Carbon isotopic compositions of long-chain n-alkanes in the Japan Sea sediments: implications for paleoenvironmental changes over the past 85 kyr

Carbon isotopic compositions of long-chain n-alkanes in cores KH-79-3 (C-3 and L-3) taken from the Oki Ridge (37°04′N, 134°42′E, water depth 935 m) in the Japan Sea were measured by gas chromatography/isotope ratio mass spectrometry. Fluctuations in the carbon isotopic composition of terrestrially derived material, as determined from δ13Cn-alkane (weighted average of δ13C values for higher plant-derived n-C29 and n-C31 alkanes), range from −29.1 to −32.1‰ for the past 85 kyr in core KH-79-3, C-3. δ13Cn-alkane values in colder climates (18O stages 2 and 4) are depleted in 13C by approximately 1‰ relative to those in warmer climates (stages 1, 3 and 5.1). This suggests that the temporal variations of the δ13C values for higher plant-derived n-alkanes exhibit a close relationship with glacial–interglacial cycles. The results over the past 30 kyr (14C age) reveal that the δ13Cn-alkane values shifted abruptly from −31.1±0.5‰ to −29.9±0.4‰ at around 10 kyr B.P., with the isotopic values remaining relatively constant thereafter. This positive (1.2‰) shift in δ13Cn-alkane values can be attributed to one or both of the following factors: (i) enhancement of primary productivity on land; and (ii) change in vegetational types (e.g. C4/C3 plants ratios and/or changes in the species of C3 plants).