Long-chain alkenones and alkyl alkenoates in the coastal and pelagic sediments of the northwest North Pacific, with special reference to the reconstruction of Emiliania huxleyi and Gephyrocapsa oceanica ratios

Long-chain (n C37---C39) alkenones and (n C37---C38) alkyl alkenoates in marine sediments are derived mainly from two marine coccolithophorids, Emiliania huxleyi and Gephyrocapsa oceanica. Laboratory cultured strains of E. huxleyi (strain EH2) and G. oceanica (strain GO1), isolated from the Great Barrier Reef and Mutsu Bay in northern Japan (41°N, 141°E), respectively, were cultured at 10°C, 15°C, 20°C, 22°C, 25°C, 28°C and analyzed for C37 to C39 alkenones, C36 methyl alkenoate (FAME) and C36 ethyl alkenoate (FAEE). Results obtained indicated significant differences in the alkenone and alkyl alkenoate profiles between these two algal species. The ratios of FAEE to C37 alkenones (EE/K37) and C38 alkenones (EE/K38) in G. oceanica were higher than those in E. huxleyi and, for both algal species, these ratios showed strong negative nonlinear exponential correlations when plotted against the unsaturation index of C37 alkenones (Uk•37). Using fitted equations for these relationships, the relative abundance of E. huxleyi and G. oceanica in the sediment samples from the northwest North Pacific and the coastal and inland waters of Japan were estimated and compared with direct microscopic determinations. E. huxleyi comprised more than 80% of the sedimentary coccolith assemblage in the open ocean, whereas G. oceanica was exclusively dominant in the inland waters. These results almost agreed with microscopic counts of these coccoliths occurring in the sediment samples. In the light of these findings, we suggest that there is a possibility that the relative abundance of E. huxleyi and G. oceanica in sediments may be estimated using nonlinear exponential calibrations based on EE/K37 to Uk•37 and EE/K38 to Uk•37 regressions.