Interglacial-glacial variations of opal and water contents in the central part of the Okhotsk Sea during the last 500 ka

In order to understand the orbital-scale glacial-interglacial variation, water content and biogenic opal content were measured from about 18-m long core PC5B, which was collected in the central part of the Okhotsk Sea during MR06-04 cruise using R/V Mirai (JAMSTEC). Age model of core PC5B was determined indirectly with a correlation of sediment color b* values to the well-dated core MD01-2415, complement to the calibration by SPECMAP. The bottommost of core was estimated to be about 550 ka BP, but the topmost part of the core was lost during the core recovery, resulting that the core-top age was about 60 ka BP. The Okhotsk Sea has experienced distinctly the cyclic variations of the glacials and interglacials and the biogenic opal content recorded the surface-water production. Both water content and opal content are high during the interglacial periods, but low during the glacial periods, showing the distinct cyclic variations. However, the water content decreases abruptly at the later stage of the glacials, whereas the opal content increases distinctly during the ealier stage of the interglacials. These orbital-scale glacial and interglacial variations are related to the eccentricity forcing with 100-kyr periodicity and by the obliquity forcing with 40-kyr periodicity. Biogenic opal content indicates that the surface water diatom production increases during the warm interglacial periods, which is attributed to the well mixture of water column enough to supply the nutrients into the surface water layer under no sea ice condition. In contrast, lower opal content during the glacial periods reflects the reduced surface water production, which results from the light limitation due to the extended sea ice distribution. However, the seasonal sea ice condition at the core collection site during the glacial periods implies that strong stratification of water column is also responsible for the decreased surface water production due to the insufficient nutrients. Because the core PC5B consists largely of the fine-grained sediments, the water content variation between the glacials and interglacials may not result from different grain size but result from different particle constituents. However for the better interpretation, more data are to be required.