5:15 PM - 6:30 PM
[AHW22-P01] Distribution of sea ice rafted detritus yielded from the sediments of Okhotsk coast of Hokkaido, northern Japan
Keywords:Okhotsk Sea, sea ice, ice rafted detritus, R/V Hakuho-maru KH20-12 expedition
The Sea of Okhotsk is a southern limit of seasonal sea ice distribution in the northern hemisphere. Okhotsk coast of Hokkaido, northern Japan, is located at exactly the southern limit and extent of the cold East Sakhalin Coastal Current here might permit an expansion of sea ice distribution to this region in winter. Spring melting of sea ice in this region drops sands and gravels into the sediment in this area and may trigger phytoplankton bloom, which forms hemipelagic diatom-rich sediments associated with abnormally coarse detrital material as ice rafted detritus (IRD). Since the sea ice extent is controlled by severeness of winter monsoon characterized by cold air mass over the Sea of Okhotsk and strong northwesterly blowing sea ice to the southeast, IRD bearing diatom-rich sediments in Hokkaido coastal area could be a suitable archive of past east Asian winter monsoon variability.
In order to examine the location of IRD deposition in the Hokkaido coastal area in the Okhotsk Sea, we collected 10 short sediment core samples using a multiple corer during the R/V Hakuho-maru KH20-12 expedition from December 8 to 25, 2020. The sediments consisted of sandy silt or diatomaceous silt at shallower (<200 m water depth) sites and of diatomaceous silt or clay to clayey diatomite at deeper sites (>200 m water depth). Large gravels considered as IRD were found only at 3 sites located ~70 km offshore during our sediment core description and subsampling.
Distribution of potential IRD in this area was also investigated by Geological Survey of Japan in 2000 and 2001 using the R/V Hakurei-maru II, and they also reported that the occurrence of gravels in surface sediments was restricted to the sites located ~70 km offshore except the area around shallow and rocky Kitami-Yamato Rise. Gravel deposition during the sea ice melting season has been well known in the further offshore area, therefore the sediments collected were utilized for paleoenvironmental reconstruction. Recently, IRD has rarely deposited in the area closer to the Hokkaido coast, suggesting that sea ice containing IRD usually melted at >70 km offshore regions and the limit of sea ice distribution does not necessarily indicate the location of its melting. In order to utilize the spatial-temporal variation of IRD and diatom-rich sediments as sea ice distribution proxies, mechanisms for connecting the spring sea ice melting with location of IRD deposition need to be further investigated.
In order to examine the location of IRD deposition in the Hokkaido coastal area in the Okhotsk Sea, we collected 10 short sediment core samples using a multiple corer during the R/V Hakuho-maru KH20-12 expedition from December 8 to 25, 2020. The sediments consisted of sandy silt or diatomaceous silt at shallower (<200 m water depth) sites and of diatomaceous silt or clay to clayey diatomite at deeper sites (>200 m water depth). Large gravels considered as IRD were found only at 3 sites located ~70 km offshore during our sediment core description and subsampling.
Distribution of potential IRD in this area was also investigated by Geological Survey of Japan in 2000 and 2001 using the R/V Hakurei-maru II, and they also reported that the occurrence of gravels in surface sediments was restricted to the sites located ~70 km offshore except the area around shallow and rocky Kitami-Yamato Rise. Gravel deposition during the sea ice melting season has been well known in the further offshore area, therefore the sediments collected were utilized for paleoenvironmental reconstruction. Recently, IRD has rarely deposited in the area closer to the Hokkaido coast, suggesting that sea ice containing IRD usually melted at >70 km offshore regions and the limit of sea ice distribution does not necessarily indicate the location of its melting. In order to utilize the spatial-temporal variation of IRD and diatom-rich sediments as sea ice distribution proxies, mechanisms for connecting the spring sea ice melting with location of IRD deposition need to be further investigated.