17:15 〜 18:45
[MIS05-P05] Preliminary XRF scanning results of three marine sediment cores off Shimokita in the northwest Pacific from MR23-05 Leg 2
キーワード：Itrax、XRF core scanner、Radiograph、Deglaciation、Laminations
The North Pacific, the terminus of global deep-ocean circulation, is one of the key regions of the earth’s modern climate system because of its influence on global carbon and nutrient cycles through high surface ocean biological productivity. Also, the North Pacific Intermediate Water formation has notably changed during the late Quaternary and hypothetically driven the changes in oxygen concentration at intermediate depths. Previous records from the northwest Pacific showed intensified ventilation during the last glacial period, while much reduced intermediate ventilation and existence of oxygen minimum zone during the Bølling-Ållerød, Preboreal, and today. Here, we present preliminary results of XRF scanning for depth-transect three piston cores (SMK1-PC from 780 m water depth; SMK2-PC from 954 m water depth; and SMK3-PC from 1356 m water depth) retrieved from continental shelf of the offshore Shimokita Peninsula in the northwest Pacific during MR23-05 Leg 2 to reconstruct the water mass structure and ventilation changes since the last glacial maximum. These cores probably cover the last deglaciation based on the comparisons of their lithostratigraphy and color reflectance data (a* and L*) to those from nearby site cores with precise age models. The parallel laminations (no bioturbation) were found in the SMK1-PC sec. 16 and SMK2-PC sec. 9–14, suggesting low oxygen level in the bottom water during the last deglaciation, probably the Bølling-Ållerød and Preboreal intervals.To investigate the environmental condition during the last deglaciation and the structure of lamination, we semiquantitatively measured elemental composition of the cores using Itrax XRF-core scanner with 2 mm step-size for all sections and 200 µm step-size for the laminated intervals. The laminated intervals show high bromine (Br) counts, suggesting increases in the proportion of marine organic carbon contents compared to other intervals. The elemental compositions and the X-ray radiograph images obtained during 200 µm step-size measurements exhibited clear fine laminations, indicating that both sediment composition and bulk density were largely different at each lamina. According to radiograph images, the lamination consists of the alternation of low- and high-density lasyers. Also, the low- and high-density layers accompanied with high-Br and high-lithogenic elements (e.g., potassium and titanium), respectively. Therefore, although it is still not clear which was in control, cyclic chages in the proportion of organic and lithogenic materials in the sediment could have produced the lamination.