5:15 PM - 6:45 PM
[MIS03-P06] Origin and burial rate of organic carbon in the sediments at IODP Site U1422, the northern Japan Sea, during the last 700,000 years
Keywords:Japan Sea, organic matter, stable carbon isotope, IODP Site U1422
In order to clarify the mechanism of organic matter deposition in the northern Japan Sea and the relationship between dark-light cycles, physical properties, and the amount of organic matter in the sediment, we measured total organic carbon (TOC) and stable carbon isotope ratios (δ13C) in the sediments at Site U1422 collected in the northern Japan Sea during Integrated Ocean Drilling Program (IODP) Expedition 346. The results were also compared with the mineral composition measured by XRD to infer the relationship between organic matter deposition and paleoenvironmental changes in the northern Japan Sea over the past 700,000 years. For this study, we collected 216 subsamples from the upper 30 m of the core at 10-20 cm intervals along the splice. Samples were freeze-dried and analyzed for the organic carbon content (TOC%) and stable carbon isotope ratios using EA-C-IRMS after removing carbonate with HCl.
TOC% and δ13C variations were compared to core photographs and brightness. Dark layers were generally associated with high TOC%, while light layers showed low TOC%. However, some dark layers with low TOC% with low δ13C values were found. The results show that the periods of low δ13C, low sea level, and high burial rate of terrestrial organic matter are synchronized, which were also corresponded to high jarosite and pyrite in the sediments. The predominance of pyrite (jarosite) is also considered to indicate a reducing environment in the deep ocean at that time. This can be interpreted as a result of the active transport of materials from land to sea during the low sea level period, while nutrient supply from the open ocean to the Japan Sea decreased due to low sea level, and marine biological production did not increase.
Comparison of the burial rates of marine-derived and terrestrial-derived organic matter shows that there were periods when the respective peaks were synchronized. In such periods, either the deep ocean was a reducing environment and organic matter was better preserved, or organic matter production in the surface ocean and on land was particularly active and exceeded the decomposition of organic matter in the deep ocean. We could interpreted it as an increase in nutrients supplied to the Japan Sea from the East China Sea due to increased summer precipitation from the east Asian summer monsoon, as well as an increase in vegetation on land in the Japanese Islands. On the other hand, when the burial rate of marine organic matter increased, we did not find any cases where the burial rate of terrestrial organic matter did not increase.
TOC% and δ13C variations were compared to core photographs and brightness. Dark layers were generally associated with high TOC%, while light layers showed low TOC%. However, some dark layers with low TOC% with low δ13C values were found. The results show that the periods of low δ13C, low sea level, and high burial rate of terrestrial organic matter are synchronized, which were also corresponded to high jarosite and pyrite in the sediments. The predominance of pyrite (jarosite) is also considered to indicate a reducing environment in the deep ocean at that time. This can be interpreted as a result of the active transport of materials from land to sea during the low sea level period, while nutrient supply from the open ocean to the Japan Sea decreased due to low sea level, and marine biological production did not increase.
Comparison of the burial rates of marine-derived and terrestrial-derived organic matter shows that there were periods when the respective peaks were synchronized. In such periods, either the deep ocean was a reducing environment and organic matter was better preserved, or organic matter production in the surface ocean and on land was particularly active and exceeded the decomposition of organic matter in the deep ocean. We could interpreted it as an increase in nutrients supplied to the Japan Sea from the East China Sea due to increased summer precipitation from the east Asian summer monsoon, as well as an increase in vegetation on land in the Japanese Islands. On the other hand, when the burial rate of marine organic matter increased, we did not find any cases where the burial rate of terrestrial organic matter did not increase.