千島弧内帯における栄養塩に富む深層水の初出現 (予報)

抄録

Our purpose is to clarify Late Oligocene to Early Miocene paleoceanographic changes in the back-arc region of the Kuril Arc. We have only a few of reports on sedimentological and organic geochemical considerations as the Paleogene formations around the Sea of Okhotsk, which prompted us to research.
Kitami area, our research field, is located on the back-arc region of the Kuril Arc (Fig. 1), and has Tatsukobu-Tsubetsu biosiliceous deposit whose thickness attains more than 2500m. Based on biostratigraphic and rediometric ages, Tatsukobu-Tsubetsu biosiliceous deposit appears to be accumulated during the period ranging from Late Oligocene to earliest Early Miocene. This biosiliceous deposit begins with a basal sandstone including glauconite, which rests unconformably upon the Wakamatsuzawa Formation including silicified woods and roots. According to lithostratigraphic correlations, andesitic volcaniclastic bed of the upper part and acidic tuff bed of the middle part of the Tatsukobu Formation in the Kitami area are correlated to biotite-rich acidic tuff bed and hornblende-rich welded tuff bed in the Ponki-Ashoro area. Radiometric ages of acidic tuff bed and welded tuff bed in the Ponki-Ashoro area indicate 23.8Ma (fission track age) and 27.4Ma (K-Ar age).
To clarify the marine primary production of Tatsukobu-Tsubetsu biosiliceous deposit, we measured organic carbon congtents, C-H-N-O compositions of kerogen (insoluble organic matter) and Sulphur contents. Atomic composition analysis of kerogen concentrates is to examine the presence of terrigenous organic carbon. Plotted in a “van Krevelen Diagram” (i.e., H/C vs. O/O), estimated marine organic carbon (EMOC) and estimated non -marine organic carbon (ENOC) were calculated.
Based on stratigraphic changes in the amount of EMOC and ENOC, Tatsukobu biosiliceous deposit was divided into three stages.
In Depth 1335m-1450m and Depth 0m-350m (Fig. 5), stratigraphic variations in the amount of EMOC are parallel to those of ENOC. However, increase in EMOC are associated blosely with decrease in ENOC within Depth 350m-1335m. EMOC/ENOC ratios indicate that organic matter within Depth 350m-1335m included higher contents of marine-origin organic materials than those within other horizon. According to sedimentary petrological investigations, this deposit also contains larger volume of non-siliceous rocks in Depth 350m-1335m than in other horizon. In the past ocean, we conclude that amount of nutrients and dissolved silica components derived from land were parallel to those of ENOC. If this had happened at the back-arc region of the Kuril Arc during Late Oligocene to earliest Early Miocene, nutrients of deep water-origin contributed more to marine primary production than those of land-origin at the time of Depth 350m-1335m. This presumption suggests that the proto form of the Kuril Basin served as a vessel of nutrient-rich and silica-poor deep water at about 27.4Ma (Fig. 2).