Revisiting the origin of organic matter and depositional environment of sediment in the central Ulleung Basin, East Sea since the late Quaternary

摘要

By analyzing sedimentary geochemical data from three piston cores in the central Ulleung Basin, East Sea, we discuss the origin of organic matter and the depositional environment of sediments from Marine Isotope Stage (MIS) 3 to 1. Downcore profiles show that the values of TOC (Total Organic Carbon), TN (Total Nitrogen), and δ~(13)C_(org) are the lowest values during MIS 2 and the highest during MIS 1. The relationship between TOC/TN and δ~(13)C_(org) shows that most data fall in the region of the marine algae irrespective of MIS stage, which illustrates that the organic matter has been predominantly produced by a marine source rather than by a terrestrial source since the late Quaternary. However, depleted δ~(13)C_(org) values are discretely observed during MIS 2 and MIS 3, particularly in dark laminated mud (DLM), which appears to reflect the influence of episodic event by the East Asian Monsoon rather than the organic matter source changing by lower sea level. Previous sedimentological studies in the Ulleung Basin reported that the depositional environment of sediment in MIS 2 was more reducing than during MIS 1 and MIS 3, but our geochemical data do not support this interpretation. The abundance of most redox-sensitive trace elements (e.g., Mn, V, U, and Mo) has not significantly varied from MIS 3 to MIS 1. In addition, the ratios of redox-sensitive trace elements (e.g., Ni/Co, V/Cr, V/Ni, U/Th, and V/(V + Ni)) indicate a predominantly oxic or suboxic environment. The bottom water condition was similarly oxic to suboxic, during times of sediment deposition since the late Quaternary. The sedimentary layers containing tephra and foraminiferal shell fragments illustrate abnormal element/Al ratios, suggesting that these allochthonous sediment sources are one of important factors controlling the geochemical composition of sediments in the Ulleung Basin.