Paleoceanographic changes in the Northern East China Sea during the last 400 kyr as inferred from radiolarian assemblages (IODP Site U1429)

摘要

The East China Sea (ECS) is a shallow marginal sea that is sensitive to glacio-eustatic sea-level changes and is influenced by warm oligotrophic water of the Kuroshio Current (KC), the nutrient-rich Taiwan Warm Current, and freshwater discharges from rivers in southern China during the East Asian summer monsoon season. In this area, local paleoceanographic changes for times prior to 40 ka remain poorly studied because of high sediment accumulation rates on the seafloor. During Integrated Ocean Drilling Program Expedition 346, long sediment cores representing the last 400 kyr were retrieved from the northern part of the ECS (Site U1429). In these cores, radiolarians are abundant and well-preserved, thus using the ecological properties of radiolarians, we analyzed how glacio-eustatic sea-level variations have influenced the paleoceanography of the ECS over the last 400 kyr, with a focus on changes in water properties at intermediate depths. Additionally, the summer sea surface temperature (SST) and intermediate water temperature at about 500?m were quantified by means of data on selected radiolarian species. The KC influenced the shallow water at Site U1429 during each interglacial period over the last 400 kyr (marine isotope stages [MISs] 1, 5, 7, 9, and 11), causing a high summer SST (about 27?°C), although inflow of the KC into the ECS was probably delayed until after the sea-level maximum of interglacial MIS 1 and MIS 5. During this lag time, ECS shelf water was the dominant influence on the system. During glacial periods (MISs 2–4, 6, and 10), our data suggest that coastal conditions prevailed, probably because of a sea-level drop of more than 90?m. At these times, the summer SST was colder, ca. 20?°C. Changes in the relative abundance of Cycladophora davisiana indicate that the most significant changes in the bottom water occurred during MIS 6, when the bottom water likely became poorer in oxygen. An increase in the shallow-water primary productivity during MIS 7 and MIS 6 was probably the key factor causing the oxygen-poor conditions.