Increased negative anomalies of sedimentary organic matter δ13C and δ15N values in the East Sea (Sea of Japan) during the full glaciation of the late Quaternary

Glacio-eustatic sea-level fluctuations and subsequent paleoceanographic changes in the East Sea (Sea of Japan) have been identified by unique sedimentary features and their geochemical properties throughout the late Quaternary sediments. These events were synchronous basin wide. The effect of sea-level change in the East Sea is fundamental to the anoxia of bottom water resulting from the stagnant, strongly stratified water column with limited ventilation during the glacial periods. The four sediment cores collected from the Japan Basin, Yamato Basin and Korea Plateau in the East Sea showed extraordinarily co-varied δ13C and δ15N values of sedimentary organic matter, representing distinct negative anomalies of both isotopes for the thick dark laminated mud layers formed during full glaciations (MIS 2 and MIS 6) of the late Quaternary. The most plausible cause to induce such peculiar isotope variations is the enhanced contribution of terrestrial organic detritus, in addition to the anoxic diagenetic condition in the bottom water during the glacial times, both of which are characterized by relatively low δ13C and δ15N values. The aeolian contribution of terrigenous particles in the form of Kosa supplied from inland China was intensified in the East Sea during the last glacial maximum. During the glacial periods, the anoxic decay of sedimentary organic matter may lead not only to the reduction of δ13C values attributed to the selective preservation of 13C-depleted organic compounds, but also to the decrease of δ15N values due to the addition of 15N-depleted organic matter by using soluble nitrogenous compounds and the bacterial growth adding 15N-depleted biomass to the residual materials. Thus, the results argue against the utility of δ13C and δ15N values of sedimentary organic matter as a proxy of paleoproductivity and atmospheric pCO2 levels in a marginal sea environment such as the East Sea.