Long-Chain Alkenones in the South Yellow Sea Sediments and Their Indicative Significance for Haptophytes Species

Long-chain alkenones (LCAs) have been widely used as important biomarkers in palaeoceanographic studies. However, the commonly used LCAs proxies are mainly based on C₃₇ alkenones, and it is still lack of the studies about the distribution and indications of LCAs with different chain lengths other than C₃₇ alkenones. Here, the composition and distribution of LCAs were analyzed in surface sediments from the southern Yellow Sea (SYS) and a sedimentary core (A02-C) from the central Yellow Sea (YS) mud area. The results showed that C₃₇, C₃₈ and C₃₉ alkenones were the major LCAs in surface sediments of the SYS, and the relative contents of C_38:2Et, C_37:2Me, C_37:3Me, C_38:2Me, C_38:3Et, C_38:3Me, C_39:2Et and C_39:3Et were 18.3%–59.8%, 22.6%–41.2%, 7.4%–23.0%, 6.6%–15.4%, 3.8%–13.3%, 3.6%–8.7%, 2.8%–6.0% and 0.7%–3.0%, respectively. Then the relationships of U _38Me ^K -U _38Et ^K and U ₃₇ ^K′ -U _38Et ^K indicate that LCAs are mainly derived from Emiliania huxleyi (E. huxleyi). High ratios of total C₃₇ alkenones to total C₃₈ alkenones (K₃₇/K₃₈) (1–1.2) were found in the central SYS, corresponding to the relatively high abundance of E. huxleyi; while low ratios of K₃₇/K₃₈ (0.7–0.9) were observed at nearshore area of the SYS where Gephyrocapsa oceanica (G. oceanica) has relatively high abundance. The spatial distribution of K₃₇/K₃₈ ratio is also consistent with that of coccolithophores nannofossil in the sediments. In addition, K₃₇/K₃₈ ratio in core A02-C varied from 0.7 to 1.1 with a gradual decreasing trend during the past 5.5 kyr. This suggests that the relative abundance of E. huxleyi decreased gradually, caused by the changes in the Yellow Sea Warm Current (YSWC) and the East Asian Winter Monsoon (EAWM) during this period.