The Sedimentary Geochemical Characteristics Of The Core S9 In The Northern Okinawa Trough And Their Environmental Significance During Holocene

High resolution sediment records during Holocene have been acquired by the AMS14 C age-testing, grain size analyzing, water content analyzing, organic carbon and nitrogen, carbon isotope and trace elements of the core S9 sediments(128°15.8230′E, 29°23.4989′N), which was taken from the northern Okinawa Trough with 251 cm length and at 1062 water depth during the Okinawa Trough Cruise in the summer of 2013. A total of 251 samples were collected from S9 at 1 cm intervals. 0.1 g dry material of one sample was used for particle size measurement by Cilas 940 L Laser particle size instrument in the key laboratory of marine geology and environment, Institute of Oceanology, Chinese Academy of Sciences. A high-resolution age model of the core was established on five samples(30 cm, 80 cm, 165 cm, 220 cm and 243 cm) on the basis of accelerator mass spectrometry(AMS) radiocarbon dating of planktonic foraminifera(Globigerinoides ruber, Globigerinoides sacculife, Pulleriatina obliquilocalata, Neogloboquadrina dutertrei, Globigerina bulloides) which are measured at Woods Hole Oceanographic Institution. The depositional age is estimated to be 13.39 ka at the bottom of the core and 1.1 ka at the top of the core. The sedimentation rate of the core is 8 cm/ka during the early Holocene and increases from 17 cm/ka to 32 cm/ka during the middle Holocene and increase to 38 cm/ka during the late Holocene. Sediments from S9 core are primarily composed of silt(60%~90%), with a mean grain size of 5.6~7.2 Φ and a median grain size of 5.2~7.2 Φ. A tephra layer in the core(159~169 cm) correlate well with the volcanic eruptions at 7.6 ka in southern Japan. The results reveal that the grain size composition of the core S9 present two-stage obvious variation in millennial scales: the mean and silt increase and reach the maximum in 10.5~8.0 ka B.P., and the mean and clay decrease in 8.0~1.0 ka B.P.. The changes of grain size of core S9 are related with the relative sea-level changes, implying the control of the relative sea-level on regional material transport. The relative sea-level has been stable since 7.5 a B.P. and the grain size varies in century scales and is controlled by regional summer monsoon rainfall in the source region.The grain size parameters of the core S9 are well correlated with the stalagmite record of δ18O in the source region. The mean has been decreased since 7.5 ka B.P., showing well consistency with the regional summer monsoon rainfall. Moreover, the core S9 records several climate events in century scales, which the grain size decreases obviously, and they are corresponding to the periods when regional monsoon decrease. Organic carbon and nitrogen and carbon isotope datas reveal that the organic matter of S9 is derived from seaweed. Trace elements datas reveal that the rare earth elements of S9 are derived from Changjiang and Huanghe.The grain size composition, organic carbon and nitrogen, carbon isotope, trace elements of the core S9 implies the influence of the relative sea-level changes and regional monsoon rainfall in the source region to the sediments composition in the Okinawa Trough, and provides a powerful tool to using marine sediments features to rebuild regional monsoon rainfall.