[B-CG04] Decoding the history of Earth: From Hadean to the present

[BCG04-P11] Reconstruction of the past sea ice expansion during the last 650,000 years based on sand and gravel contents in the sediments from IODP Site U1422, northern Japan Sea

*Ryuki Saijo¹, Song Lu², Tomohisa Irino² (1.Graduate School of Environmental Science, Hokkaido University, 2.Faculty of Environmental Earth Science, Hokkaido University)

The East Asian Winter Monsoon (EAWM) is a monsoon caused by the difference in pressure between the Siberian High and the Aleutian Low. EAWM cools the coastal surface water off the Sikhote-alin and causes the formation of sea ice. The surface water cooled by EAWM and high-salinity water associated with the formation of sea ice ventilates the deep Japan Sea basin, generating the Japan Sea Proper Water (JSPW). Therefore, EAWM have influenced the paleoenvironment and ocean circulation of the Japan Seaalthough its variability of intensity has not been well understood. In order to the intensity of EAWM with high temporal resolution we focused on Ice Rafted Debris (IRD) found in the Japan Sea sediments, which could be a proxy of sea ice. For this purpose, we used sediment cores collected from Integrated Ocean Drilling Program (IODP) Site U1422. This site was located at the highest latitude during the IODP Exp. 346. For IRD quantification, we used a sieve to extract >63 μm fraction followed by examination of the volume of detrital particles using a binocular stereomicroscope. The fluctuation of IRD content was compared with the fluctuation of sediment brightness (RGB-G) which is mainly determined by the organic matter content in response to the fluctuation of summer monsoon, and bulk density (Gamma Ray Attenuation, GRA) which is mainly determined by the diatom frustule content in response to sea-level change. The results show that EAWM (IRD content) tends to be strong during glacial periods (low sea level) and weak during interglacial periods (high sea level).the relationship between RGB-G and GRA suggests the two groups of oceanographic variabilities. One is that the amount of organic matter (low RGB-G) and diatoms (low GRA) show a strong positive correlation reflecting biological production. The other is associated with low RGB-G (dark due to pyrite etc.) regardless of GRA. The correlation between IRD content and RGB-G values in each case revealed that there was little relationship between EAWM and the degree of organic matter preservation (intensity of ventilation).