The hydroclimate changes in the northern Okinawa Trough during middle Holocene: Evidence from planktonic foraminiferal assemblages

Planktonic foraminifera assemblages in the northern East China Sea are significantly influenceed by both the Changjiang Diluted Water and Kuroshio Current. A total of 28 species in the coaser faction (>150μm) are identified based on planktic foraminifera analysis of 178 samples from core NOKT-3 in the northern Okinawa Trough. The results show that variations in percentage of Globigerina quinqueloba is characterized by sharp fluctuation in early Mid-Holocene and gradual decline in late Mid-Holocene. This low temperature and low salt salinity species is an indicator of the Changjiang Diluted Water, which is controlled by the East Asian summer monsoon precipitation. Therefore, the evolution of precipetation in the middle and lower reaches of the Yangtze River between 6330 and 3890aBP can be divided into three major stages based on variations in its content: (1) high precipetation during the period of 6330~4950aBP, which correspondes to the Holocene optimum; (2) a decreased trend of precipetation from 4950 to 4600aBP, implying termination of the Holocene optimum; (3) a marked decrease in the monsoon precipitation between 4600 and 3890aBP, indicating an arid climate condition. This precipetation pattern could be related to the general weakening and southeastward retreat of the East Asian summer monsoon resulted from the decreased solar-radiation in the northern hemisphere during the Middle Holocene. Moreover, remarkably high abundance of P. obliquiloculat during 6330~5100aBP, which is normally an indicator species of the Kuroshio Current, suggests that influence of the Kuroshio Current in the Okinawa Through was persistently strong. The abundance of P. obliquiloculata sharply decreased during 5100~4000aBP, well corresponding to the so-called Pulleniatina minimum event (PME). Our results show that there were not a cold climate condition corresponding to the PME during the period of late Mid-Holocene. In addition, enhancement of the El Niño-like condition in the tropical Pacific region during this time window also can not fully explain the occurrence of PME in the northern Okinawa Trough.