this section should have originated from cyanobacteria. The second group which is strongly depleted in ¹³C (-53.1 ± 5.3‰ 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.