Paleoenvironmental changes in the northern area of the East China Sea during the past 42,000 years

Abstract

A 33.65-m-long IMAGES (MD982195) core with very high sedimentation rate (ca. 80 cm/kyr), retrieved from the northern part of the East China Sea, was analyzed at 10-cm intervals (about 125 years) for oxygen–carbon isotope of Globigerinoides ruber (sensu stricto), sea surface temperature (SST) from alkenones (U 37′ K), and principal component analysis of planktonic foraminiferal assemblage. The general pattern of the δ 18O curve, resembling the alkenone SST curve, is similar to the standard open ocean oxygen isotope curve. It reveals many light δ 18O peaks apparently corresponding to Dansgaard–Oeschger (D–O) cycles as observed in Greenland ice cores for the last 42 ka. The alkenone SST at the core top is 24 °C, which is close to the modern annual mean SST around the core site. On the other hand, SST during the last glacial maximum (LGM) was about 5 °C lower than that in the present day. Alkenone-derived SST does not correlate with the light negative δ 18O peaks corresponding to the D–O cycles. The oxygen isotope ratios for the surface waters, based on oxygen isotopes of G. ruber and alkenone SSTs, indicate approximately 1.0‰ increase on average during the LGM and many light peaks at the warm D–O events. This suggests that fresh water was supplied to the core site during these events. The meandering of the westerlies during the cold and warm D–O cycles would have caused an atmospheric tele-connection between East Asia and Greenland. The core was divided into four periods based on Q-mode factor analysis of the planktonic foraminiferal assemblage, i.e. (1) a strong upwelling period from 42 to 24 ka, indicated by abundant occurrence of Globigerina bulloides, (2) a cold and low salinity water mass period from 24 to 14 ka, suggested by increase of Neogloboquadrina pachyderma, Neogloboquadrina incompta, and Globigerina quinqueloba, (3) a transitional period from cold to warm water masses from 14 to 8 ka, as indicated by the increase of warm water species such as Pulleniatina obliquiloculata and Globigerinoides ruber, and (4) the present-day warm Kuroshio Current condition after 8 ka. The Kuroshio Current flowed into the ECS since the last 42 ka and weakened during the last glacial maximum (LGM) due to the expansion of the coastal water in the northern part of the ECS.