A biomarker approach for assessing marine and terrigenous inputs to the sediments of Sea of Okhotsk for the last 27,000 years
A sediment core taken from the Sea of Okhotsk, which is located in the western North Pacific rim with an extensive seasonal sea-ice cover, was investigated for lipid class compounds, total organic carbon, total nitrogen, and δ 13C of bulk organic matter to reconstruct changes in sedimentary regimes of marine and terrestrial organic matter over the last 27 kyrs. Higher molecular weight n-alkanes, fatty acids, and fatty alcohols showed an increase in terrigenous inputs in glacial and, to a greater extent, in early deglacial periods. The enhanced terrestrial input in the deglaciation stages has, for the first time, been found and never been reported in the open ocean. Biomarker profiles, together with the coarse-grain size distribution in nearby sediment cores, suggested that sea-ice rafting was the main transport mechanism of continental material to the Okhotsk Sea sediments. However, the ice-rafting inputs have been weakened from 14 kyrs BP to the Holocene probably due to a rapid reduction of the sea-ice cover and continental shelf erosion. Profiles of C 37 alkenones, phytol, and brassicasterol, as well as temporal variation in calcium carbonate and organic carbon contents, indicated that the marine inputs were enhanced during the deglaciation. The enhanced marine inputs were likely caused by an intensification of the primary production induced by a possibly increased nutrient supply from the continental shelves in early deglaciation, and by the deglacial retreat of the sea-ice cover at 14 kyrs BP. This study demonstrates that biomarkers in the marginal marine sediments had recorded changes in the biogeochemical processes in the Sea of Okhotsk over the last 27 kyrs, which may be significantly affected by changes in the Okhotsk Water exchange processes such as sea-ice activity and sea-ice transport of terrigenous materials.