Stable Isotope Analysis of Japan Sea and East China Sea Sediments: Late Pleistocene Paleoceanographic Reconstructions

The East Asian Monsoon system is an important dynamic of East Asian climates, affecting over one-third of the world’s population. Marginal seas within East Asia are ideal environments to study past fluctuations of monsoon intensities and durations as they are sensitive to climatic and glacio-eustatic sea level changes. This study focuses on continuous sedimentary sequences collected from three Integrated Oceanic Drilling Program Expedition 346 sites; Sites U1426 and U1427 in the Japan Sea and Site U1429 in the East China Sea. Elemental concentration (%TOC, %TN, and %CaCO3) and stable isotope ratios (δ13C and δ15N) are viable proxies to reconstruct past relative productivity rates. Japan Sea sediments show clear differentiation between glacial and interglacial periods in the proxies studied with increased elemental concentrations and isotopic enrichment indicative of increased relative paleoproductivity rates occurring in interglacial periods with sea levels \u3e70 m. Glacial periods, in comparison, generally have decreased relative paleoproductivity rates with decreased elemental concentrations and isotopic depletion. Nitrogen isotopes (δ15N) do not follow the same enrichment cycles as the other geochemical proxies and generally show the most enriched values during glacial low stands, likely indicating anoxic bottom water conditions and denitrification through bacterial processes. The Mid-Pleistocene Transition (MPT) was also identified in Site U1426 sediments through the use of continuous wavelet analysis and multitaper method spectral analysis. Before the MPT, the higher frequency orbital periods of precession and obliquity dominated the paleoproductivity cyclicities while a lower frequency 100,000-year cycle developed at the MPT and dominated the cyclicity to the present. East China Sea sediments do not show clear differentiation between glacial and interglacial periods and instead have relatively constant elemental and isotopic values during the last 350,000 years with the exception of negative excursions during stadial events during Marine Isotope Stage (MIS) 5 and MIS 7. The abrupt negative excursions likely resulted from decreased flow of the Kuroshio Current and reduced upwelling of the Kuroshio Intermediate Water throughout the Okinawa Trough during periods of decreased sea levels. Reduced flow of the Kuroshio Current likely led to the deposition of gravity flow layers during these stadial events