Mineralogical and geochemical changes in the sediments of the Okhotsk Sea during deglacial periods in the past 500 kyrs

Characterized by extended seasonal sea-ice cover, the Okhotsk Sea is widely considered one possible source of the North Pacific Intermediate Water (NPIW). Therefore, reconstructing the characteristics of the sediments that resulted from the ice-melting pulses of this northeastern Asian sea during glacial–interglacial cycles is crucial to understanding past climatic changes and NPIW formation in the northwest Pacific Ocean. Here, we produced the detailed mineralogy and geochemistry of the upper 20 m of the sediments of IMAGES Core MD012414, which was drilled in the central part of the Okhotsk Sea. This depth covers the last 500 kyrs. The mineralogical data show that in glacial periods, the sediments were predominantly composed of quartz and plagioclase with a volume of about 80%, indicating that they were mainly from the surrounding landmasses. During interglacial periods, however, biogenetic calcite and amorphous opal drastically increased from less than 5% to a remarkable 40–60%, while the element ratios Mg/Al, Ca/Al and Si/Al also had anomalous increases from 0.2 to 0.5, 0.12 to 1.2 and 3.5 to 10, respectively. These characteristics of the sediments in interglacial periods strongly suggest that the melting of permanent ice opened a gateway, thereby letting the northwest Pacific warmer water flow into the Okhotsk Sea, which subsequently increased biogenetic productivity during the deglacial periods. In addition, in early interglacial periods, the bottom water became anoxic, as evidenced by the presence of dolomite and the enrichment of the Mn/Al and P/Al ratios more than 25 to 200 times. The fact that these trends did not occur in stage 7 strongly suggests that stage 8 may have been a warmer glacial period.